Merge branch 'master' into checkpointing

Add option to skip branch-history check

.github/workflows/builds-and-tests.yml

@@ -142,6 +142,10 @@ jobs:
 
     continue-on-error: ${{ matrix.enzyme && true || false }}
 
+    # Enable ccache for all jobs except Windows (would need sccache).
+    env:
+      USE_CCACHE: ${{ matrix.os != 'windows-latest' }}
+
     steps:
       # Fix 'No space left on device' errors for Ubuntu builds.
       - name: Run Actions Cleaner
@@ -290,6 +294,52 @@ jobs:
           echo "OMPI_CC=$LLVM_PREFIX/bin/clang" >> $GITHUB_ENV
           echo "OMPI_CXX=$LLVM_PREFIX/bin/clang++" >> $GITHUB_ENV
 
+      # Restore the compiler cache (ccache). The key embeds the run id, so new
+      # runs save a fresh snapshot; the restore-keys prefix warm-starts from the
+      # most recent prior run (incl. the base branch for PRs).
+      - name: cache ccache
+        if: ${{ env.USE_CCACHE == 'true' }}
+        uses: actions/cache@v5
+        with:
+          path: .ccache
+          key: ccache-${{ matrix.os }}-${{ matrix.build-system }}-${{ matrix.target }}-${{ matrix.mpi }}-${{ matrix.hypre-target }}-${{ matrix.precision }}${{ matrix.enzyme && '-enzyme' || '' }}-${{ github.run_id }}
+          restore-keys: |
+            ccache-${{ matrix.os }}-${{ matrix.build-system }}-${{ matrix.target }}-${{ matrix.mpi }}-${{ matrix.hypre-target }}-${{ matrix.precision }}${{ matrix.enzyme && '-enzyme' || '' }}-
+
+      # Configure ccache and select how it is injected into the MFEM build:
+      # - make:  set CXX="ccache g++"; for MPI, OMPI_CXX="ccache g++" so mpicxx
+      #          runs ccache around g++ (not ccache around the mpicxx wrapper).
+      # - cmake: set CMAKE_<LANG>_COMPILER_LAUNCHER=ccache.
+      # - enzyme: wrap the brew clang++ via OMPI_CXX.
+      # The chosen options are passed through build-mfem's 'config-options'
+      # input (see the build step below).
+      - name: configure ccache
+        if: ${{ env.USE_CCACHE == 'true' }}
+        run: |
+          command -v ccache >/dev/null 2>&1 || {
+            if [[ "${{ runner.os }}" == "Linux" ]]; then
+              sudo apt-get update && sudo apt-get install -y ccache
+            else
+              brew install ccache
+            fi
+          }
+          echo "CCACHE_DIR=${{ github.workspace }}/.ccache" >> $GITHUB_ENV
+          echo "CCACHE_MAXSIZE=1G" >> $GITHUB_ENV
+          echo "CCACHE_COMPILERCHECK=content" >> $GITHUB_ENV
+          # Ignore header timestamps (restamped by each checkout) so direct mode hits.
+          echo "CCACHE_SLOPPINESS=include_file_mtime,include_file_ctime,time_macros" >> $GITHUB_ENV
+          # Hash absolute paths relative to the workspace.
+          echo "CCACHE_BASEDIR=${{ github.workspace }}" >> $GITHUB_ENV
+          if [[ "${{ matrix.enzyme }}" == "true" ]]; then
+            echo "OMPI_CXX=ccache $LLVM_PREFIX/bin/clang++" >> $GITHUB_ENV
+          elif [[ "${{ matrix.build-system }}" == "cmake" ]]; then
+            echo 'CCACHE_CONFIG_OPTS=-DCMAKE_CXX_COMPILER_LAUNCHER=ccache -DCMAKE_C_COMPILER_LAUNCHER=ccache' >> $GITHUB_ENV
+          else
+            echo "OMPI_CXX=ccache g++" >> $GITHUB_ENV
+            echo 'CCACHE_CONFIG_OPTS=CXX="ccache g++" MPICXX="mpicxx"' >> $GITHUB_ENV
+          fi
+        shell: bash
+
       # MFEM build and test
       - name: build
         uses: mfem/github-actions/build-mfem@v2.7
@@ -305,9 +355,14 @@ jobs:
           metis-dir: ${{ env.METIS_TOP_DIR }}
           mfem-dir: ${{ env.MFEM_TOP_DIR }}
           precision: ${{ matrix.precision }}
-          config-options: ${{ matrix.config-opts }}
+          config-options: ${{ matrix.config-opts }} ${{ env.CCACHE_CONFIG_OPTS }}
           library-only: ${{ matrix.target == 'dbg' && matrix.os != 'ubuntu-latest' }}
 
+      - name: ccache stats
+        if: ${{ env.USE_CCACHE == 'true' }}
+        run: ccache -s
+        shell: bash
+
       # Run checks (and only checks) on debug targets
       - name: checks
         if: matrix.build-system == 'make' && matrix.target == 'dbg'

.github/workflows/cleanup-pr-caches.yml

@@ -0,0 +1,42 @@
+# Copyright (c) 2010-2025, Lawrence Livermore National Security, LLC. Produced
+# at the Lawrence Livermore National Laboratory. All Rights reserved. See files
+# LICENSE and NOTICE for details. LLNL-CODE-806117.
+#
+# This file is part of the MFEM library. For more information and source code
+# availability visit https://mfem.org.
+#
+# MFEM is free software; you can redistribute it and/or modify it under the
+# terms of the BSD-3 license. We welcome feedback and contributions, see file
+# CONTRIBUTING.md for details.
+---
+# A closed PR's caches can never be restored again, so delete them to free
+# space against the 10 GB per-repo cache limit.
+name: Cleanup PR caches
+
+on:
+  pull_request:
+    types: [closed]
+
+permissions:
+  actions: write
+
+jobs:
+  cleanup:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Delete caches for the closed PR
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GH_REPO: ${{ github.repository }}
+          PR_REF: refs/pull/${{ github.event.pull_request.number }}/merge
+        run: |
+          echo "Deleting caches for $PR_REF"
+          while :; do
+            ids=$(gh cache list --ref "$PR_REF" --limit 100 --json id --jq '.[].id')
+            [ -n "$ids" ] || break
+            echo "$ids" | while read -r id; do
+              [ -n "$id" ] || continue
+              echo "Deleting cache $id"
+              gh cache delete "$id" || echo "  (already gone)"
+            done
+          done

.github/workflows/repo-check.yml

@@ -13,6 +13,7 @@ name: "Checks"
 
 permissions:
   actions: write
+  pull-requests: read
 
 on:
   push:
@@ -29,6 +30,11 @@ concurrency:
 # by checking if the workflow trigger is 'push' ("github.event_name == 'push'")
 # and if we are in a fork ("github.event.pull_request.head.repo.full_name !=
 # github.repository").
+#
+# The logic for the branch-history check is slightly different, since that check
+# also inspects the PR's labels to allow for overriding failures. In this case,
+# we run on all 'pull_request' triggers, but only run for 'push' triggers that
+# do not correspond to any open PRs.
 
 jobs:
   file-headers-check:
@@ -128,10 +134,7 @@ jobs:
 
   branch-history:
     if: |
-      github.ref != 'refs/heads/next' &&
-      github.ref != 'refs/heads/master' &&
-      (github.event_name == 'push' ||
-       github.event.pull_request.head.repo.full_name != github.repository)
+      github.ref != 'refs/heads/next' && github.ref != 'refs/heads/master'
     runs-on: ubuntu-latest
     steps:
       - name: checkout mfem
@@ -139,7 +142,27 @@ jobs:
         with:
           fetch-depth: 0
 
+      - name: check for pull request
+        id: check_pr
+        if: github.event_name == 'push'
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          pr_exists=$(gh pr list --repo "$GITHUB_REPOSITORY" \
+            --head "$GITHUB_REF_NAME" \
+            --state open \
+            --json number \
+            --jq 'length > 0')
+          echo "pr_exists=$pr_exists" >> "$GITHUB_OUTPUT"
+
       - name: branch-history
+        id: branch_history
+        if: |
+          (github.event_name == 'pull_request' ||
+           github.event_name == 'workflow_dispatch' ||
+           steps.check_pr.outputs.pr_exists == 'false')
+        continue-on-error: ${{ contains(github.event.pull_request.labels.*.name,
+                                        'branch-history-override') }}
         run: |
           # We override origin to make sure we point to the main repo.
           # This is to have consistent test results on PRs from forks.
@@ -147,3 +170,9 @@ jobs:
           git remote add origin https://github.com/mfem/mfem.git
           git checkout -b gh-actions-branch-history
           ./config/githooks/pre-push --history
+
+      - name: report branch-history override
+        if: steps.branch_history.outcome == 'failure'
+        run: |
+          echo "::warning::branch-history check failed, but the" \
+            "'branch-history-override' label is set."

config/githooks/README.md

@@ -39,3 +39,8 @@ when a picture was added for documentation.
 If that is the case, make sure the failure is indeed justified, and rerun the
 push command with the `--no-verify` option. This will skip the hooks, allowing
 you to push those changes.
+
+The `branch-history` check is run automatically through GitHub Actions. If a
+branch is known to have a large number of changes that are legitimate, the
+check can be overridden by setting  the label 'branch-history-override' on the
+pull request.

fem/datacollection.hpp

@@ -25,35 +25,21 @@
 namespace mfem
 {
 
-/// Lightweight adaptor over an std::map from type K to type to V
-template<typename K, typename V,
-         typename = typename std::enable_if<std::is_default_constructible<V>::value>::type>
-class GenericFieldMap
+/// Lightweight adaptor over an std::map from strings to pointer to T
+template<typename T>
+class NamedFieldsMap
 {
-private:
-   static constexpr bool ValueIsPointer = std::is_pointer<V>::value;
-
 public:
-   typedef std::map<K, V> MapType;
+   typedef std::map<std::string, T*> MapType;
    typedef typename MapType::iterator iterator;
    typedef typename MapType::const_iterator const_iterator;
 
-   /// Register field @a field with name @a key
-   /// Only enabled if the template parameter V is not a pointer
-   template<typename = std::enable_if<!ValueIsPointer, bool>>
-   void Register(const K& key, V field)
+   /// Register field @a field with name @a fname
+   /** Replace existing field associated with @a fname (and optionally
+       delete associated pointer if @a own_data is true) */
+   void Register(const std::string& fname, T* field, bool own_data)
    {
-      field_map[key] = field;
-   }
-
-   /// Register field @a field with name @a key
-   /** Replace existing field associated with @a key (and optionally
-       delete associated pointer if @a own_data is true).
-       Only enabled if the template parameter V is a pointer*/
-   template<typename = std::enable_if<ValueIsPointer, bool>>
-   void Register(const K& key, V field, bool own_data)
-   {
-      V& ref = field_map[key];
+      T*& ref = field_map[fname];
       if (own_data)
       {
          delete ref; // if newly allocated -> ref is null -> OK
@@ -61,40 +47,23 @@ public:
       ref = field;
    }
 
-   /// Unregister association between field @a field and name @a key
-   /// Only enabled if the template parameter V is not a pointer
-   template<typename = std::enable_if<!ValueIsPointer, bool>>
-   void Deregister(const K& key)
+   /// Unregister association between field @a field and name @a fname
+   /** Optionally delete associated pointer if @a own_data is true */
+   void Deregister(const std::string& fname, bool own_data)
    {
-      iterator it = field_map.find(key);
-      if ( it != field_map.end() )
-      {
-         field_map.erase(it);
-      }
-   }
-
-   /// Unregister association between field @a field and name @a key
-   /** Optionally delete associated pointer if @a own_data is true.
-       Only enabled if the template parameter V is a pointer */
-   template<typename = std::enable_if<ValueIsPointer, bool>>
-   void Deregister(const K& key, bool own_data)
-   {
-      iterator it = field_map.find(key);
+      iterator it = field_map.find(fname);
       if ( it != field_map.end() )
       {
          if (own_data)
          {
             delete it->second;
-            it->second = nullptr;
          }
          field_map.erase(it);
       }
    }
 
    /// Clear all associations between names and fields
-   /** Delete associated pointers when @a own_data is true.
-       Only enabled if the template parameter V is a pointer */
-   template<typename = std::enable_if<ValueIsPointer, bool>>
+   /** Delete associated pointers when @a own_data is true */
    void DeleteData(bool own_data)
    {
       for (iterator it = field_map.begin(); it != field_map.end(); ++it)
@@ -107,37 +76,22 @@ public:
       }
    }
 
-   /// Predicate to check if a field is associated with name @a key
-   bool Has(const K& key) const
+   /// Predicate to check if a field is associated with name @a fname
+   bool Has(const std::string& fname) const
    {
-      return field_map.find(key) != field_map.end();
+      return field_map.find(fname) != field_map.end();
    }
 
-   /// Get a pointer to the field associated with name @a key
-   /** @return Field associated with @a key or NULL,
-       if value is pointer and key not found */
-   V Get(const K& key) const
+   /// Get a pointer to the field associated with name @a fname
+   /** @return Pointer to field associated with @a fname or NULL */
+   T* Get(const std::string& fname) const
    {
-      const_iterator it = field_map.find(key);
-      if (it != field_map.end())
-      {
-         return it->second;
-      }
-      else
-      {
-         if constexpr (ValueIsPointer)
-         {
-            return nullptr;
-         }
-         else
-         {
-            return V(); // Return default-constructed value for non-pointer types
-         }
-      }
+      const_iterator it = field_map.find(fname);
+      return it != field_map.end() ? it->second : NULL;
    }
 
    /// Returns a const reference to the underlying map
-   const MapType &GetMap() const { return field_map; }
+   const MapType& GetMap() const { return field_map; }
 
    /// Returns the number of registered fields
    int NumFields() const { return field_map.size(); }
@@ -152,24 +106,21 @@ public:
    /// Returns an end const iterator to the registered fields
    const_iterator end() const { return field_map.end(); }
 
-   /// Returns an iterator to the field @a key
-   iterator find(const K& key)
-   { return field_map.find(key); }
+   /// Returns an iterator to the field @a fname
+   iterator find(const std::string& fname)
+   { return field_map.find(fname); }
 
-   /// Returns a const iterator to the field @a key
-   const_iterator find(const K& key) const
-   { return field_map.find(key); }
+   /// Returns a const iterator to the field @a fname
+   const_iterator find(const std::string& fname) const
+   { return field_map.find(fname); }
 
-   /// Clears the map of registered fields
+   /// Clears the map of registered fields without reclaiming memory
    void clear() { field_map.clear(); }
 
 protected:
    MapType field_map;
 };
 
-/// Lightweight adaptor over an std::map from strings to pointer to T
-template<typename T>
-using NamedFieldsMap = GenericFieldMap<std::string, T*>;
 
 /** A class for collecting finite element data that is part of the same
     simulation. Currently, this class groups together grid functions (fields),

fem/dfem/util.hpp

@@ -671,20 +671,6 @@ public:
                     MPI_COMM_WORLD);
    }
 
-   Operator& GetGradient(const Vector &x0) const override
-   {
-      x = x0;
-      f.UseDevice(x.UseDevice());
-      xpev.UseDevice(x.UseDevice());
-
-      op.Mult(x, f);
-      const real_t xnorm_local = x.Norml2();
-      MPI_Allreduce(&xnorm_local, &xnorm, 1, MPITypeMap<real_t>::mpi_type, MPI_SUM,
-                    MPI_COMM_WORLD);
-
-      return const_cast<FDJacobian&>(*this);
-   }
-
    void Mult(const Vector &v, Vector &y) const override
    {
       // See [1] for choice of eps.
@@ -739,11 +725,11 @@ public:
 
 private:
    const Operator &op;
-   mutable Vector x, f;
+   Vector x, f;
    mutable Vector xpev;
    real_t lambda = 1.0e-6;
    real_t fixed_eps;
-   mutable real_t xnorm;
+   real_t xnorm;
 };
 
 /// @brief Find the index of a field descriptor in a vector of field descriptors.

fem/nonlinearform.hpp

@@ -224,9 +224,6 @@ public:
    /** @see GetGradient(const Vector &) */
    Operator &GetGradient(const Vector &x, bool finalize) const;
 
-   /// Suppress a warning about hiding overloaded virtual function.
-   using Operator::GetGradient;
-
    /// Update the NonlinearForm to propagate updates of the associated FE space.
    /** After calling this method, the essential boundary conditions need to be
        set again. */

general/optparser.cpp

@@ -12,7 +12,9 @@
 #include "optparser.hpp"
 #include "../linalg/vector.hpp"
 #include "../general/communication.hpp"
+#include <cerrno>
 #include <cctype>
+#include <cstdlib>
 
 namespace mfem
 {
@@ -48,6 +50,19 @@ int isValidAsInt(char * s)
    return 1;
 }
 
+int isValidAsLongLong(char * s)
+{
+   if (s == NULL || *s == '\0')
+   {
+      return 0;
+   }
+
+   char *end = NULL;
+   errno = 0;
+   std::strtoll(s, &end, 10);
+   return errno == 0 && end != s && *end == '\0';
+}
+
 int isValidAsDouble(char * s)
 {
    // A valid floating point number for atof using the "C" locale is formed by
@@ -200,6 +215,10 @@ void OptionsParser::Parse()
                   isValid = isValidAsInt(argv[i]);
                   *(int *)(options[j].var_ptr) = atoi(argv[i++]);
                   break;
+               case LONG:
+                  isValid = isValidAsLongLong(argv[i]);
+                  *(long long *)(options[j].var_ptr) = std::strtoll(argv[i++], NULL, 10);
+                  break;
                case DOUBLE:
                   isValid = isValidAsDouble(argv[i]);
                   *(real_t *)(options[j].var_ptr) = atof(argv[i++]);
@@ -279,6 +298,10 @@ void OptionsParser::WriteValue(const Option &opt, std::ostream &os)
          os << *(int *)(opt.var_ptr);
          break;
 
+      case LONG:
+         os << *(long long *)(opt.var_ptr);
+         break;
+
       case DOUBLE:
          os << *(real_t *)(opt.var_ptr);
          break;
@@ -410,7 +433,7 @@ void OptionsParser::PrintHelp(ostream &os) const
    static const char *line_sep = "";
    static const char *types[] = { " <int>", " <double>", " <string>",
                                   " <string>", "", "", " '<int>...'",
-                                  " '<double>...'"
+                                  " '<double>...'", " <long long>"
                                 };
 
    os << indent << "-h" << seprtr << "--help" << descr_sep

general/optparser.hpp

@@ -31,7 +31,7 @@ class Vector;
 class OptionsParser
 {
 public:
-   enum OptionType { INT, DOUBLE, STRING, STD_STRING, ENABLE, DISABLE, ARRAY, VECTOR };
+   enum OptionType { INT, DOUBLE, STRING, STD_STRING, ENABLE, DISABLE, ARRAY, VECTOR, LONG };
 
 private:
    struct Option
@@ -98,6 +98,14 @@ public:
                             required));
    }
 
+   /// Add a long integer option and set 'var' to receive the value.
+   void AddOption(long long *var, const char *short_name, const char *long_name,
+                  const char *description, bool required = false)
+   {
+      options.Append(Option(LONG, var, short_name, long_name, description,
+                            required));
+   }
+
    /// Add a double option and set 'var' to receive the value.
    void AddOption(real_t *var, const char *short_name, const char *long_name,
                   const char *description, bool required = false)

linalg/CMakeLists.txt

@@ -27,7 +27,6 @@ list(APPEND SRCS
   handle.cpp
   matrix.cpp
   mma.cpp
-  multivector.cpp
   ode.cpp
   operator.cpp
   ordering.cpp
@@ -64,7 +63,6 @@ list(APPEND HDRS
   linalg.hpp
   matrix.hpp
   mma.hpp
-  multivector.hpp
   ode.hpp
   operator.hpp
   ordering.hpp
@@ -78,6 +76,12 @@ list(APPEND HDRS
   ttensor.hpp
   tensor.hpp
   vector.hpp
+  chpt/chpt.hpp
+  chpt/dynamic_checkpointing.hpp
+  chpt/file_checkpoint_storage.hpp
+  chpt/fixed_slot_checkpoint_storage.hpp
+  chpt/revolve_checkpointing.hpp
+  chpt/revolve_file_storage.hpp
   )
 
 if (MFEM_USE_MPI)

linalg/chpt/chpt.hpp

@@ -0,0 +1,21 @@
+// Copyright (c) 2010-2025, Lawrence Livermore National Security, LLC. Produced
+// at the Lawrence Livermore National Laboratory. All Rights reserved. See files
+// LICENSE and NOTICE for details. LLNL-CODE-806117.
+//
+// This file is part of the MFEM library. For more information and source code
+// availability visit https://mfem.org.
+//
+// MFEM is free software; you can redistribute it and/or modify it under the
+// terms of the BSD-3 license. We welcome feedback and contributions, see file
+// CONTRIBUTING.md for details.
+
+#ifndef MFEM_CHPT_HPP
+#define MFEM_CHPT_HPP
+
+#include "dynamic_checkpointing.hpp"
+#include "file_checkpoint_storage.hpp"
+#include "fixed_slot_checkpoint_storage.hpp"
+#include "revolve_checkpointing.hpp"
+#include "revolve_file_storage.hpp"
+
+#endif // MFEM_CHPT_HPP

linalg/chpt/dynamic_checkpointing.hpp

@@ -0,0 +1,309 @@
+#ifndef MFEM_DYNAMIC_CHECKPOINTING_HPP
+#define MFEM_DYNAMIC_CHECKPOINTING_HPP
+
+#pragma once
+
+
+#include <map>
+#include <memory>
+#include <limits>
+#include <vector>
+#include <utility>
+
+namespace mfem
+{
+
+template <typename Snapshot>
+class InMemoryCheckpointStorage
+{
+public:
+   using Handle = int;
+
+   Handle InvalidHandle() const { return -1; }
+   bool IsValid(const Handle &h) const { return h >= 0; }
+
+   Handle Store(Snapshot &&snap)
+   {
+      Handle id = InvalidHandle();
+      if (!free_.empty())
+      {
+         id = free_.back();
+         free_.pop_back();
+         MFEM_VERIFY(!slots_[id],
+                     "InMemoryCheckpointStorage: free slot unexpectedly occupied.");
+
+         // Reuse the handle ID only; erased Snapshot objects are not reused.
+         slots_[id].reset(new Snapshot(std::move(snap)));
+      }
+      else
+      {
+         id = (Handle) slots_.size();
+         slots_.push_back(std::unique_ptr<Snapshot>(new Snapshot(std::move(snap))));
+      }
+      return id;
+   }
+
+
+   template <typename Func>
+   void Read(const Handle &h, Func &&f) const
+   {
+      MFEM_VERIFY(IsValid(h), "InMemoryCheckpointStorage: invalid handle.");
+      MFEM_VERIFY(h < (Handle) slots_.size(),
+                  "InMemoryCheckpointStorage: handle out of range.");
+      MFEM_VERIFY(slots_[h].get() != nullptr,
+                  "InMemoryCheckpointStorage: empty slot.");
+      f(*slots_[h]);
+   }
+
+   void Erase(Handle &h)
+   {
+      if (!IsValid(h)) { h = InvalidHandle(); return; }
+      MFEM_VERIFY(h < (Handle) slots_.size(),
+                  "InMemoryCheckpointStorage: handle out of range.");
+      slots_[h].reset();
+      free_.push_back(h);
+      h = InvalidHandle();
+   }
+
+private:
+   std::vector<std::unique_ptr<Snapshot>> slots_;
+   std::vector<Handle> free_;
+};
+
+
+/**
+ * Dynamic checkpointing manager (Wang–Moin–Iaccarino 2009), with pluggable storage.
+ *
+ * Snapshot: checkpointed object (often the primal State itself).
+ * Storage : must provide:
+ *   using Handle = ...
+ *   Handle InvalidHandle() const;
+ *   bool   IsValid(const Handle&) const;
+ *   Handle Store(Snapshot&&);
+ *   template<class F> void Read(const Handle&, F&&) const; // calls f(const Snapshot&)
+ *   void Erase(Handle&);
+ */
+template <typename Snapshot,
+          typename Storage = InMemoryCheckpointStorage<Snapshot>>
+class DynamicCheckpointing
+{
+public:
+   using Step = long long;
+   using Handle = typename Storage::Handle;
+
+   struct Checkpoint
+   {
+      int level = 0;
+      Handle h; // InvalidHandle => placeholder
+      Checkpoint() = default;
+      Checkpoint(int lvl, const Handle &inv) : level(lvl), h(inv) {}
+   };
+
+   explicit DynamicCheckpointing(int s)
+      : s_(s),
+        owned_storage_(new Storage()),
+        storage_(owned_storage_.get())
+   {
+      MFEM_VERIFY(s_ > 0, "DynamicCheckpointing: s must be > 0.");
+      Reset();
+   }
+
+   DynamicCheckpointing(int s, Storage &external_storage)
+      : s_(s),
+        owned_storage_(nullptr),
+        storage_(&external_storage)
+   {
+      MFEM_VERIFY(s_ > 0, "DynamicCheckpointing: s must be > 0.");
+      Reset();
+   }
+
+   ~DynamicCheckpointing() { ReleaseAll(); }
+
+   void Reset()
+   {
+      ReleaseAll();
+      cps_.emplace(Step(0), Checkpoint(InfLevel(), storage_->InvalidHandle()));
+   }
+
+   struct CheckpointInfo
+   {
+      Step step;
+      int level;
+      bool stored;
+   };
+
+   std::vector<CheckpointInfo> GetCheckpointInfo() const
+   {
+      std::vector<CheckpointInfo> info;
+      info.reserve(cps_.size());
+      for (const auto &kv : cps_)
+      {
+         info.push_back({kv.first, kv.second.level, storage_->IsValid(kv.second.h)});
+      }
+      return info;
+   }
+
+   Step GetMaxStep() const
+   {
+      MFEM_VERIFY(!cps_.empty(), "DynamicCheckpointing: checkpoint map is empty.");
+      return cps_.rbegin()->first;
+   }
+
+   template <typename State, typename PrimalStepFn, typename MakeSnapshotFn>
+   void ForwardStep(const Step i,
+                    State &u_i_inout,
+                    PrimalStepFn &&primal_step,
+                    MakeSnapshotFn &&make_snapshot)
+   {
+      AllocateCheckpointForNextStep(i);
+
+      auto it = cps_.find(i);
+      if (it != cps_.end())
+      {
+         storage_->Erase(it->second.h);
+         Snapshot snap = make_snapshot(u_i_inout);
+         it->second.h = storage_->Store(std::move(snap));
+      }
+
+      primal_step(u_i_inout, i);
+   }
+
+   template <typename State,
+             typename AdjState,
+             typename PrimalStepFn,
+             typename AdjointStepFn,
+             typename MakeSnapshotFn,
+             typename RestoreSnapshotFn>
+   void BackwardStep(const Step i,
+                     AdjState &q_ip1_inout,
+                     State &u_work_inout,
+                     PrimalStepFn &&primal_step,
+                     AdjointStepFn &&adjoint_step,
+                     MakeSnapshotFn &&make_snapshot,
+                     RestoreSnapshotFn &&restore_snapshot)
+   {
+      // remove placeholder at i+1
+      const Step ph = i + 1;
+      auto it_ph = cps_.find(ph);
+
+      MFEM_VERIFY(it_ph != cps_.end(),
+                  "DynamicCheckpointing: expected checkpoint at i+1 before BackwardStep.");
+      storage_->Erase(it_ph->second.h);
+      cps_.erase(it_ph);
+
+      MFEM_ASSERT(GetMaxStep() <= i,
+                  "DynamicCheckpointing: found a checkpoint beyond current adjoint step.");
+
+      auto restore_from_handle = [&](Handle &h)
+      {
+         storage_->Read(h, [&](const Snapshot &snap)
+         {
+            restore_snapshot(snap, u_work_inout);
+         });
+         storage_->Erase(h); // retrieved => placeholder (Algorithm 4 semantics)
+      };
+
+      if (GetMaxStep() == i)
+      {
+         Handle h = TakeHandleMakePlaceholder(i);
+         restore_from_handle(h);
+      }
+      else
+      {
+         const Step k = GetMaxStep();
+         Handle hk = TakeHandleMakePlaceholder(k);
+         restore_from_handle(hk);
+
+         for (Step t = k; t < i; ++t)
+         {
+            ForwardStep(t, u_work_inout, primal_step, make_snapshot);
+         }
+      }
+
+      adjoint_step(q_ip1_inout, u_work_inout, i);
+   }
+
+private:
+   int s_ = 0;
+   std::unique_ptr<Storage> owned_storage_;
+   Storage *storage_ = nullptr;
+   std::map<Step, Checkpoint> cps_;
+
+   static int InfLevel() { return std::numeric_limits<int>::max(); }
+
+   void ReleaseAll()
+   {
+      if (!storage_) { cps_.clear(); return; }
+      for (auto &kv : cps_) { storage_->Erase(kv.second.h); }
+      cps_.clear();
+   }
+
+   bool FindDispensableLargestStep(Step &out_step) const
+   {
+      int max_level_seen = std::numeric_limits<int>::min();
+      for (auto it = cps_.rbegin(); it != cps_.rend(); ++it)
+      {
+         const Step step = it->first;
+         const int lvl = it->second.level;
+         if (max_level_seen > lvl) { out_step = step; return true; }
+         max_level_seen = (lvl > max_level_seen) ? lvl : max_level_seen;
+      }
+      return false;
+   }
+
+   void AllocateCheckpointForNextStep(const Step i)
+   {
+      const Step new_step = i + 1;
+      MFEM_VERIFY(cps_.find(new_step) == cps_.end(),
+                  "DynamicCheckpointing: checkpoint at i+1 already exists.");
+
+      const Handle inv = storage_->InvalidHandle();
+
+      // allow growth to s+1 entries (incl. placeholder)
+      if ((int)cps_.size() <= s_)
+      {
+         cps_.emplace(new_step, Checkpoint(0, inv));
+         return;
+      }
+
+      Step disp = -1;
+      if (FindDispensableLargestStep(disp))
+      {
+         auto it = cps_.find(disp);
+         MFEM_ASSERT(it != cps_.end(),
+                     "Internal error: dispensable checkpoint not found.");
+         storage_->Erase(it->second.h);
+         cps_.erase(it);
+         cps_.emplace(new_step, Checkpoint(0, inv));
+         return;
+      }
+
+      auto it_i = cps_.find(i);
+      MFEM_VERIFY(it_i != cps_.end(),
+                  "DynamicCheckpointing: promotion expected checkpoint at step i but none found.");
+      MFEM_VERIFY(i != 0,
+                  "DynamicCheckpointing: attempted to remove step 0 checkpoint.");
+
+      const int l = it_i->second.level;
+      storage_->Erase(it_i->second.h);
+      cps_.erase(it_i);
+      cps_.emplace(new_step, Checkpoint(l + 1, inv));
+   }
+
+   Handle TakeHandleMakePlaceholder(const Step i)
+   {
+      auto it = cps_.find(i);
+      MFEM_VERIFY(it != cps_.end(),
+                  "DynamicCheckpointing: TakeHandle requested a non-existent checkpoint.");
+      MFEM_VERIFY(storage_->IsValid(it->second.h),
+                  "DynamicCheckpointing: TakeHandle requested a checkpoint with no snapshot.");
+
+      Handle h = std::move(it->second.h);
+      it->second.h = storage_->InvalidHandle();
+      return h;
+   }
+};
+
+} // namespace mfem
+
+#endif // MFEM_DYNAMIC_CHECKPOINTING_HPP

linalg/chpt/file_checkpoint_storage.hpp

@@ -0,0 +1,304 @@
+#ifndef MFEM_FILE_CHECKPOINT_STORAGE_HPP
+#define MFEM_FILE_CHECKPOINT_STORAGE_HPP
+
+#pragma once
+#include "../vector.hpp"
+
+#include <cerrno>
+#include <cstdint>
+#include <cstdio>    // std::remove, std::rename
+#include <cstring>
+#include <string>
+#include <vector>
+#include <sstream>
+#include <iomanip>
+#include <fstream>
+#include <type_traits>
+
+#if __has_include(<filesystem>)
+#include <filesystem>
+namespace mfem_fs = std::filesystem;
+#define MFEM_HAVE_FILESYSTEM 1
+#else
+#define MFEM_HAVE_FILESYSTEM 0
+#endif
+
+namespace mfem
+{
+
+/**
+ * @brief Default binary serializer for checkpoint snapshots.
+ *
+ * Contract:
+ *   - static void     Write(std::ostream&, const Snapshot&)
+ *   - static Snapshot Read (std::istream&)
+ *
+ * Default implementation supports trivially-copyable POD types.
+ * Specialization for mfem::Vector is provided below.
+ *
+ * Portability note:
+ *   This binary format is NOT portable across endianness or differing sizeof(real_t).
+ *   If you need portability, provide a custom SnapshotIO (e.g., text, XDR, HDF5).
+ */
+template <typename Snapshot, typename Enable = void>
+struct DefaultCheckpointBinaryIO
+{
+   static void Write(std::ostream &, const Snapshot &)
+   {
+      static_assert(sizeof(Snapshot) == 0,
+                    "DefaultCheckpointBinaryIO: no implementation for this Snapshot type. "
+                    "Provide a custom SnapshotIO with Write/Read.");
+   }
+
+   static Snapshot Read(std::istream &)
+   {
+      static_assert(sizeof(Snapshot) == 0,
+                    "DefaultCheckpointBinaryIO: no implementation for this Snapshot type. "
+                    "Provide a custom SnapshotIO with Write/Read.");
+      return Snapshot();
+   }
+};
+
+// POD / trivially-copyable types (double, int, structs of POD, etc.)
+template <typename Snapshot>
+struct DefaultCheckpointBinaryIO<
+   Snapshot,
+   typename std::enable_if<std::is_trivially_copyable<Snapshot>::value>::type>
+{
+   static void Write(std::ostream &os, const Snapshot &x)
+   {
+      os.write(reinterpret_cast<const char*>(&x), sizeof(Snapshot));
+      MFEM_VERIFY(os.good(),
+                  "DefaultCheckpointBinaryIO: failed to write POD snapshot.");
+   }
+
+   static Snapshot Read(std::istream &is)
+   {
+      Snapshot x;
+      is.read(reinterpret_cast<char*>(&x), sizeof(Snapshot));
+      MFEM_VERIFY(is.good(),
+                  "DefaultCheckpointBinaryIO: failed to read POD snapshot.");
+      return x;
+   }
+};
+
+// Specialization for mfem::Vector
+template <>
+struct DefaultCheckpointBinaryIO<mfem::Vector, void>
+{
+   static void Write(std::ostream &os, const mfem::Vector &v)
+   {
+      const std::int64_t n = (std::int64_t) v.Size();
+      os.write(reinterpret_cast<const char*>(&n), sizeof(n));
+      MFEM_VERIFY(os.good(), "VectorBinaryIO: failed to write vector size.");
+
+      if (n > 0)
+      {
+         const mfem::real_t *data = v.HostRead();
+         os.write(reinterpret_cast<const char*>(data),
+                  (std::streamsize)(n * (std::int64_t)sizeof(mfem::real_t)));
+         MFEM_VERIFY(os.good(), "VectorBinaryIO: failed to write vector data.");
+      }
+   }
+
+   static mfem::Vector Read(std::istream &is)
+   {
+      std::int64_t n = 0;
+      is.read(reinterpret_cast<char*>(&n), sizeof(n));
+      MFEM_VERIFY(is.good(), "VectorBinaryIO: failed to read vector size.");
+      MFEM_VERIFY(n >= 0, "VectorBinaryIO: invalid negative vector size.");
+
+      mfem::Vector v((int)n);
+      if (n > 0)
+      {
+         mfem::real_t *data = v.HostWrite();
+         is.read(reinterpret_cast<char*>(data),
+                 (std::streamsize)(n * (std::int64_t)sizeof(mfem::real_t)));
+         v.Read();
+         MFEM_VERIFY(is.good(), "VectorBinaryIO: failed to read vector data.");
+      }
+      return v;
+   }
+};
+
+
+/**
+ * @brief File-based checkpoint storage backend.
+ *
+ * Stores each snapshot in a separate file:
+ *   <directory>/<prefix><id><extension>
+ *
+ * Handle is a monotonically-increasing integer id (reused if keep_files==false).
+ *
+ * Template parameters:
+ *  - Snapshot   : stored snapshot type
+ *  - SnapshotIO : serializer with static Write/Read methods (see DefaultCheckpointBinaryIO)
+ *
+ * Threading:
+ *  - Not thread-safe. Typical adjoint/checkpointing usage is single-threaded control flow.
+ */
+template <typename Snapshot,
+          typename SnapshotIO = DefaultCheckpointBinaryIO<Snapshot>>
+class FileCheckpointStorage
+{
+public:
+   using Handle = std::int64_t;
+
+   /**
+    * @param directory   directory where checkpoint files live (created if create_dir==true)
+    * @param prefix      filename prefix (e.g. "ckpt_")
+    * @param extension   filename extension (e.g. ".bin")
+    * @param create_dir  create directory if missing (requires <filesystem>)
+    * @param keep_files  if true, Erase() will NOT delete files (useful for debugging),
+    *                    and ids are NOT reused.
+    */
+   FileCheckpointStorage(const std::string &directory,
+                         const std::string &prefix = "ckpt_",
+                         const std::string &extension = ".bin",
+                         bool create_dir = true,
+                         bool keep_files = false)
+      : dir_(directory),
+        prefix_(prefix),
+        ext_(extension),
+        keep_files_(keep_files)
+   {
+      MFEM_VERIFY(!dir_.empty(),
+                  "FileCheckpointStorage: directory must be non-empty.");
+      MFEM_VERIFY(!prefix_.empty(),
+                  "FileCheckpointStorage: prefix must be non-empty.");
+      MFEM_VERIFY(!ext_.empty(),
+                  "FileCheckpointStorage: extension must be non-empty.");
+
+      if (create_dir)
+      {
+#if MFEM_HAVE_FILESYSTEM
+         std::error_code ec;
+         mfem_fs::create_directories(mfem_fs::path(dir_), ec);
+         MFEM_VERIFY(!ec, "FileCheckpointStorage: failed to create directory: " << dir_);
+#else
+         MFEM_ABORT("FileCheckpointStorage: create_dir=true requires <filesystem> support.");
+#endif
+      }
+   }
+
+   Handle InvalidHandle() const { return (Handle)-1; }
+   bool IsValid(const Handle &h) const { return h >= 0; }
+
+   /**
+    * @brief Store snapshot to file, return handle.
+    *
+    * Uses atomic-ish pattern:
+    *  - write to "<path>.tmp"
+    *  - rename to "<path>"
+    */
+   Handle Store(Snapshot &&snap)
+   {
+      const Handle id = AllocateId_();
+      const std::string path = Path_(id);
+      const std::string tmp  = path + ".tmp";
+
+      {
+         std::ofstream os(tmp, std::ios::binary | std::ios::trunc);
+         MFEM_VERIFY(os.is_open(),
+                     "FileCheckpointStorage: failed to open for write: " << tmp);
+
+         // Write payload
+         SnapshotIO::Write(os, snap);
+         MFEM_VERIFY(os.good(), "FileCheckpointStorage: write failed for: " << tmp);
+      }
+
+      // Rename tmp -> final
+      const int rc = std::rename(tmp.c_str(), path.c_str());
+      MFEM_VERIFY(rc == 0, "FileCheckpointStorage: rename failed: " << tmp << " -> "
+                  << path);
+
+      return id;
+   }
+
+   /**
+    * @brief Read snapshot from file and pass it to callback f(const Snapshot&).
+    *
+    * The reference passed to f is valid only during the call.
+    */
+   template <typename Func>
+   void Read(const Handle &h, Func &&f) const
+   {
+      MFEM_VERIFY(IsValid(h),
+                  "FileCheckpointStorage: Read called with invalid handle.");
+      const std::string path = Path_(h);
+
+      std::ifstream is(path, std::ios::binary);
+      MFEM_VERIFY(is.is_open(),
+                  "FileCheckpointStorage: failed to open for read: " << path);
+
+      Snapshot snap = SnapshotIO::Read(is);
+      MFEM_VERIFY(is.good(), "FileCheckpointStorage: read failed for: " << path);
+
+      f(snap);
+   }
+
+   /**
+    * @brief Erase snapshot (delete file unless keep_files==true), set handle invalid.
+    *
+    * Id reuse policy:
+    *  - if keep_files_ == false: deleted ids are reused to avoid creating huge numbers of files
+    *  - if keep_files_ == true : ids are not reused (avoid overwriting old debug files)
+    */
+   void Erase(Handle &h)
+   {
+      if (!IsValid(h)) { h = InvalidHandle(); return; }
+
+      if (!keep_files_)
+      {
+         const std::string path = Path_(h);
+         const int rc = std::remove(path.c_str());
+         MFEM_VERIFY(rc == 0 || errno == ENOENT,
+                     "FileCheckpointStorage: failed to remove file: " << path);
+
+         free_.push_back(h);
+      }
+
+      h = InvalidHandle();
+   }
+
+private:
+   std::string dir_;
+   std::string prefix_;
+   std::string ext_;
+   bool keep_files_ = false;
+
+   mutable Handle next_id_ = 0;
+   mutable std::vector<Handle> free_;
+
+   Handle AllocateId_()
+   {
+      if (!keep_files_ && !free_.empty())
+      {
+         const Handle id = free_.back();
+         free_.pop_back();
+         return id;
+      }
+      return next_id_++;
+   }
+
+   std::string Path_(Handle id) const
+   {
+      MFEM_VERIFY(IsValid(id),
+                  "FileCheckpointStorage: Path_ called with invalid id.");
+
+      std::ostringstream oss;
+      oss << prefix_ << std::setw(12) << std::setfill('0') << id << ext_;
+
+#if MFEM_HAVE_FILESYSTEM
+      mfem_fs::path p = mfem_fs::path(dir_) / mfem_fs::path(oss.str());
+      return p.string();
+#else
+      // Fallback: simple concatenation; assumes dir_ ends without trailing slash if needed.
+      return dir_ + "/" + oss.str();
+#endif
+   }
+};
+
+} // namespace mfem
+
+#endif // MFEM_FILE_CHECKPOINT_STORAGE_HPP

linalg/chpt/fixed_slot_checkpoint_storage.hpp

@@ -0,0 +1,408 @@
+#ifndef MFEM_FIXED_SLOT_CHECKPOINT_STORAGE_HPP
+#define MFEM_FIXED_SLOT_CHECKPOINT_STORAGE_HPP
+
+#pragma once
+
+#include "../vector.hpp"
+
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <string>
+#include <vector>
+#include <fstream>
+#include <limits>
+#include <type_traits>
+
+namespace mfem
+{
+
+// ------------------------------------------------------------
+// Packer 1: trivially-copyable snapshots (memcpy)
+// ------------------------------------------------------------
+template <typename Snapshot>
+class TrivialFixedPacker
+{
+public:
+   TrivialFixedPacker()
+   {
+      static_assert(std::is_trivially_copyable<Snapshot>::value,
+                    "TrivialFixedPacker requires Snapshot to be trivially copyable.");
+   }
+
+   std::size_t SlotBytes() const { return sizeof(Snapshot); }
+
+   void Pack(const Snapshot &snap, void *dst) const
+   {
+      std::memcpy(dst, &snap, sizeof(Snapshot));
+   }
+
+   void Unpack(const void *src, Snapshot &snap) const
+   {
+      std::memcpy(&snap, src, sizeof(Snapshot));
+   }
+};
+
+
+// ------------------------------------------------------------
+// Packer 2: mfem::Vector of fixed length n
+// (Serialized size is fixed: n*sizeof(real_t))
+// ------------------------------------------------------------
+class FixedVectorPacker
+{
+public:
+   explicit FixedVectorPacker(int n) : n_(n)
+   {
+      MFEM_VERIFY(n_ > 0, "FixedVectorPacker: n must be > 0.");
+   }
+
+   std::size_t SlotBytes() const
+   {
+      return (std::size_t)n_ * sizeof(mfem::real_t);
+   }
+
+   void Pack(const mfem::Vector &v, void *dst) const
+   {
+      MFEM_VERIFY(v.Size() == n_, "FixedVectorPacker: vector size mismatch.");
+      std::memcpy(dst, v.HostRead(), SlotBytes());
+   }
+
+   void Unpack(const void *src, mfem::Vector &v) const
+   {
+      v.SetSize(n_);
+      std::memcpy(v.HostWrite(), src, SlotBytes());
+   }
+
+   int Size() const { return n_; }
+
+private:
+   int n_ = 0;
+};
+
+
+// ------------------------------------------------------------
+// Fixed-slot MEMORY storage: one big RAM block
+// ------------------------------------------------------------
+template <typename Snapshot, typename Packer = TrivialFixedPacker<Snapshot>>
+class FixedSlotMemoryCheckpointStorage
+{
+public:
+   using Handle = int;
+
+   FixedSlotMemoryCheckpointStorage(int max_slots, const Packer &packer = Packer())
+      : max_slots_(max_slots), packer_(packer)
+   {
+      MFEM_VERIFY(max_slots_ > 0,
+                  "FixedSlotMemoryCheckpointStorage: max_slots must be > 0.");
+
+      slot_bytes_ = packer_.SlotBytes();
+      MFEM_VERIFY(slot_bytes_ > 0,
+                  "FixedSlotMemoryCheckpointStorage: SlotBytes must be > 0.");
+
+      // Single contiguous block
+      bytes_.resize((std::size_t)max_slots_ * slot_bytes_);
+
+      in_use_.assign(max_slots_, 0);
+      free_.reserve(max_slots_);
+      for (int i = 0; i < max_slots_; ++i) { free_.push_back(i); }
+   }
+
+   Handle InvalidHandle() const { return -1; }
+   bool IsValid(const Handle &h) const { return h >= 0; }
+
+   int MaxSlots() const { return max_slots_; }
+   std::size_t SlotBytes() const { return slot_bytes_; }
+
+   Handle Store(Snapshot &&snap)
+   {
+      MFEM_VERIFY(!free_.empty(),
+                  "FixedSlotMemoryCheckpointStorage: out of slots (increase max_slots).");
+
+      const int slot = free_.back();
+      free_.pop_back();
+      in_use_[slot] = 1;
+
+      void *dst = SlotPtr_(slot);
+      packer_.Pack(snap, dst);
+
+      return slot;
+   }
+
+   template <typename Func>
+   void Read(const Handle &h, Func &&f) const
+   {
+      MFEM_VERIFY(IsValid(h),
+                  "FixedSlotMemoryCheckpointStorage: Read invalid handle.");
+      MFEM_VERIFY(h < max_slots_,
+                  "FixedSlotMemoryCheckpointStorage: Read handle out of range.");
+      MFEM_VERIFY(in_use_[h] == 1,
+                  "FixedSlotMemoryCheckpointStorage: Read from free slot.");
+
+      Snapshot tmp;
+      const void *src = SlotPtrConst_(h);
+      packer_.Unpack(src, tmp);
+
+      f(tmp);
+   }
+
+   void Erase(Handle &h)
+   {
+      if (!IsValid(h)) { h = InvalidHandle(); return; }
+
+      MFEM_VERIFY(h < max_slots_,
+                  "FixedSlotMemoryCheckpointStorage: Erase handle out of range.");
+      MFEM_VERIFY(in_use_[h] == 1,
+                  "FixedSlotMemoryCheckpointStorage: double-free / invalid erase.");
+
+      in_use_[h] = 0;
+      free_.push_back(h);
+      h = InvalidHandle();
+   }
+
+   /// Optional: return all slots to the free list (does not zero memory).
+   void Reset()
+   {
+      free_.clear();
+      for (int i = 0; i < max_slots_; ++i) { in_use_[i] = 0; free_.push_back(i); }
+   }
+
+private:
+   int max_slots_ = 0;
+   std::size_t slot_bytes_ = 0;
+   Packer packer_;
+
+   std::vector<unsigned char> bytes_; // single block
+   std::vector<unsigned char> in_use_;
+   std::vector<int> free_;
+
+   void *SlotPtr_(int slot)
+   {
+      return (void*)(&bytes_[(std::size_t)slot * slot_bytes_]);
+   }
+
+   const void *SlotPtrConst_(int slot) const
+   {
+      return (const void*)(&bytes_[(std::size_t)slot * slot_bytes_]);
+   }
+};
+
+
+// ------------------------------------------------------------
+// Fixed-slot FILE storage: one single pre-sized file
+// ------------------------------------------------------------
+template <typename Snapshot, typename Packer = TrivialFixedPacker<Snapshot>>
+class FixedSlotFileCheckpointStorage
+{
+public:
+   using Handle = int;
+
+   struct Header
+   {
+      char magic[8];              // "MFCKPTFS"
+      std::uint64_t version;      // 1
+      std::uint64_t slot_bytes;
+      std::uint64_t max_slots;
+      std::uint64_t reserved[4];  // future use / padding
+   };
+
+   FixedSlotFileCheckpointStorage(const std::string &path,
+                                  int max_slots,
+                                  const Packer &packer = Packer(),
+                                  bool truncate = true,
+                                  bool flush_on_store = false)
+      : path_(path),
+        max_slots_(max_slots),
+        packer_(packer),
+        flush_on_store_(flush_on_store)
+   {
+      MFEM_VERIFY(!path_.empty(), "FixedSlotFileCheckpointStorage: empty file path.");
+      MFEM_VERIFY(max_slots_ > 0,
+                  "FixedSlotFileCheckpointStorage: max_slots must be > 0.");
+
+      slot_bytes_ = packer_.SlotBytes();
+      MFEM_VERIFY(slot_bytes_ > 0,
+                  "FixedSlotFileCheckpointStorage: SlotBytes must be > 0.");
+
+      Open_(truncate);
+
+      in_use_.assign(max_slots_, 0);
+      free_.reserve(max_slots_);
+      for (int i = 0; i < max_slots_; ++i) { free_.push_back(i); }
+
+      scratch_.resize(slot_bytes_);
+   }
+
+   ~FixedSlotFileCheckpointStorage()
+   {
+      if (file_.is_open()) { file_.close(); }
+   }
+
+   Handle InvalidHandle() const { return -1; }
+   bool IsValid(const Handle &h) const { return h >= 0; }
+
+   int MaxSlots() const { return max_slots_; }
+   std::size_t SlotBytes() const { return slot_bytes_; }
+   const std::string &Path() const { return path_; }
+
+   Handle Store(Snapshot &&snap)
+   {
+      MFEM_VERIFY(!free_.empty(),
+                  "FixedSlotFileCheckpointStorage: out of slots (increase max_slots).");
+
+      const int slot = free_.back();
+      free_.pop_back();
+      in_use_[slot] = 1;
+
+      // Pack into scratch buffer then write into fixed slot offset
+      packer_.Pack(snap, scratch_.data());
+
+      const std::uint64_t off = SlotOffset_(slot);
+      file_.seekp((std::streamoff)off, std::ios::beg);
+      MFEM_VERIFY(file_.good(), "FixedSlotFileCheckpointStorage: seekp failed.");
+
+      file_.write(reinterpret_cast<const char*>(scratch_.data()),
+                  (std::streamsize)slot_bytes_);
+      MFEM_VERIFY(file_.good(), "FixedSlotFileCheckpointStorage: write failed.");
+
+      if (flush_on_store_) { file_.flush(); }
+
+      return slot;
+   }
+
+   template <typename Func>
+   void Read(const Handle &h, Func &&f) const
+   {
+      MFEM_VERIFY(IsValid(h), "FixedSlotFileCheckpointStorage: Read invalid handle.");
+      MFEM_VERIFY(h < max_slots_,
+                  "FixedSlotFileCheckpointStorage: Read handle out of range.");
+      MFEM_VERIFY(in_use_[h] == 1,
+                  "FixedSlotFileCheckpointStorage: Read from free slot.");
+
+      const std::uint64_t off = SlotOffset_(h);
+      file_.seekg((std::streamoff)off, std::ios::beg);
+      MFEM_VERIFY(file_.good(), "FixedSlotFileCheckpointStorage: seekg failed.");
+
+      file_.read(reinterpret_cast<char*>(scratch_.data()),
+                 (std::streamsize)slot_bytes_);
+      MFEM_VERIFY(file_.good(), "FixedSlotFileCheckpointStorage: read failed.");
+
+      Snapshot tmp;
+      packer_.Unpack(scratch_.data(), tmp);
+      f(tmp);
+   }
+
+   void Erase(Handle &h)
+   {
+      if (!IsValid(h)) { h = InvalidHandle(); return; }
+
+      MFEM_VERIFY(h < max_slots_,
+                  "FixedSlotFileCheckpointStorage: Erase handle out of range.");
+      MFEM_VERIFY(in_use_[h] == 1,
+                  "FixedSlotFileCheckpointStorage: double-free / invalid erase.");
+
+      // No file deletion; just return slot to free list.
+      in_use_[h] = 0;
+      free_.push_back(h);
+      h = InvalidHandle();
+   }
+
+   /// Optional: return all slots to free list (file contents remain).
+   void Reset()
+   {
+      free_.clear();
+      for (int i = 0; i < max_slots_; ++i) { in_use_[i] = 0; free_.push_back(i); }
+   }
+
+private:
+   std::string path_;
+   int max_slots_ = 0;
+   std::size_t slot_bytes_ = 0;
+   Packer packer_;
+   bool flush_on_store_ = false;
+
+   // mutable because Read() is const but needs to seek/read
+   mutable std::fstream file_;
+   mutable std::vector<unsigned char> scratch_;
+
+   std::vector<unsigned char> in_use_;
+   std::vector<int> free_;
+
+   static Header MakeHeader_(std::uint64_t slot_bytes, std::uint64_t max_slots)
+   {
+      Header h;
+      std::memset(&h, 0, sizeof(h));
+      h.magic[0] = 'M'; h.magic[1] = 'F'; h.magic[2] = 'C'; h.magic[3] = 'K';
+      h.magic[4] = 'P'; h.magic[5] = 'T'; h.magic[6] = 'F'; h.magic[7] = 'S';
+      h.version = 1;
+      h.slot_bytes = slot_bytes;
+      h.max_slots  = max_slots;
+      return h;
+   }
+
+   void Open_(bool truncate)
+   {
+      const std::ios::openmode mode =
+         std::ios::binary | std::ios::in | std::ios::out | (truncate ?
+                                                            std::ios::trunc : (std::ios::openmode)0);
+
+      file_.open(path_.c_str(), mode);
+      MFEM_VERIFY(file_.is_open(),
+                  "FixedSlotFileCheckpointStorage: failed to open file.");
+
+      const Header expected = MakeHeader_((std::uint64_t)slot_bytes_,
+                                          (std::uint64_t)max_slots_);
+
+      if (truncate)
+      {
+         // Write header
+         file_.seekp(0, std::ios::beg);
+         file_.write(reinterpret_cast<const char*>(&expected), sizeof(expected));
+         MFEM_VERIFY(file_.good(),
+                     "FixedSlotFileCheckpointStorage: header write failed.");
+
+         // Pre-size file to: header + max_slots*slot_bytes
+         const std::uint64_t total = (std::uint64_t)sizeof(Header)
+                                     + (std::uint64_t)max_slots_ * (std::uint64_t)slot_bytes_;
+
+         MFEM_VERIFY(total > 0,
+                     "FixedSlotFileCheckpointStorage: invalid total file size.");
+         file_.seekp((std::streamoff)(total - 1), std::ios::beg);
+         MFEM_VERIFY(file_.good(),
+                     "FixedSlotFileCheckpointStorage: seekp for resize failed.");
+
+         const char zero = 0;
+         file_.write(&zero, 1);
+         MFEM_VERIFY(file_.good(),
+                     "FixedSlotFileCheckpointStorage: resize write failed.");
+         file_.flush();
+      }
+      else
+      {
+         // Validate existing header
+         Header got;
+         file_.seekg(0, std::ios::beg);
+         file_.read(reinterpret_cast<char*>(&got), sizeof(got));
+         MFEM_VERIFY(file_.good(),
+                     "FixedSlotFileCheckpointStorage: header read failed.");
+
+         MFEM_VERIFY(std::memcmp(got.magic, expected.magic, 8) == 0,
+                     "FixedSlotFileCheckpointStorage: magic mismatch.");
+         MFEM_VERIFY(got.version == expected.version,
+                     "FixedSlotFileCheckpointStorage: version mismatch.");
+         MFEM_VERIFY(got.slot_bytes == expected.slot_bytes,
+                     "FixedSlotFileCheckpointStorage: slot_bytes mismatch.");
+         MFEM_VERIFY(got.max_slots  == expected.max_slots,
+                     "FixedSlotFileCheckpointStorage: max_slots mismatch.");
+      }
+   }
+
+   std::uint64_t SlotOffset_(int slot) const
+   {
+      return (std::uint64_t)sizeof(Header) + (std::uint64_t)slot *
+             (std::uint64_t)slot_bytes_;
+   }
+};
+
+} // namespace mfem
+
+#endif // MFEM_FIXED_SLOT_CHECKPOINT_STORAGE_HPP

linalg/chpt/revolve_checkpointing.hpp

@@ -0,0 +1,537 @@
+#ifndef MFEM_REVOLVE_CHECKPOINTING_HPP
+#define MFEM_REVOLVE_CHECKPOINTING_HPP
+
+#pragma once
+
+#include <cstdint>
+#include <cstring>
+#include <fstream>
+#include <string>
+#include <vector>
+#include <algorithm>
+
+namespace mfem
+{
+
+// -------------------------------
+// Fixed-slot storage: Memory
+// -------------------------------
+class RevolveFixedSlotMemoryStorage
+{
+private:
+   int max_slots_ = 0;
+   size_t slot_bytes_ = 0;
+   std::vector<uint8_t> data_;
+
+public:
+   RevolveFixedSlotMemoryStorage(int max_slots, size_t slot_bytes)
+      : max_slots_(max_slots), slot_bytes_(slot_bytes),
+        data_(size_t(max_slots)*slot_bytes, 0)
+   {
+      MFEM_VERIFY(max_slots_ > 0, "max_slots must be > 0");
+      MFEM_VERIFY(slot_bytes_ > 0, "slot_bytes must be > 0");
+   }
+
+   int MaxSlots() const { return max_slots_; }
+   size_t SlotBytes() const { return slot_bytes_; }
+
+   void Save(int slot, const uint8_t *src, size_t bytes)
+   {
+      MFEM_VERIFY(0 <= slot && slot < max_slots_, "Save: slot out of range");
+      MFEM_VERIFY(bytes == slot_bytes_, "Save: bytes mismatch");
+      MFEM_VERIFY(src != nullptr, "Save: src is null");
+      std::memcpy(data_.data() + size_t(slot)*slot_bytes_, src, slot_bytes_);
+   }
+
+   void Load(int slot, uint8_t *dst, size_t bytes) const
+   {
+      MFEM_VERIFY(0 <= slot && slot < max_slots_, "Load: slot out of range");
+      MFEM_VERIFY(bytes == slot_bytes_, "Load: bytes mismatch");
+      MFEM_VERIFY(dst != nullptr, "Load: dst is null");
+      std::memcpy(dst, data_.data() + size_t(slot)*slot_bytes_, slot_bytes_);
+   }
+};
+
+// -------------------------------
+// Fixed-slot storage: Single file
+// -------------------------------
+class RevolveFixedSlotFileStorage
+{
+private:
+   int max_slots_ = 0;
+   size_t slot_bytes_ = 0;
+   std::string filename_;
+   mutable std::fstream file_;
+
+   std::streamoff Offset(int slot) const
+   {
+      return std::streamoff(size_t(slot) * slot_bytes_);
+   }
+
+public:
+   RevolveFixedSlotFileStorage(const std::string &filename,
+                               int max_slots,
+                               size_t slot_bytes)
+      : max_slots_(max_slots), slot_bytes_(slot_bytes), filename_(filename)
+   {
+      MFEM_VERIFY(max_slots_ > 0, "max_slots must be > 0");
+      MFEM_VERIFY(slot_bytes_ > 0, "slot_bytes must be > 0");
+      MFEM_VERIFY(!filename_.empty(), "filename must not be empty");
+
+      // Create/truncate file and size it.
+      {
+         std::ofstream ofs(filename_, std::ios::binary | std::ios::trunc);
+         MFEM_VERIFY(ofs.good(), "Failed to create checkpoint file");
+         const size_t total = size_t(max_slots_) * slot_bytes_;
+         if (total > 0)
+         {
+            ofs.seekp(std::streamoff(total - 1));
+            char zero = 0;
+            ofs.write(&zero, 1);
+         }
+      }
+
+      file_.open(filename_, std::ios::binary | std::ios::in | std::ios::out);
+      MFEM_VERIFY(file_.good(), "Failed to open checkpoint file");
+   }
+
+   ~RevolveFixedSlotFileStorage()
+   {
+      if (file_.is_open()) { file_.close(); }
+   }
+
+   int MaxSlots() const { return max_slots_; }
+   size_t SlotBytes() const { return slot_bytes_; }
+
+   void Save(int slot, const uint8_t *src, size_t bytes)
+   {
+      MFEM_VERIFY(0 <= slot && slot < max_slots_, "Save: slot out of range");
+      MFEM_VERIFY(bytes == slot_bytes_, "Save: bytes mismatch");
+      MFEM_VERIFY(src != nullptr, "Save: src is null");
+
+      file_.seekp(Offset(slot));
+      MFEM_VERIFY(file_.good(), "Save: seekp failed");
+      file_.write(reinterpret_cast<const char*>(src), std::streamsize(slot_bytes_));
+      MFEM_VERIFY(file_.good(), "Save: write failed");
+      file_.flush();
+   }
+
+   void Load(int slot, uint8_t *dst, size_t bytes) const
+   {
+      MFEM_VERIFY(0 <= slot && slot < max_slots_, "Load: slot out of range");
+      MFEM_VERIFY(bytes == slot_bytes_, "Load: bytes mismatch");
+      MFEM_VERIFY(dst != nullptr, "Load: dst is null");
+
+      file_.seekg(Offset(slot));
+      MFEM_VERIFY(file_.good(), "Load: seekg failed");
+      file_.read(reinterpret_cast<char*>(dst), std::streamsize(slot_bytes_));
+      MFEM_VERIFY(file_.good(), "Load: read failed");
+   }
+};
+
+// -------------------------------
+// REVOLVE controller (actions)
+// (Transcribed from revolve.c used in ADOL-C; Algorithm 799.)
+// -------------------------------
+enum class RevolveAction
+{
+   advance,
+   takeshot,
+   restore,
+   firsturn,
+   youturn,
+   terminate
+};
+
+class RevolveController
+{
+private:
+   int snaps_ = 0;
+   int check_ = -1;
+   int capo_  = 0;
+   int fine_  = 0;
+
+   int turn_ = 0;
+   int oldfine_ = 0;
+
+   // Stack of checkpoint times, indexed by 'check_'.
+   std::vector<int> ch_;
+
+public:
+   RevolveController() = default;
+
+   RevolveController(int snaps, int capo0, int fine0)
+      : snaps_(snaps), check_(-1), capo_(capo0), fine_(fine0),
+        turn_(0), oldfine_(fine0), ch_(snaps, 0)
+   {
+      MFEM_VERIFY(snaps_ > 0, "REVOLVE snaps must be > 0");
+      MFEM_VERIFY(capo_ <= fine_, "REVOLVE: capo must be <= fine");
+
+      // Match revolve.c initialization behavior.
+      if (check_ == -1 && capo_ < fine_)
+      {
+         turn_ = 0;
+         ch_[0] = capo_ - 1;
+      }
+   }
+
+   int Snaps() const { return snaps_; }
+   int Check() const { return check_; }
+   int Capo() const  { return capo_; }
+   int Fine() const  { return fine_; }
+   const std::vector<int>& CheckpointTimes() const { return ch_; }
+
+   RevolveAction Next()
+   {
+      MFEM_VERIFY(!(check_ < -1), "REVOLVE: check < -1");
+      MFEM_VERIFY(!(capo_ > fine_), "REVOLVE: capo > fine");
+
+      if ((check_ == -1) && (capo_ < fine_))
+      {
+         turn_ = 0;
+         ch_[0] = capo_ - 1;
+      }
+
+      const int diff = fine_ - capo_;
+      switch (diff)
+      {
+         case 0:
+         {
+            // Terminate or restore to next checkpoint on the stack.
+            if (check_ == -1 || capo_ == ch_[0])
+            {
+               check_ -= 1; // mirror revolve.c behavior
+               return RevolveAction::terminate;
+            }
+            else
+            {
+               capo_ = ch_[check_];
+               oldfine_ = fine_;
+               return RevolveAction::restore;
+            }
+         }
+         case 1:
+         {
+            // One adjoint step available.
+            fine_ -= 1;
+            if (check_ >= 0 && ch_[check_] == capo_) { check_ -= 1; }
+
+            if (turn_ == 0)
+            {
+               turn_ = 1;
+               oldfine_ = fine_;
+               return RevolveAction::firsturn;
+            }
+            else
+            {
+               oldfine_ = fine_;
+               return RevolveAction::youturn;
+            }
+         }
+         default:
+         {
+            // diff > 1
+            if (check_ == -1 || ch_[check_] != capo_)
+            {
+               // Take a new checkpoint at current capo.
+               check_ += 1;
+               MFEM_VERIFY(check_ + 1 <= snaps_, "REVOLVE: exceeded snaps");
+               ch_[check_] = capo_;
+               oldfine_ = fine_;
+               return RevolveAction::takeshot;
+            }
+            else
+            {
+               // Advance capo forward within (capo, fine).
+               // This follows the binomial logic in revolve.c.
+               MFEM_VERIFY(!((oldfine_ < fine_) && (snaps_ == check_ + 1)),
+                           "REVOLVE: fine increased unexpectedly with full stack");
+
+               const int oldcapo = capo_;
+               const int ds = snaps_ - check_;
+               MFEM_VERIFY(ds >= 1, "REVOLVE: ds < 1");
+
+               int reps = 0;
+               long long range = 1;
+               while (range < (fine_ - capo_))
+               {
+                  reps += 1;
+                  // range = range*(reps+ds)/reps  (integer arithmetic)
+                  range = range * (reps + ds) / reps;
+               }
+               MFEM_VERIFY(reps >= 1, "REVOLVE: reps < 1");
+
+               // Binomial helper values (integer)
+               const long long bino1 = range * reps / (ds + reps);
+               const long long bino2 = (ds > 1) ? (bino1 * ds / (ds + reps - 1)) : 1;
+               const long long bino3 =
+                  (ds == 1) ? 0 :
+                  (ds > 2)  ? (bino2 * (ds - 1) / (ds + reps - 2)) : 1;
+               const long long bino4 = bino2 * (reps - 1) / ds;
+               const long long bino5 =
+                  (ds < 3) ? 0 :
+                  (ds > 3) ? (bino3 * (ds - 2) / reps) : 1;
+
+               // Kowarz "new version": keep l^ as small as possible
+               const long long bino6 = bino1 * ds / reps;
+
+               const long long gap = fine_ - capo_;
+               if (gap <= bino1 + bino3)
+               {
+                  capo_ += int(bino4);
+               }
+               else if (gap < bino1 + bino2)
+               {
+                  capo_ = fine_ - int(bino2 + bino3);
+               }
+               else if (gap <= bino1 + bino2 + bino5)
+               {
+                  capo_ += int(bino1 - bino3);
+               }
+               else
+               {
+                  capo_ = fine_ - int(bino6);
+               }
+
+               if (capo_ == oldcapo) { capo_ = oldcapo + 1; }
+
+               oldfine_ = fine_;
+               return RevolveAction::advance;
+            }
+         }
+      }
+   }
+};
+
+// -------------------------------
+// Fixed-step REVOLVE checkpointing
+// -------------------------------
+template <typename StorageT>
+class FixedStepRevolveCheckpointing
+{
+public:
+   struct Shot
+   {
+      int time = 0;   // state index
+      int slot = 0;   // checkpoint slot index
+   };
+
+private:
+   int num_steps_ = 0;        // total number of primal steps (0..num_steps)
+   int num_checkpoints_ = 0;  // REVOLVE "snaps"
+   size_t snapshot_bytes_ = 0;
+
+   StorageT *storage_ = nullptr;
+
+   std::vector<Shot> forward_shots_;
+   int forward_shot_cursor_ = 0;
+
+   // Controller state at the beginning of the reverse sweep (pre-firsturn).
+   RevolveController ctrl_init_;
+   RevolveController ctrl_;
+
+   // Two scratch buffers for (de)serialization.
+   std::vector<uint8_t> io_buf_;
+   std::vector<uint8_t> prefinal_buf_;
+   bool prefinal_valid_ = false;
+
+   // Reverse sweep bookkeeping.
+   bool reverse_started_ = false;
+   int u_work_time_ = -1;
+
+public:
+   FixedStepRevolveCheckpointing(int num_steps,
+                                 int num_checkpoints,
+                                 size_t snapshot_bytes,
+                                 StorageT &storage)
+      : num_steps_(num_steps),
+        num_checkpoints_(num_checkpoints),
+        snapshot_bytes_(snapshot_bytes),
+        storage_(&storage),
+        io_buf_(snapshot_bytes, 0),
+        prefinal_buf_(snapshot_bytes, 0)
+   {
+      MFEM_VERIFY(num_steps_ >= 0, "num_steps must be >= 0");
+      MFEM_VERIFY(num_checkpoints_ > 0, "num_checkpoints must be > 0");
+      MFEM_VERIFY(snapshot_bytes_ > 0, "snapshot_bytes must be > 0");
+
+      MFEM_VERIFY(storage_->MaxSlots() == num_checkpoints_,
+                  "Storage MaxSlots() must match num_checkpoints");
+      MFEM_VERIFY(storage_->SlotBytes() == snapshot_bytes_,
+                  "Storage SlotBytes() must match snapshot_bytes");
+
+      BuildForwardPlanAndInitialControllerState();
+      Reset();
+   }
+
+   void Reset()
+   {
+      forward_shot_cursor_ = 0;
+      prefinal_valid_ = false;
+      reverse_started_ = false;
+      u_work_time_ = -1;
+      ctrl_ = ctrl_init_;
+   }
+
+   int NumSteps() const { return num_steps_; }
+   int NumCheckpoints() const { return num_checkpoints_; }
+   size_t SnapshotBytes() const { return snapshot_bytes_; }
+
+   // ForwardStep: called for i=0..num_steps-1
+   template <typename State, typename PrimalStep, typename MakeSnapshot>
+   void ForwardStep(int i,
+                    State &u,
+                    PrimalStep &&primal_step,
+                    MakeSnapshot &&make_snapshot)
+   {
+      MFEM_VERIFY(0 <= i && i < num_steps_, "ForwardStep: i out of range");
+
+      // Take any planned shots at time i (before advancing).
+      while (forward_shot_cursor_ < (int)forward_shots_.size() &&
+             forward_shots_[forward_shot_cursor_].time == i)
+      {
+         const int slot = forward_shots_[forward_shot_cursor_].slot;
+         make_snapshot(u, io_buf_.data(), snapshot_bytes_);
+         storage_->Save(slot, io_buf_.data(), snapshot_bytes_);
+         forward_shot_cursor_++;
+      }
+
+      // Cache u_{num_steps-1} so reverse can start there (REVOLVE expects capo=num_steps-1).
+      if (i == num_steps_ - 1)
+      {
+         make_snapshot(u, prefinal_buf_.data(), snapshot_bytes_);
+         prefinal_valid_ = true;
+      }
+
+      // Advance one step.
+      primal_step(i, u);
+   }
+
+   // BackwardStep: called for i=num_steps-1..0
+   template <typename State, typename AdjointState,
+             typename PrimalStep, typename AdjointStep,
+             typename MakeSnapshot, typename RestoreSnapshot>
+   void BackwardStep(int i,
+                     AdjointState &lambda,
+                     State &u_work,
+                     PrimalStep &&primal_step,
+                     AdjointStep &&adjoint_step,
+                     MakeSnapshot &&make_snapshot,
+                     RestoreSnapshot &&restore_snapshot)
+   {
+      MFEM_VERIFY(0 <= i && i < num_steps_, "BackwardStep: i out of range");
+
+      if (!reverse_started_)
+      {
+         MFEM_VERIFY(prefinal_valid_ || num_steps_ == 0,
+                     "Reverse started but prefinal state was not captured. "
+                     "Did you run the forward loop through i=num_steps-1?");
+
+         ctrl_ = ctrl_init_;
+         reverse_started_ = true;
+
+         if (num_steps_ > 0)
+         {
+            // Restore u_{num_steps-1} into u_work.
+            restore_snapshot(u_work, prefinal_buf_.data(), snapshot_bytes_);
+            u_work_time_ = num_steps_ - 1;
+         }
+      }
+
+      // Execute controller actions until we perform exactly one adjoint step.
+      for (;;)
+      {
+         RevolveAction act = ctrl_.Next();
+
+         switch (act)
+         {
+            case RevolveAction::takeshot:
+            {
+               const int slot = ctrl_.Check();
+               MFEM_VERIFY(u_work_time_ == ctrl_.Capo(),
+                           "takeshot: u_work_time must equal capo");
+               make_snapshot(u_work, io_buf_.data(), snapshot_bytes_);
+               storage_->Save(slot, io_buf_.data(), snapshot_bytes_);
+               break;
+            }
+            case RevolveAction::restore:
+            {
+               const int slot = ctrl_.Check();
+               storage_->Load(slot, io_buf_.data(), snapshot_bytes_);
+               restore_snapshot(u_work, io_buf_.data(), snapshot_bytes_);
+               u_work_time_ = ctrl_.Capo();
+               break;
+            }
+            case RevolveAction::advance:
+            {
+               const int target = ctrl_.Capo();
+               MFEM_VERIFY(u_work_time_ >= 0, "advance: u_work_time not initialized");
+               MFEM_VERIFY(target >= u_work_time_, "advance: target < current time");
+
+               for (int t = u_work_time_; t < target; ++t)
+               {
+                  primal_step(t, u_work);
+               }
+               u_work_time_ = target;
+               break;
+            }
+            case RevolveAction::firsturn:
+            case RevolveAction::youturn:
+            {
+               // After firsturn/youturn, ctrl_.Fine() has been decremented and equals ctrl_.Capo().
+               const int step = ctrl_.Fine();
+               MFEM_VERIFY(step == ctrl_.Capo(), "youturn: fine != capo");
+               MFEM_VERIFY(step == u_work_time_, "youturn: u_work_time != step");
+               MFEM_VERIFY(step == i,
+                           "BackwardStep called with i that doesn't match REVOLVE schedule");
+
+               // One adjoint step.
+               adjoint_step(step, u_work, lambda);
+               return;
+            }
+            case RevolveAction::terminate:
+            {
+               MFEM_ABORT("REVOLVE terminated early: BackwardStep called after completion?");
+               break;
+            }
+         }
+      }
+   }
+
+private:
+   void BuildForwardPlanAndInitialControllerState()
+   {
+      forward_shots_.clear();
+
+      RevolveController sim(num_checkpoints_, /*capo=*/0, /*fine=*/num_steps_);
+
+      // Forward-plan phase: only TAKESHOT/ADVANCE should occur while fine-capo>1.
+      while (sim.Fine() - sim.Capo() > 1)
+      {
+         RevolveAction a = sim.Next();
+         if (a == RevolveAction::takeshot)
+         {
+            forward_shots_.push_back({sim.Capo(), sim.Check()});
+         }
+         else if (a == RevolveAction::advance)
+         {
+            // nothing to record; capo moved forward inside sim
+         }
+         else
+         {
+            MFEM_ABORT("Unexpected REVOLVE action during forward planning phase");
+         }
+      }
+
+      // At this point, sim is in the pre-firsturn state (capo = num_steps-1, fine = num_steps)
+      // for num_steps>=1. For num_steps<=1, fine-capo<=1 from the start.
+      ctrl_init_ = sim;
+   }
+};
+
+
+}
+
+
+
+#endif //MFEM_REVOLVE_CHECKPOINTING_HPP

linalg/chpt/revolve_file_storage.hpp

@@ -0,0 +1,362 @@
+#ifndef MFEM_REVOLVE_SEPARATE_FILE_STORAGE_HPP
+#define MFEM_REVOLVE_SEPARATE_FILE_STORAGE_HPP
+
+#pragma once
+
+#include "../vector.hpp"
+
+#include <cstdint>
+#include <cstdio>    // std::remove
+#include <fstream>
+#include <iomanip>
+#include <limits>
+#include <sstream>
+#include <string>
+#include <vector>
+
+#if __cplusplus >= 201703L
+#include <filesystem>
+#include <system_error>
+#else
+#if defined(_WIN32)
+#include <direct.h>   // _mkdir
+#else
+#include <sys/stat.h> // mkdir
+#include <sys/types.h>
+#include <errno.h>
+#endif
+#endif
+
+namespace mfem
+{
+
+/**
+ * @brief Separate-file checkpoint storage for REVOLVE: one file per checkpoint slot.
+ *
+ * This storage is "slot indexed":
+ *   slot = 0..max_slots-1
+ *
+ * File naming:
+ *   <dir>/<prefix><slot as zero-padded integer><ext>
+ *
+ * Example:
+ *   dir="ckpt", prefix="rev_", ext=".bin", max_slots=8
+ *   -> ckpt/rev_00.bin ... ckpt/rev_07.bin
+ *
+ * Intended REVOLVE interface:
+ *   int    MaxSlots() const;
+ *   size_t SlotBytes() const;
+ *   void   Save(int slot, const uint8_t *src, size_t bytes);
+ *   void   Load(int slot, uint8_t *dst, size_t bytes) const;
+ *
+ * Notes:
+ * - Slot files are pre-created and resized on construction if truncate_files=true.
+ * - Save() overwrites the full slot file content from offset 0.
+ * - Load() reads exactly SlotBytes() from offset 0.
+ * - If keep_files=false, destructor deletes the slot files.
+ * - If keep_open=true, keeps N files open (faster, but uses file descriptors).
+ */
+class RevolveSeparateFileCheckpointStorage
+{
+public:
+   RevolveSeparateFileCheckpointStorage(const std::string &directory,
+                                        const std::string &prefix,
+                                        const std::string &ext,
+                                        int max_slots,
+                                        std::size_t slot_bytes,
+                                        bool create_dir = true,
+                                        bool truncate_files = true,
+                                        bool keep_files = true,
+                                        bool keep_open = false,
+                                        bool flush_on_save = false)
+      : dir_(directory),
+        prefix_(prefix),
+        ext_(ext),
+        max_slots_(max_slots),
+        slot_bytes_(slot_bytes),
+        keep_files_(keep_files),
+        keep_open_(keep_open),
+        flush_on_save_(flush_on_save)
+   {
+      MFEM_VERIFY(max_slots_ > 0,
+                  "RevolveSeparateFileCheckpointStorage: max_slots must be > 0.");
+      MFEM_VERIFY(slot_bytes_ > 0,
+                  "RevolveSeparateFileCheckpointStorage: slot_bytes must be > 0.");
+      MFEM_VERIFY(!prefix_.empty(),
+                  "RevolveSeparateFileCheckpointStorage: prefix must not be empty.");
+
+      if (create_dir) { EnsureDirectory_(dir_); }
+
+      // Precompute slot paths.
+      slot_paths_.resize((std::size_t)max_slots_);
+      for (int s = 0; s < max_slots_; ++s)
+      {
+         slot_paths_[(std::size_t)s] = MakeSlotPath_(s);
+      }
+
+      if (truncate_files)
+      {
+         PrecreateAll_();
+      }
+      else
+      {
+         // Optional light sanity check: try opening one file. (User may want to reuse existing.)
+         // We'll rely on Save/Load verification otherwise.
+      }
+
+      if (keep_open_)
+      {
+         OpenAll_();
+      }
+   }
+
+   ~RevolveSeparateFileCheckpointStorage()
+   {
+      CloseAll_();
+
+      if (!keep_files_)
+      {
+         RemoveAllFiles_();
+      }
+   }
+
+   int MaxSlots() const { return max_slots_; }
+   std::size_t SlotBytes() const { return slot_bytes_; }
+
+   /// Return the full path for a given slot (useful for debugging).
+   const std::string &SlotPath(int slot) const
+   {
+      MFEM_VERIFY(0 <= slot && slot < max_slots_, "SlotPath: slot out of range.");
+      return slot_paths_[(std::size_t)slot];
+   }
+
+   /**
+    * @brief Save a checkpoint image into slot file.
+    *
+    * Requirements:
+    * - slot in [0, MaxSlots())
+    * - bytes == SlotBytes()
+    * - src != nullptr
+    */
+   void Save(int slot, const std::uint8_t *src, std::size_t bytes)
+   {
+      MFEM_VERIFY(0 <= slot && slot < max_slots_, "Save: slot out of range.");
+      MFEM_VERIFY(bytes == slot_bytes_, "Save: bytes mismatch.");
+      MFEM_VERIFY(src != nullptr, "Save: src is null.");
+
+      if (keep_open_)
+      {
+         std::fstream &f = files_[(std::size_t)slot];
+         MFEM_VERIFY(f.is_open(), "Save: file not open (keep_open).");
+
+         f.clear();
+         f.seekp(0, std::ios::beg);
+         MFEM_VERIFY(f.good(), "Save: seekp failed (keep_open).");
+
+         f.write(reinterpret_cast<const char*>(src), (std::streamsize)slot_bytes_);
+         MFEM_VERIFY(f.good(), "Save: write failed (keep_open).");
+
+         if (flush_on_save_) { f.flush(); }
+         return;
+      }
+
+      // Open on demand
+      std::fstream f(SlotPath(slot).c_str(),
+                     std::ios::binary | std::ios::in | std::ios::out);
+      if (!f.is_open())
+      {
+         // If missing, create it sized correctly, then reopen.
+         PrecreateOne_(slot);
+         f.open(SlotPath(slot).c_str(), std::ios::binary | std::ios::in | std::ios::out);
+      }
+      MFEM_VERIFY(f.is_open(), "Save: failed to open slot file.");
+
+      f.seekp(0, std::ios::beg);
+      MFEM_VERIFY(f.good(), "Save: seekp failed.");
+
+      f.write(reinterpret_cast<const char*>(src), (std::streamsize)slot_bytes_);
+      MFEM_VERIFY(f.good(), "Save: write failed.");
+
+      if (flush_on_save_) { f.flush(); }
+   }
+
+   /**
+    * @brief Load a checkpoint image from slot file.
+    *
+    * Requirements:
+    * - slot in [0, MaxSlots())
+    * - bytes == SlotBytes()
+    * - dst != nullptr
+    */
+   void Load(int slot, std::uint8_t *dst, std::size_t bytes) const
+   {
+      MFEM_VERIFY(0 <= slot && slot < max_slots_, "Load: slot out of range.");
+      MFEM_VERIFY(bytes == slot_bytes_, "Load: bytes mismatch.");
+      MFEM_VERIFY(dst != nullptr, "Load: dst is null.");
+
+      if (keep_open_)
+      {
+         std::fstream &f = files_[(std::size_t)slot];
+         MFEM_VERIFY(f.is_open(), "Load: file not open (keep_open).");
+
+         f.clear();
+         f.seekg(0, std::ios::beg);
+         MFEM_VERIFY(f.good(), "Load: seekg failed (keep_open).");
+
+         f.read(reinterpret_cast<char*>(dst), (std::streamsize)slot_bytes_);
+         MFEM_VERIFY(f.good(), "Load: read failed (keep_open).");
+         return;
+      }
+
+      std::ifstream f(SlotPath(slot).c_str(), std::ios::binary);
+      MFEM_VERIFY(f.is_open(), "Load: failed to open slot file.");
+
+      f.read(reinterpret_cast<char*>(dst), (std::streamsize)slot_bytes_);
+      MFEM_VERIFY(f.good(), "Load: read failed.");
+   }
+
+private:
+   std::string dir_;
+   std::string prefix_;
+   std::string ext_;
+   int max_slots_ = 0;
+   std::size_t slot_bytes_ = 0;
+
+   bool keep_files_ = true;
+   bool keep_open_ = false;
+   bool flush_on_save_ = false;
+
+   std::vector<std::string> slot_paths_;
+   mutable std::vector<std::fstream> files_; // only used if keep_open_==true
+
+   static std::string JoinPath_(const std::string &dir, const std::string &file)
+   {
+      if (dir.empty()) { return file; }
+      const char last = dir.back();
+      if (last == '/' || last == '\\') { return dir + file; }
+      return dir + "/" + file;
+   }
+
+   int SlotDigits_() const
+   {
+      int x = max_slots_ - 1;
+      int d = 1;
+      while (x >= 10) { x /= 10; ++d; }
+      return d;
+   }
+
+   std::string MakeSlotFilename_(int slot) const
+   {
+      std::ostringstream os;
+      os << prefix_
+         << std::setw(SlotDigits_()) << std::setfill('0') << slot
+         << ext_;
+      return os.str();
+   }
+
+   std::string MakeSlotPath_(int slot) const
+   {
+      return JoinPath_(dir_, MakeSlotFilename_(slot));
+   }
+
+   void PrecreateOne_(int slot) const
+   {
+      MFEM_VERIFY(0 <= slot && slot < max_slots_, "PrecreateOne: slot out of range.");
+
+      // Create/truncate and set file size to slot_bytes_ by writing last byte.
+      std::ofstream ofs(SlotPath(slot).c_str(), std::ios::binary | std::ios::trunc);
+      MFEM_VERIFY(ofs.is_open(), "PrecreateOne: failed to create slot file.");
+
+      const std::uint64_t sb = (std::uint64_t)slot_bytes_;
+      MFEM_VERIFY(sb <= (std::uint64_t)std::numeric_limits<std::streamoff>::max(),
+                  "PrecreateOne: slot_bytes too large for streamoff.");
+
+      if (sb > 0)
+      {
+         ofs.seekp((std::streamoff)(sb - 1), std::ios::beg);
+         MFEM_VERIFY(ofs.good(), "PrecreateOne: seekp failed.");
+
+         const char zero = 0;
+         ofs.write(&zero, 1);
+         MFEM_VERIFY(ofs.good(), "PrecreateOne: size write failed.");
+      }
+   }
+
+   void PrecreateAll_() const
+   {
+      for (int slot = 0; slot < max_slots_; ++slot)
+      {
+         PrecreateOne_(slot);
+      }
+   }
+
+   void OpenAll_()
+   {
+      files_.resize((std::size_t)max_slots_);
+
+      for (int slot = 0; slot < max_slots_; ++slot)
+      {
+         std::fstream &f = files_[(std::size_t)slot];
+         f.open(SlotPath(slot).c_str(), std::ios::binary | std::ios::in | std::ios::out);
+         if (!f.is_open())
+         {
+            PrecreateOne_(slot);
+            f.open(SlotPath(slot).c_str(), std::ios::binary | std::ios::in | std::ios::out);
+         }
+         MFEM_VERIFY(f.is_open(), "OpenAll: failed to open slot file.");
+      }
+   }
+
+   void CloseAll_()
+   {
+      if (!files_.empty())
+      {
+         for (auto &f : files_) { if (f.is_open()) { f.close(); } }
+         files_.clear();
+      }
+   }
+
+   void RemoveAllFiles_() const
+   {
+      for (int slot = 0; slot < max_slots_; ++slot)
+      {
+         // Ignore remove errors (e.g., already removed), but you can tighten if desired.
+         std::remove(SlotPath(slot).c_str());
+      }
+   }
+
+   static void EnsureDirectory_(const std::string &dir)
+   {
+      if (dir.empty()) { return; }
+
+#if __cplusplus >= 201703L
+      namespace fs = std::filesystem;
+      std::error_code ec;
+
+      if (!fs::exists(dir, ec))
+      {
+         fs::create_directories(dir, ec);
+      }
+      MFEM_VERIFY(!ec, "EnsureDirectory: failed to create directory: " + dir);
+#else
+#if defined(_WIN32)
+      const int rc = _mkdir(dir.c_str());
+      if (rc != 0)
+      {
+         // If directory already exists, _mkdir fails. We accept that.
+         // There's no reliable portable "exists" check in pre-C++17 without more code.
+      }
+#else
+      const int rc = mkdir(dir.c_str(), 0755);
+      if (rc != 0 && errno != EEXIST)
+      {
+         MFEM_ABORT("EnsureDirectory: failed to create directory: " + dir);
+      }
+#endif
+#endif
+   }
+};
+
+} // namespace mfem
+
+#endif // MFEM_REVOLVE_SEPARATE_FILE_STORAGE_HPP

linalg/linalg.hpp

@@ -15,7 +15,6 @@
 // Linear algebra header file
 
 #include "vector.hpp"
-#include "multivector.hpp"
 #include "operator.hpp"
 #include "matrix.hpp"
 #include "sparsemat.hpp"
@@ -41,6 +40,7 @@
 #include "filteredsolver.hpp"
 #include "ordering.hpp"
 #include "particlevector.hpp"
+#include "chpt/chpt.hpp"
 
 #ifdef MFEM_USE_AMGX
 #include "amgxsolver.hpp"

linalg/multivector.cpp

@@ -1,60 +0,0 @@
-// Copyright (c) 2010-2025, Lawrence Livermore National Security, LLC. Produced
-// at the Lawrence Livermore National Laboratory. All Rights reserved. See files
-// LICENSE and NOTICE for details. LLNL-CODE-806117.
-//
-// This file is part of the MFEM library. For more information and source code
-// availability visit https://mfem.org.
-//
-// MFEM is free software; you can redistribute it and/or modify it under the
-// terms of the BSD-3 license. We welcome feedback and contributions, see file
-// CONTRIBUTING.md for details.
-
-#include "multivector.hpp"
-
-namespace mfem
-{
-
-MultiVector::MultiVector(const Array<int> &vector_sizes)
-{
-   SetSizes(vector_sizes);
-}
-
-MultiVector::MultiVector(const Array<int> &vector_sizes, MemoryType mt)
-{
-   SetSizes(vector_sizes, mt);
-}
-
-MultiVector::MultiVector(Vector &base, const Array<int> &vector_sizes)
-{
-   MakeRef(base, vector_sizes);
-}
-
-void MultiVector::SetSizes(const Array<int> &vector_sizes)
-{
-   blocks.resize(vector_sizes.Size());
-   for (int i = 0; i < vector_sizes.Size(); i++)
-   {
-      operator[](i).SetSize(vector_sizes[i]);
-   }
-}
-
-void MultiVector::SetSizes(const Array<int> &vector_sizes, MemoryType mt)
-{
-   blocks.resize(vector_sizes.Size());
-   for (int i = 0; i < vector_sizes.Size(); i++)
-   {
-      operator[](i).SetSize(vector_sizes[i], mt);
-   }
-}
-
-void MultiVector::MakeRef(Vector &base, const Array<int> &vector_sizes)
-{
-   blocks.resize(vector_sizes.Size());
-   for (int offset = 0, i = 0; i < vector_sizes.Size(); i++)
-   {
-      blocks[i].emplace<0>(base, offset, vector_sizes[i]);
-      offset += vector_sizes[i];
-   }
-}
-
-} // namespace mfem

linalg/multivector.hpp

@@ -1,198 +0,0 @@
-// Copyright (c) 2010-2025, Lawrence Livermore National Security, LLC. Produced
-// at the Lawrence Livermore National Laboratory. All Rights reserved. See files
-// LICENSE and NOTICE for details. LLNL-CODE-806117.
-//
-// This file is part of the MFEM library. For more information and source code
-// availability visit https://mfem.org.
-//
-// MFEM is free software; you can redistribute it and/or modify it under the
-// terms of the BSD-3 license. We welcome feedback and contributions, see file
-// CONTRIBUTING.md for details.
-
-#ifndef MFEM_MULTIVECTOR_HPP
-#define MFEM_MULTIVECTOR_HPP
-
-#include "../general/array.hpp"
-#include "vector.hpp"
-#include <vector>
-#include <array>
-#include <variant>
-
-namespace mfem
-{
-
-/// Class representing an array of Vectors with generally different sizes.
-/** This class is similar to BlockVector with the following two main
-    differences:
-    - the data for the individual Vector blocks does not need to be part of one
-      big contiguous memory allocation;
-    - this class does not inherit from class Vector (as a consequence of the
-      first bullet).
-
-    Internally, each Vector block is represented as either:
-    - (default) a Vector object constructed and owned by this class; this
-      object, in turn, as any Vector object, can own its Memory allocation or
-      refer to a sub-Memory of another Memory object; or
-    - a pointer to an externally allocated Vector or classes derived from
-      Vector. */
-class MultiVector
-{
-private:
-   std::vector<std::variant<Vector,Vector*>> blocks;
-
-public:
-   /// Create an empty MultiVector with zero blocks.
-   MultiVector() = default;
-
-   /** @brief Create a MultiVector with @a num_blocks blocks. The individual
-       Vector blocks are default initialized, i.e. they all have size zero. */
-   MultiVector(int num_blocks)
-      : blocks(num_blocks) { }
-
-   /** @brief Construct a MultiVector with number of blocks and individual block
-       Vector sizes given by @a vector_sizes.
-
-       @note The memory of the individual Vector blocks is NOT initialized. */
-   MultiVector(const Array<int> &vector_sizes);
-
-   /** @brief Construct a MultiVector with number of blocks and individual block
-       Vector sizes given by @a vector_sizes. All Vector blocks use the
-       MemoryType @a mt.
-
-       @note The memory of the individual Vector blocks is NOT initialized. */
-   MultiVector(const Array<int> &vector_sizes, MemoryType mt);
-
-   /** @brief Construct a MultiVector referencing data within a given monolithic
-       Vector @a base.
-
-       With this constructor, the Memory flags of @a base and of the individual
-       Vector blocks may need to be explicitly synchronized when data is moved
-       between host and device. */
-   MultiVector(Vector &base, const Array<int> &vector_sizes);
-
-   /** @brief Construct a MultiVector referencing multiple Vectors given as
-       arguments.
-
-       The VectorTypes reference arguments are expected to be static_cast-able
-       to (Vector &) which is the case if the types are derived from Vector,
-       e.g. HypreParVector, GridFunction, etc.
-
-       With this constructor, operations on individual Vector blocks are
-       performed directly on the objects @a vs. In particular, there is no need
-       to synchronize the Memory flags of @a vs and the ones of the individual
-       Vector blocks when data is moved between host and device. */
-   template <typename... VectorTypes,
-             std::enable_if_t<
-                std::conjunction_v<
-                   std::is_convertible<VectorTypes&,Vector&>...>, bool> = true>
-   MultiVector(VectorTypes &...vs) { MakeRef(vs...); }
-
-   /// Return the number of Vectors in the MultiVector.
-   int NumBlocks() const { return blocks.size(); }
-
-   /** @brief Set the number of Vectors in the MultiVector. Existing Vector
-       blocks will remain unmodified. New Vector blocks will be default
-       initialized, i.e. they all have size zero. */
-   void SetNumBlocks(int num_blocks) { blocks.resize(num_blocks); }
-
-   /// Read-write access to the i-th Vector.
-   inline Vector &operator[](int i);
-
-   /// Read-only access to the i-th Vector.
-   inline const Vector &operator[](int i) const;
-
-   /** @brief Update the MultiVector according to the given @a vector_sizes.
-
-       This method can be used to add or remove blocks. The individual Vector
-       sizes are updated using the method Vector::SetSize(int). */
-   void SetSizes(const Array<int> &vector_sizes);
-
-   /** @brief Update the MultiVector according to the given @a vector_sizes and
-       MemoryType @a mt.
-
-       This method can be used to add or remove blocks. The individual Vector
-       sizes and MemoryType are updated using the method
-       Vector::SetSize(int, MemoryType). */
-   void SetSizes(const Array<int> &vector_sizes, MemoryType mt);
-
-   /** @brief Update the MultiVector to reference data within a given monolithic
-       Vector @a base.
-
-       After calling this method, the Memory flags of @a base and of the
-       individual Vector blocks may need to be explicitly synchronized when data
-       is moved between host and device.*/
-   void MakeRef(Vector &base, const Array<int> &vector_sizes);
-
-   /** @brief Update the @a i-th MultiVector block to reference data within the
-       given monolithic Vector @a base at the given @a offset and with the given
-       @a size.
-
-       After calling this method, the Memory flags of @a base and of the @a i-th
-       Vector block may need to be explicitly synchronized when data is moved
-       between host and device.*/
-   inline void MakeRef(int i, Vector &base, int offset, int size)
-   {
-      blocks[i].emplace<0>(base, offset, size);
-   }
-
-   /** @brief Update the MultiVector to reference multiple Vectors given as
-       arguments.
-
-       The VectorTypes reference arguments are expected to be static_cast-able
-       to (Vector &) which is the case if the types are derived from Vector,
-       e.g. HypreParVector, GridFunction, etc.
-
-       After calling this method, operations on individual Vector blocks are
-       performed directly on the objects @a vs. In particular, there is no need
-       to synchronize the Memory flags of @a vs and the ones of the individual
-       Vector blocks when data is moved between host and device. */
-   template <typename... VectorTypes,
-             std::enable_if_t<
-                std::conjunction_v<
-                   std::is_convertible<VectorTypes&,Vector&>...>, bool> = true>
-   inline void MakeRef(VectorTypes &...vs);
-
-   /** @brief Update the @a i-th MultiVector block to reference the given
-       Vector @a v.
-
-       After calling this method, operations on the @a i-th Vector block are
-       performed directly on the Vector @a v. In particular, there is no need
-       to synchronize the Memory flags of @a v and the ones of the @a i-th
-       Vector blocks when data is moved between host and device. */
-   inline void MakeRef(int i, Vector &v) { blocks[i] = &v; }
-};
-
-// Inline and template methods
-
-inline Vector &MultiVector::operator[](int i)
-{
-   auto &bi = blocks[i];
-   return (bi.index() == 0) ? std::get<0>(bi) : *std::get<1>(bi);
-}
-
-inline const Vector &MultiVector::operator[](int i) const
-{
-   auto &bi = blocks[i];
-   return (bi.index() == 0) ? std::get<0>(bi) : *std::get<1>(bi);
-}
-
-template <typename... VectorTypes,
-          std::enable_if_t<
-             std::conjunction_v<
-                std::is_convertible<VectorTypes&,Vector&>...>, bool>>
-inline void MultiVector::MakeRef(VectorTypes &...vs)
-{
-   blocks.resize(sizeof...(vs));
-   if constexpr (sizeof...(vs) > 0)
-   {
-      const std::array vs_p{&static_cast<Vector&>(vs)...};
-      for (std::size_t i = 0; i < sizeof...(vs); i++)
-      {
-         blocks[i] = vs_p[i];
-      }
-   }
-}
-
-} // namespace mfem
-
-#endif // MFEM_MULTIVECTOR_HPP

linalg/operator.cpp

@@ -111,16 +111,6 @@ void Operator::ArrayAddMultTranspose(const Array<const Vector *> &X,
    }
 }
 
-void Operator::Mult(const MultiVector &, MultiVector &)
-{
-   MFEM_ABORT("this method is not overriden for this class!");
-}
-
-Operator &Operator::GetGradient(const MultiVector &) const
-{
-   MFEM_ABORT("this method is not overriden for this class!");
-}
-
 void Operator::FormLinearSystem(const Array<int> &ess_tdof_list,
                                 Vector &x, Vector &b,
                                 Operator* &Aout, Vector &X, Vector &B,

linalg/operator.hpp

@@ -13,7 +13,6 @@
 #define MFEM_OPERATOR
 
 #include "vector.hpp"
-#include "multivector.hpp"
 
 namespace mfem
 {
@@ -130,16 +129,6 @@ public:
    virtual void ArrayAddMultTranspose(const Array<const Vector *> &X,
                                       Array<Vector *> &Y, const real_t a = 1.0) const;
 
-   /** @brief Operator application, y = A(x), where the input @a x and the
-       output @a y are MultiVector objects, i.e. they generally use
-       non-contiguous memory representation.
-
-       The typical use case for this method are block operators like
-       DifferentiableOperator.
-
-       The base class implementation for the method is to generate an error. */
-   virtual void Mult(const MultiVector &x, MultiVector &y);
-
    /** @brief Evaluate the gradient operator at the point @a x. The default
        behavior in class Operator is to generate an error. */
    virtual Operator &GetGradient(const Vector &x) const
@@ -148,16 +137,6 @@ public:
       return const_cast<Operator &>(*this);
    }
 
-   /** @brief Evaluate the gradient operator at the point @a x. The input @a x
-       is provided as a MultiVector, i.e. it generally uses non-contiguous
-       memory representation.
-
-       The typical use case for this method are block operators like
-       DifferentiableOperator.
-
-       The base class implementation for the method is to generate an error. */
-   virtual Operator &GetGradient(const MultiVector &x) const;
-
    /** @brief Computes the diagonal entries into @a diag. Typically, this
        operation only makes sense for linear Operator%s. In some cases, only an
        approximation of the diagonal is computed. */

makefile

@@ -811,6 +811,7 @@ MINIAPPS_SUBDIRS = dpg/util hooke/operators hooke/preconditioners \
 FORMAT_FILES += $(foreach dir,$(TESTS_SUBDIRS),tests/$(dir)/*.?pp)
 FORMAT_FILES += $(foreach dir,$(UNIT_TESTS_SUBDIRS),tests/unit/$(dir)/*.?pp)
 FORMAT_FILES += $(foreach dir,$(MINIAPPS_SUBDIRS),miniapps/$(dir)/*.?pp)
+FORMAT_FILES += linalg/chpt/*.?pp miniapps/mtop/chpt/*.?pp
 FORMAT_FILES += config/cmake/config.hpp.in config/config.hpp.in mfem*.hpp
 FORMAT_EXCLUDE = general/tinyxml2.cpp tests/unit/catch.hpp
 FORMAT_LIST = $(filter-out $(FORMAT_EXCLUDE),$(wildcard $(FORMAT_FILES)))

miniapps/CMakeLists.txt

@@ -22,7 +22,6 @@ add_subdirectory(common)
 add_subdirectory(contact)
 add_subdirectory(dfem)
 add_subdirectory(diag-smoothers)
-add_subdirectory(multiapp)
 add_subdirectory(dpg)
 add_subdirectory(electromagnetics)
 add_subdirectory(fluids/navier)
@@ -32,6 +31,7 @@ add_subdirectory(hdiv-linear-solver)
 add_subdirectory(hooke)
 add_subdirectory(meshing)
 add_subdirectory(mtop)
+add_subdirectory(mtop/chpt)
 add_subdirectory(multidomain)
 add_subdirectory(nurbs)
 add_subdirectory(parelag)

miniapps/hdiv-linear-solver/change_basis.hpp

@@ -80,8 +80,6 @@ public:
    // limitations
    void MultRT_2D(const Vector &x, Vector &y, Mode mode) const;
    void MultRT_3D(const Vector &x, Vector &y, Mode mode) const;
-   // suppress warning about hiding overloaded virtual function:
-   using Operator::Mult;
 };
 
 } // namespace mfem

miniapps/mtop/chpt/CMakeLists.txt

@@ -0,0 +1,68 @@
+# Copyright (c) 2010-2025, Lawrence Livermore National Security, LLC. Produced
+# at the Lawrence Livermore National Laboratory. All Rights reserved. See files
+# LICENSE and NOTICE for details. LLNL-CODE-806117.
+#
+# This file is part of the MFEM library. For more information and source code
+# availability visit https://mfem.org.
+#
+# MFEM is free software; you can redistribute it and/or modify it under the
+# terms of the BSD-3 license. We welcome feedback and contributions, see file
+# CONTRIBUTING.md for details.
+
+list(APPEND CHPT_SOURCES )
+list(APPEND CHPT_HEADERS )
+
+convert_filenames_to_full_paths(CHPT_SOURCES)
+convert_filenames_to_full_paths(CHPT_HEADERS)
+
+set(CHPT_COMMON_FILES
+    EXTRA_SOURCES ${CHPT_SOURCES}
+    EXTRA_HEADERS ${CHPT_HEADERS})
+
+add_mfem_miniapp(mtop_ex_scalar_chpt
+                 MAIN ex_scalar_field.cpp
+                 ${CHPT_COMMON_FILES}
+                 LIBRARIES mfem)
+
+add_mfem_miniapp(mtop_ex_adapt_chpt
+                 MAIN ex_scalar_adaptive.cpp
+                 ${CHPT_COMMON_FILES}
+                 LIBRARIES mfem)
+
+
+add_mfem_miniapp(mtop_ex_vector_chpt
+                 MAIN ex_vector_field.cpp
+                 ${CHPT_COMMON_FILES}
+                 LIBRARIES mfem)
+
+add_mfem_miniapp(mtop_ex_fixedslot
+                 MAIN ex_fixedslot_scalar_vector.cpp
+                 ${CHPT_COMMON_FILES}
+                 LIBRARIES mfem)
+
+# add_mfem_miniapp(mtop_ex_segmentio
+#                  MAIN ex_segmentio_scalar_vector_mfem.cpp
+#                  ${CHPT_COMMON_FILES}
+#                  LIBRARIES mfem)
+
+add_mfem_miniapp(mtop_ex_file
+                 MAIN ex_fileio_scalar_vector_mfem.cpp
+                 ${CHPT_COMMON_FILES}
+                 LIBRARIES mfem)
+
+
+add_mfem_miniapp(mtop_ex_revolve
+                 MAIN ex_revolve.cpp
+                 ${CHPT_COMMON_FILES}
+                 LIBRARIES mfem)
+
+add_mfem_miniapp(mtop_ex_dyn
+                 MAIN ex_dynamic_chpt.cpp
+                 ${CHPT_COMMON_FILES}
+                 LIBRARIES mfem)
+
+add_mfem_miniapp(mtop_ex_rev_fixedslot
+                 MAIN ex_revolve_chpt_fixedslot.cpp
+                 ${CHPT_COMMON_FILES}
+                 LIBRARIES mfem)
+

miniapps/mtop/chpt/ex_dynamic_chpt.cpp

@@ -0,0 +1,718 @@
+#include "mfem.hpp"
+
+#include <cmath>
+#include <iomanip>
+#include <string>
+
+using namespace mfem;
+
+
+/************************************************************************
+ * The forward state consists of vector and several additional values.
+ * The goal is to demonstrate how different storages can be used together
+ * with DynamicCheckpointing in order to avoid unnecessary memory
+ * allocations, data copies, and deallocations.
+ * *********************************************************************/
+struct State
+{
+   mfem::real_t time = 0.0;
+   mfem::real_t obj  = 0.0;
+   mfem::Vector v;
+};
+
+// ---------------------------
+// Snapshot type used by storage:
+// a lightweight view (non-owning).
+// ---------------------------
+struct StateSnapshotView
+{
+   mfem::real_t time = 0.0;
+   mfem::real_t obj  = 0.0;
+
+   // Points to n*sizeof(real_t) bytes.
+   // - during Store(): points to State::v host data
+   // - during Read(): points into storage's internal scratch buffer
+   const unsigned char *v_bytes = nullptr;
+};
+
+// ---------------------------
+// Packer for fixed-slot storage
+// Layout in slot bytes:
+//   [ time | obj | v[0..n-1] ]
+// ---------------------------
+class StateSnapshotViewPacker
+{
+public:
+   explicit StateSnapshotViewPacker(int n) : n_(n)
+   {
+      MFEM_VERIFY(n_ > 0, "StateSnapshotViewPacker: n must be > 0.");
+   }
+
+   std::size_t SlotBytes() const
+   {
+      return (std::size_t)(2 + n_) * sizeof(mfem::real_t);
+   }
+
+   void Pack(const StateSnapshotView &s, void *dst) const
+   {
+      MFEM_VERIFY(dst != nullptr, "Pack: dst is null.");
+      MFEM_VERIFY(s.v_bytes != nullptr, "Pack: v_bytes is null.");
+
+      unsigned char *b = static_cast<unsigned char*>(dst);
+
+      std::memcpy(b + 0*sizeof(mfem::real_t), &s.time, sizeof(mfem::real_t));
+      std::memcpy(b + 1*sizeof(mfem::real_t), &s.obj,  sizeof(mfem::real_t));
+      std::memcpy(b + 2*sizeof(mfem::real_t),
+                  s.v_bytes,
+                  (std::size_t)n_ * sizeof(mfem::real_t));
+   }
+
+   // IMPORTANT:
+   // Unpack produces a view into the provided src buffer.
+   // The resulting pointer is only valid as long as src remains unchanged.
+   void Unpack(const void *src, StateSnapshotView &snap) const
+   {
+      MFEM_VERIFY(src != nullptr, "Unpack: src is null.");
+
+      const unsigned char *b = static_cast<const unsigned char*>(src);
+
+      std::memcpy(&snap.time, b + 0*sizeof(mfem::real_t), sizeof(mfem::real_t));
+      std::memcpy(&snap.obj,  b + 1*sizeof(mfem::real_t), sizeof(mfem::real_t));
+      snap.v_bytes = b + 2*sizeof(mfem::real_t);
+   }
+
+   int N() const { return n_; }
+
+private:
+   int n_ = 0;
+};
+
+
+/*****************************************************************************
+ * Provides a recipe for runing dynamic checkpointing with  memory storage
+ * alocated as one single big block.
+ * **************************************************************************/
+void run_fixed_slot_memory_storage( mfem::real_t Tfinal /*Final time*/,
+                                    mfem::real_t dtime  /*time step*/,
+                                    int s /* checkpoint budget (real checkpoints) */,
+                                    int n  /*State vector size*/,
+                                    bool print=true)
+{
+   StateSnapshotViewPacker packer(n);
+
+   using Storage = mfem::FixedSlotMemoryCheckpointStorage<StateSnapshotView,
+         StateSnapshotViewPacker>;
+   Storage storage(s, packer);
+
+   // Snapshot type is StateSnapshotView
+   using Checkpointing = mfem::DynamicCheckpointing<StateSnapshotView, Storage>;
+   Checkpointing ckpt(s, storage);
+
+   // Returns view of the State and avoids data transfer
+   auto make_snapshot = [&](const State &u) -> StateSnapshotView
+   {
+      MFEM_VERIFY(u.v.Size() == n, "make_snapshot: State.v size changed!");
+      const mfem::real_t *vh = u.v.HostRead(); // host pointer
+
+      StateSnapshotView snap;
+      snap.time = u.time;
+      snap.obj  = u.obj;
+      snap.v_bytes = reinterpret_cast<const unsigned char*>(vh);
+      return snap;
+   };
+
+   //Transfers data from the snaphot view to the State u_out.
+   auto restore_snapshot = [&](const StateSnapshotView &snap, State &u_out)
+   {
+      u_out.time = snap.time;
+      u_out.obj  = snap.obj;
+
+      if (u_out.v.Size() != n) { u_out.v.SetSize(n); }
+      mfem::real_t *vh = u_out.v.HostWrite();
+
+      std::memcpy(vh,
+                  snap.v_bytes,
+                  (std::size_t)n * sizeof(mfem::real_t));
+   };
+
+   using Step = Checkpointing::Step;
+
+   State u;
+   u.v.SetSize(n); u.v=0.0;
+
+   auto primal_step = [&](State &u_inout, Step i)
+   {
+      // advance u_inout.v, u_inout.time, u_inout.obj
+      if (print)
+      {
+         mfem::out<<"Forward step: "<<i<<" time="<<u_inout.time<<" obj="<<u_inout.obj<<std::endl;
+      }
+
+      u_inout.time=i*0.1;
+      u_inout.obj=i*0.2;
+   };
+
+   struct AdjointState { /* ... */ };
+   AdjointState q;
+
+   auto adjoint_step = [&](AdjointState &q_ip1_inout, const State &u_i, Step i)
+   {
+      // use u_i.time/u_i.obj/u_i.v to update adjoint
+      if (print)
+      {
+         mfem::out<<"Adjoint step: "<<i<<" time="<<u_i.time<<" obj="<<u_i.obj<<std::endl;
+      }
+
+   };
+
+   // Forward sweep (unknown number of steps)
+   mfem::real_t t=0.0;
+   Step i=0;
+   while (t<Tfinal)
+   {
+      // Store snapshot of u_i when manager decides; then primal_step u_i -> u_{i+1}
+      ckpt.ForwardStep(i, u, primal_step, make_snapshot);
+      t+=dtime;
+      ++i;
+   }
+
+   mfem::out << "Total number of steps m="<<i<<"\n";
+   mfem::out << "\nBackward sweep.\n";
+
+   const Step m=i;
+   // Backward sweep
+   State u_work;
+   u_work.v.SetSize(n); // allocate once
+
+   for (Step i = m - 1; i >= 0; --i)
+   {
+      ckpt.BackwardStep(i, q, u_work,
+                        primal_step, adjoint_step,
+                        make_snapshot, restore_snapshot);
+      if (i == 0) { break; }
+   }
+}
+
+/****************************************************************************
+ * Provides a recipe for runing dynamic checkpointing with  file storage
+ * alocated as one single big file.
+ ****************************************************************************/
+
+/****************************************************************************
+ * The dynamic algorithm conceptually maintains s + 1 checkpoints, but the
+ * last one is a placeholder checkpoint that “stores no solution and takes
+ * little memory”; only s are real stored solutions.
+ * So FixedSlotFileCheckpointStorage needs only s slots.
+ * File behavior for FixedSlotFileCheckpointStorage:
+        - Writes a small header + s * slot_bytes payload region,
+        - Erase(handle) just returns the slot to the free list
+          (file contents remain, will be overwritten later),
+          no filesystem metadata churn.
+
+ * Device memory: Runs on GPU -packing to a host file requires a host pointer.
+    Using:
+        u.v.HostRead() for packing,
+        u_out.v.HostWrite() for restore,
+    keeps memmory access correct.
+ * Snapshot pointer lifetime: In the read path, StateSnapshotViewPacker::Unpack
+   sets snap.v_bytes to point into the storage’s internal read buffer. That
+   pointer is only valid until the storage performs another Read/Store
+   (and in general, one should treat it as valid only during restore_snapshot).
+ *****************************************************************************/
+
+void run_fixed_slot_file_storage(mfem::real_t Tfinal /*Final time*/,
+                                 mfem::real_t dtime  /*time step*/,
+                                 int s /* checkpoint budget (real checkpoints) */,
+                                 int n  /*State vector size*/,
+                                 const std::string &file_path,
+                                 bool print=true)
+{
+   // s = number of REAL checkpoints (placeholder doesn't store a snapshot)
+   MFEM_VERIFY(s > 0, "Need s > 0.");
+   MFEM_VERIFY(n > 0, "Need n > 0.");
+
+   StateSnapshotViewPacker packer(n);
+
+   // Fixed-slot FILE storage:
+   // - single file (file_path)
+   // - max_slots = s (real checkpoints)
+   // - slot_bytes = packer.SlotBytes()
+   // - truncate=true to start fresh
+   // - flush_on_store=false for performance (set true if you want extra safety)
+   using Storage =
+      mfem::FixedSlotFileCheckpointStorage<StateSnapshotView, StateSnapshotViewPacker>;
+   Storage storage(file_path,
+                   /*max_slots=*/s,
+                   /*packer=*/packer,
+                   /*truncate=*/true,
+                   /*flush_on_store=*/false);
+
+   // Dynamic checkpointing manager using this storage.
+   using Checkpointing = mfem::DynamicCheckpointing<StateSnapshotView, Storage>;
+   Checkpointing ckpt(s, storage);
+
+   // ---------------------------
+   // Callbacks
+   // ---------------------------
+
+   // Convert State -> SnapshotView (no allocation).
+   auto make_snapshot = [&](const State &u) -> StateSnapshotView
+   {
+      MFEM_VERIFY(u.v.Size() == n, "make_snapshot: State.v size changed!");
+
+      // Ensure host pointer valid even when MFEM is using device memory.
+      const mfem::real_t *vh = u.v.HostRead();
+
+      StateSnapshotView snap;
+      snap.time = u.time;
+      snap.obj  = u.obj;
+      snap.v_bytes = reinterpret_cast<const unsigned char*>(vh);
+      return snap;
+   };
+
+   // Convert SnapshotView -> State (must COPY out of v_bytes).
+   auto restore_snapshot = [&](const StateSnapshotView &snap, State &u_out)
+   {
+      u_out.time = snap.time;
+      u_out.obj  = snap.obj;
+
+      if (u_out.v.Size() != n) { u_out.v.SetSize(n); }
+
+      mfem::real_t *vh = u_out.v.HostWrite();
+      std::memcpy(vh,
+                  snap.v_bytes,
+                  (std::size_t)n * sizeof(mfem::real_t));
+   };
+
+   // Example primal/adjoint step signatures (replace with your own):
+   using Step = Checkpointing::Step;
+
+   auto primal_step = [&](State &u_inout, Step i)
+   {
+      if (print)
+      {
+         mfem::out<<"Forward step: "<<i<<" time="<<u_inout.time<<" obj="<<u_inout.obj<<std::endl;
+      }
+
+      u_inout.time=i*0.1;
+      u_inout.obj=i*0.2;
+      // update u_inout.time, u_inout.obj, u_inout.v
+   };
+
+   struct AdjointState
+   {
+      // your adjoint variables, e.g., mfem::Vector lambda;
+   };
+
+   auto adjoint_step = [&](AdjointState &q_ip1_inout, const State &u_i, Step i)
+   {
+      // update adjoint using u_i
+      if (print)
+      {
+         mfem::out<<"Adjoint step: "<<i<<" time="<<u_i.time<<" obj="<<u_i.obj<<std::endl;
+      }
+   };
+
+   // ---------------------------
+   // Forward/backward skeleton
+   // ---------------------------
+   State u;
+   u.v.SetSize(n);
+   u.time = 0.0;
+   u.obj  = 0.0;
+   u.v = 0.0;
+
+   // Forward sweep (unknown number of steps)
+   mfem::real_t t=0.0;
+   Step i=0;
+   while (t<Tfinal)
+   {
+      ckpt.ForwardStep(i, u, primal_step, make_snapshot);
+      t+=dtime;
+      ++i;
+   }
+
+   mfem::out << "Total number of steps m="<<i-1<<"\n";
+   mfem::out << "\nBackward sweep.\n";
+
+   // Backward sweep
+   AdjointState q;
+   const Step m=i;
+   State u_work;
+   u_work.v.SetSize(n);
+
+   for (Step i = m - 1; i >= 0; --i)
+   {
+      ckpt.BackwardStep(i, q, u_work,
+                        primal_step, adjoint_step,
+                        make_snapshot, restore_snapshot);
+      if (i == 0) { break; }
+   }
+}
+
+
+/******************************************************************************
+   The following classes implement the IO and the data necessery for dynamic
+   checkpointing with mfem::FileCheckpointStorage (one file per stored snapshot).
+   With FileCheckpointStorage, the Snapshot must own the data which will be
+   written, i.e. it cannot be a “view” containing pointers into State::v
+   as those pointers would be meaningless when read back.
+*******************************************************************************/
+
+// The StateSnapshot is just a copy of the State
+using StateSnapshot=State;
+
+struct StateSnapshotBinaryIO
+{
+   static void Write(std::ostream &os, const StateSnapshot &s)
+   {
+      os.write(reinterpret_cast<const char*>(&s.time), sizeof(mfem::real_t));
+      os.write(reinterpret_cast<const char*>(&s.obj),  sizeof(mfem::real_t));
+      MFEM_VERIFY(os.good(), "StateSnapshotBinaryIO: write time/obj failed.");
+
+      const std::int64_t n = (std::int64_t)s.v.Size();
+      os.write(reinterpret_cast<const char*>(&n), sizeof(n));
+      MFEM_VERIFY(os.good(), "StateSnapshotBinaryIO: write vector size failed.");
+
+      if (n > 0)
+      {
+         const mfem::real_t *vh = s.v.HostRead(); // ensure host pointer
+         os.write(reinterpret_cast<const char*>(vh),
+                  (std::streamsize)(n * (std::int64_t)sizeof(mfem::real_t)));
+         MFEM_VERIFY(os.good(), "StateSnapshotBinaryIO: write vector payload failed.");
+      }
+   }
+
+   static StateSnapshot Read(std::istream &is)
+   {
+      StateSnapshot s;
+
+      is.read(reinterpret_cast<char*>(&s.time), sizeof(mfem::real_t));
+      is.read(reinterpret_cast<char*>(&s.obj),  sizeof(mfem::real_t));
+      MFEM_VERIFY(is.good(), "StateSnapshotBinaryIO: read time/obj failed.");
+
+      std::int64_t n = 0;
+      is.read(reinterpret_cast<char*>(&n), sizeof(n));
+      MFEM_VERIFY(is.good(), "StateSnapshotBinaryIO: read vector size failed.");
+      MFEM_VERIFY(n >= 0, "StateSnapshotBinaryIO: invalid negative vector size.");
+
+      s.v.SetSize((int)n);
+      if (n > 0)
+      {
+         mfem::real_t *vh = s.v.HostWrite(); // ensure host pointer
+         is.read(reinterpret_cast<char*>(vh),
+                 (std::streamsize)(n * (std::int64_t)sizeof(mfem::real_t)));
+         MFEM_VERIFY(is.good(), "StateSnapshotBinaryIO: read vector payload failed.");
+      }
+      return s;
+   }
+};
+
+/***************************************************************************
+    Important differences vs FixedSlotFileCheckpointStorage
+    Metadata overhead
+
+    FileCheckpointStorage typically:
+        *creates a new file for each Store() (plus a temp file rename),
+        *deletes a file on each Erase() (unless keep_files=true).
+
+    Dynamic checkpointing can perform many store/erase operations during
+    forward and during recomputation in reverse, so this can hammer filesystem
+    metadata on parallel filesystems.
+
+    If that’s a concern, prefer:
+        *fixed-slot single-file storage, or
+        *segmented “range file” storage (few files + offsets).
+
+    Correctness note about pointers:
+    This is why we used an owning snapshot (StateSnapshot): storing a “view”
+    with v_bytes into file storage would store pointer values, which are
+    meaningless when read back.
+*****************************************************************************/
+
+void run_file_storage(mfem::real_t Tfinal /*Final time*/,
+                      mfem::real_t dtime  /*time step*/,
+                      int s /* checkpoint budget (real checkpoints) */,
+                      int n  /*State vector size*/,
+                      const std::string &directory,
+                      bool print=true)
+{
+   MFEM_VERIFY(s > 0, "Need s > 0.");
+   MFEM_VERIFY(n > 0, "Need n > 0.");
+
+   // One file per snapshot (create_dir=true). keep_files=false means Erase() removes files.
+   using Storage =
+      mfem::FileCheckpointStorage<StateSnapshot, StateSnapshotBinaryIO>;
+   Storage storage(directory, "ckpt_",
+                   ".bin", /*create_dir=*/true, /*keep_files=*/false);
+
+   // Snapshot type = StateSnapshot (owning)
+   using Checkpointing = mfem::DynamicCheckpointing<StateSnapshot, Storage>;
+   Checkpointing ckpt(s, storage);
+
+   using Step = Checkpointing::Step;
+
+   // ---- Callbacks ----
+
+   // Make an owning snapshot from the current State (deep copy of vector payload)
+   auto make_snapshot = [&](const State &u) -> StateSnapshot
+   {
+      MFEM_VERIFY(u.v.Size() == n, "make_snapshot: State.v size changed!");
+
+      StateSnapshot snap;
+      snap.time = u.time;
+      snap.obj  = u.obj;
+
+      snap.v.SetSize(n);
+      const mfem::real_t *src = u.v.HostRead();
+      mfem::real_t *dst = snap.v.HostWrite();
+      std::memcpy(dst, src, (std::size_t)n * sizeof(mfem::real_t));
+
+      return snap; // move into storage
+   };
+
+   // Restore from snapshot into an actual State (copy payload)
+   auto restore_snapshot = [&](const StateSnapshot &snap, State &u_out)
+   {
+      u_out.time = snap.time;
+      u_out.obj  = snap.obj;
+
+      MFEM_VERIFY(snap.v.Size() == n,
+                  "restore_snapshot: snapshot vector size mismatch.");
+
+      if (u_out.v.Size() != n) { u_out.v.SetSize(n); }
+
+      const mfem::real_t *src = snap.v.HostRead();
+      mfem::real_t *dst = u_out.v.HostWrite();
+      std::memcpy(dst, src, (std::size_t)n * sizeof(mfem::real_t));
+   };
+
+   // Example primal step signature (replace with your integrator)
+   auto primal_step = [&](State &u_inout, Step i)
+   {
+      // advance u_inout.time, u_inout.obj, u_inout.v
+      if (print)
+      {
+         mfem::out<<"Forward step: "<<i<<" time="<<u_inout.time<<" obj="<<u_inout.obj<<std::endl;
+      }
+
+      u_inout.time=i*0.1;
+      u_inout.obj=i*0.2;
+   };
+
+   // Example adjoint state and adjoint step signature (replace with yours)
+   struct AdjointState
+   {
+      // e.g., mfem::Vector lambda;
+   };
+
+   auto adjoint_step = [&](AdjointState &q_ip1_inout, const State &u_i, Step i)
+   {
+      // update adjoint using u_i
+      if (print)
+      {
+         mfem::out<<"Adjoint step: "<<i<<" time="<<u_i.time<<" obj="<<u_i.obj<<std::endl;
+      }
+   };
+
+   // ---- Forward/backward skeleton ----
+
+   State u;
+   u.v.SetSize(n);
+   u.time = 0.0;
+   u.obj  = 0.0;
+   u.v = 0.0;
+
+
+
+   // Forward sweep (unknown number of steps)
+   mfem::real_t t=0.0;
+   Step i=0;
+   while (t<Tfinal)
+   {
+      ckpt.ForwardStep(i, u, primal_step, make_snapshot);
+      t+=dtime;
+      ++i;
+   }
+
+   mfem::out << "Total number of steps m="<<i-1<<"\n";
+   mfem::out << "\nBackward sweep.\n";
+
+   // backward
+   const Step m=i;
+   AdjointState q;
+   State u_work;
+   u_work.v.SetSize(n);
+
+   for (Step i = m - 1; i >= 0; --i)
+   {
+      ckpt.BackwardStep(i, q, u_work,
+                        primal_step, adjoint_step,
+                        make_snapshot, restore_snapshot);
+      if (i == 0) { break; }
+   }
+}
+
+/****************************************************************************
+ * Using standard in memory storage. Works with variable snaphot sizes.
+ * The size of the vector (n) is provided in order to compare the run to
+ * the other implementations.
+ ***************************************************************************/
+
+void run_in_memory_storage(mfem::real_t Tfinal /*Final time*/,
+                           mfem::real_t dtime  /*time step*/,
+                           int s /* checkpoint budget (real checkpoints) */,
+                           int n  /*State vector size*/,
+                           bool print=true)
+{
+   InMemoryCheckpointStorage<StateSnapshot> storage;
+
+   using Checkpointing = mfem::DynamicCheckpointing<StateSnapshot,
+         InMemoryCheckpointStorage<StateSnapshot>>;
+
+   Checkpointing ckpt(s, storage);
+
+   auto make_snapshot = [&](const State &u) -> StateSnapshot
+   {
+      StateSnapshot snap;
+      snap.time = u.time;
+      snap.obj  = u.obj;
+      snap.v    = u.v;   // deep copy (MFEM handles memory)
+      return snap;       // moved into storage by Store()
+   };
+
+   auto restore_snapshot = [&](const StateSnapshot &snap, State &u_out)
+   {
+      u_out.time = snap.time;
+      u_out.obj  = snap.obj;
+      u_out.v    = snap.v; // deep copy back into working state
+   };
+
+   using Step = Checkpointing::Step;
+
+   // Primal step: u_i -> u_{i+1}
+   auto primal_step = [&](State &u, Step i)
+   {
+      // ... update u.time, u.obj, u.v ...
+      if (print)
+      {
+         mfem::out<<"Forward step: "<<i<<" time="<<u.time<<" obj="<<u.obj<<std::endl;
+      }
+
+      u.time=i*0.1;
+      u.obj=i*0.2;
+   };
+
+   // Adjoint step: q_{i+1} -> q_i using u_i
+   struct AdjointState
+   {
+      // e.g. mfem::Vector lambda;
+   };
+
+   auto adjoint_step = [&](AdjointState &q, const State &u_i, Step i)
+   {
+      // ... update q using u_i ...
+      if (print)
+      {
+         mfem::out<<"Adjoint step: "<<i<<" time="<<u_i.time<<" obj="<<u_i.obj<<std::endl;
+      }
+   };
+
+   State u;
+   u.v.SetSize(n);
+   u.v=0.0;
+   u.time = 0.0;
+   u.obj  = 0.0;
+
+   // Forward sweep (unknown number of steps)
+   mfem::real_t t=0.0;
+   Step i=0;
+   while (t<Tfinal)
+   {
+      ckpt.ForwardStep(i, u, primal_step, make_snapshot);
+      t+=dtime;
+      ++i;
+   }
+
+   mfem::out << "Total number of steps m="<<i-1<<"\n";
+   mfem::out << "\nBackward sweep.\n";
+
+   // Backward
+   const Step m=i;
+   AdjointState q;
+   State u_work;  // used to restore/recompute primal state at step i
+   u_work.v.SetSize(u.v.Size());
+
+   for (Step i = m - 1; i >= 0; --i)
+   {
+      ckpt.BackwardStep(i, q, u_work,
+                        primal_step, adjoint_step,
+                        make_snapshot, restore_snapshot);
+      if (i == 0) { break; }
+   }
+}
+
+int main(int argc, char *argv[])
+{
+   int s = 5/* checkpoint budget (real checkpoints) */;
+   int n = 30 /* fixed State.v size */;
+   std::string filepath=
+      "dynamic_ckpts.bin"; /*file name for fixed slot single file*/
+   std::string directory="dyn_ckpts";
+
+   mfem::real_t Tfinal=1;
+   mfem::real_t dt=0.1;
+   int print=true;
+
+   OptionsParser args(argc, argv);
+   args.AddOption(&s,      "-s",
+                  "--checkpoints", "Checkpoint budget s (real checkpoints).");
+   args.AddOption(&n,      "-n",
+                  "--size", "Size of the state vector.");
+   args.AddOption(&Tfinal, "-T",
+                  "--tfinal",      "Terminate when accumulated time reaches Tfinal.");
+   args.AddOption(&dt,    "-dt", "--dt",         "Time step.");
+   args.AddOption(&filepath, "-file", "--file",
+                  "File name for storing the checkpoints.");
+   args.AddOption(&directory, "-dir", "--dir",
+                  "Directory for storing the checkpoints.");
+   args.AddOption(&print, "-print", "--print",
+                  "Turn on/off printing." );
+
+   args.Parse();
+   if (!args.Good())
+   {
+      args.PrintUsage(mfem::out);
+      return 1;
+   }
+   args.PrintOptions(mfem::out);
+
+   double run_time;
+
+   mfem::out<<"\nFixedSlotMemoryStorage Start\n";
+   mfem::tic();
+   run_fixed_slot_memory_storage(Tfinal, dt, s, n, print);
+   run_time=mfem::toc();
+   mfem::out<<"FixedSlotMemoryStorage time= "<<run_time<<std::endl;
+
+   mfem::out<<"\nFixedSlotFileStorage Start\n";
+   mfem::tic();
+   run_fixed_slot_file_storage(Tfinal,dt, s,n, filepath,print);
+   run_time=mfem::toc();
+   mfem::out<<"FixedSlotFileStorage time= "<<run_time<<std::endl;
+
+   mfem::out<<"\nFileStorage Start\n";
+   mfem::tic();
+   run_file_storage(Tfinal,dt, s,n, directory, print);
+   run_time=mfem::toc();
+   mfem::out<<"FileStorage time= "<<run_time<<std::endl;
+
+   mfem::out<<"\nInMemoryStorage\n";
+   mfem::tic();
+   run_in_memory_storage(Tfinal,dt, s,n, print);
+   run_time=mfem::toc();
+   mfem::out<<"InMemoryStorage time= "<<run_time<<std::endl;
+
+   mfem::out << "\nDone.\n";
+   return 0;
+}

miniapps/mtop/chpt/ex_fileio_scalar_vector_mfem.cpp

@@ -0,0 +1,313 @@
+#include "mfem.hpp"
+
+#include <cmath>
+#include <iomanip>
+
+using namespace mfem;
+
+static double LogisticStep(double u, double alpha, double dt)
+{
+   return u + dt * alpha * u * (1.0 - u);
+}
+
+static double LogisticJac(double u, double alpha, double dt)
+{
+   // d/du [u + dt*alpha*u*(1-u)] = 1 + dt*alpha*(1 - 2u)
+   return 1.0 + dt * alpha * (1.0 - 2.0*u);
+}
+
+#if MFEM_HAVE_FILESYSTEM
+static void PurgeDir(const std::string &dir)
+{
+   std::error_code ec;
+   mfem_fs::remove_all(mfem_fs::path(dir), ec);
+   mfem_fs::create_directories(mfem_fs::path(dir), ec);
+   MFEM_VERIFY(!ec, "Failed to purge/create directory.");
+}
+#endif
+
+int main(int argc, char *argv[])
+{
+   // ---------- shared options ----------
+   int s = 3;
+   double dt0 = 0.02;
+   double omega = 0.2;
+   double Tfinal = 1.0;
+   bool keep_files = false;
+   bool purge_dirs = true;
+
+   // scalar problem params
+   double alpha = 2.0;
+   double u0 = 0.2;
+   double target_s = 0.7;
+   double eps = 1e-7;
+
+   // vector problem params
+   int n = 16;
+   double target_v = 0.7;
+
+   std::string dir_scalar = "chk_scalar_files";
+   std::string dir_vector = "chk_vector_files";
+
+   OptionsParser args(argc, argv);
+   args.AddOption(&s, "-s", "--checkpoints",
+                  "Checkpoint budget s (real checkpoints).");
+   args.AddOption(&dt0, "-dt0", "--dt0",
+                  "Base dt for dt(i)=dt0*(1+0.5*sin(omega*i)).");
+   args.AddOption(&omega, "-om", "--omega",
+                  "Omega for dt(i)=dt0*(1+0.5*sin(omega*i)).");
+   args.AddOption(&Tfinal, "-T", "--tfinal",
+                  "Stop when accumulated time reaches Tfinal.");
+   args.AddOption(&keep_files, "-k", "--keep-files", "-nk", "--no-keep-files",
+                  "Keep checkpoint files (debug).");
+   args.AddOption(&purge_dirs, "-p", "--purge-dirs", "-np", "--no-purge-dirs",
+                  "Purge checkpoint directories at start (recommended).");
+
+   args.AddOption(&alpha, "-a", "--alpha", "Logistic growth alpha.");
+   args.AddOption(&u0, "-u0", "--u0", "Scalar initial u0.");
+   args.AddOption(&target_s, "-ts", "--target-scalar", "Scalar target.");
+   args.AddOption(&eps, "-eps", "--fd-eps",
+                  "FD epsilon (scalar and vector directional).");
+
+   args.AddOption(&n, "-n", "--size", "Vector dimension.");
+   args.AddOption(&target_v, "-tv", "--target-vector",
+                  "Vector target value per component.");
+
+   args.AddOption(&dir_scalar, "-ds", "--dir-scalar",
+                  "Directory for scalar checkpoint files.");
+   args.AddOption(&dir_vector, "-dv", "--dir-vector",
+                  "Directory for vector checkpoint files.");
+
+   args.Parse();
+   if (!args.Good())
+   {
+      args.PrintUsage(mfem::out);
+      return 1;
+   }
+   args.PrintOptions(mfem::out);
+
+   MFEM_VERIFY(s > 0, "Need s > 0.");
+   MFEM_VERIFY(dt0 > 0.0, "Need dt0 > 0.");
+   MFEM_VERIFY(Tfinal > 0.0, "Need Tfinal > 0.");
+
+#if MFEM_HAVE_FILESYSTEM
+   if (purge_dirs)
+   {
+      PurgeDir(dir_scalar);
+      PurgeDir(dir_vector);
+   }
+#else
+   MFEM_VERIFY(!purge_dirs, "purge_dirs requires <filesystem> support.");
+#endif
+
+   using StepS =
+      mfem::DynamicCheckpointing<double, mfem::FileCheckpointStorage<double>>::Step;
+   auto dt_func = [&](StepS i)
+   {
+      const double dt = dt0 * (1.0 + 0.5 * std::sin(omega * double(i)));
+      MFEM_VERIFY(dt > 0.0, "dt_func produced non-positive dt.");
+      return dt;
+   };
+
+   mfem::out << std::setprecision(15);
+
+   // ============================================================
+   // A) Scalar run (double) with FileCheckpointStorage
+   // ============================================================
+   {
+      mfem::FileCheckpointStorage<double> storage(dir_scalar, "ckpt_", ".bin", true,
+                                                  keep_files);
+      mfem::DynamicCheckpointing<double, mfem::FileCheckpointStorage<double>> ckpt(s,
+                                                                                   storage);
+
+      auto make_snapshot = [](const double &u) { return u; };
+      auto restore_snapshot = [](const double &snap, double &state) { state = snap; };
+
+      auto primal_step = [&](double &u, StepS i)
+      {
+         const double dt = dt_func(i);
+         u = LogisticStep(u, alpha, dt);
+      };
+
+      auto adjoint_step = [&](double &lambda, const double &u_i, StepS i)
+      {
+         const double dt = dt_func(i);
+         const double dF = LogisticJac(u_i, alpha, dt);
+         lambda *= dF;
+      };
+
+      double u = u0;
+      double t_phys = 0.0;
+      StepS i = 0;
+
+      while (t_phys < Tfinal)
+      {
+         ckpt.ForwardStep(i, u, primal_step, make_snapshot);
+         t_phys += dt_func(i);
+         ++i;
+      }
+      const StepS m = i;
+      const double u_m = u;
+      const double J = 0.5 * (u_m - target_s) * (u_m - target_s);
+
+      double lambda = (u_m - target_s); // terminal adjoint
+      double u_work = 0.0;
+
+      for (StepS j = m - 1; j >= 0; --j)
+      {
+         ckpt.BackwardStep(j, lambda, u_work,
+                           primal_step, adjoint_step,
+                           make_snapshot, restore_snapshot);
+         if (j == 0) { break; }
+      }
+
+      const double dJ_du0_adj = lambda;
+
+      // FD check
+      auto forward_only_J = [&](double u_init)
+      {
+         double uu = u_init;
+         double tt = 0.0;
+         StepS k = 0;
+         while (tt < Tfinal)
+         {
+            uu = LogisticStep(uu, alpha, dt_func(k));
+            tt += dt_func(k);
+            ++k;
+         }
+         const double r = uu - target_s;
+         return 0.5 * r * r;
+      };
+
+      const double Jp = forward_only_J(u0 + eps);
+      const double Jm = forward_only_J(u0 - eps);
+      const double dJ_du0_fd = (Jp - Jm) / (2.0 * eps);
+
+      mfem::out << "\n[FileIO][Scalar]\n";
+      mfem::out << "  m steps        = " << m << "\n";
+      mfem::out << "  u_m            = " << u_m << "\n";
+      mfem::out << "  J              = " << J << "\n";
+      mfem::out << "  dJ/du0 adjoint  = " << dJ_du0_adj << "\n";
+      mfem::out << "  dJ/du0 FD       = " << dJ_du0_fd << "\n";
+      mfem::out << "  abs err         = " << std::abs(dJ_du0_adj - dJ_du0_fd) << "\n";
+   }
+
+   // ============================================================
+   // B) Vector run (mfem::Vector) with FileCheckpointStorage
+   // ============================================================
+   {
+      mfem::FileCheckpointStorage<mfem::Vector> storage(dir_vector, "ckpt_", ".bin",
+                                                        true, keep_files);
+      mfem::DynamicCheckpointing<mfem::Vector, mfem::FileCheckpointStorage<mfem::Vector>>
+            ckpt(s, storage);
+
+      using StepV =
+         mfem::DynamicCheckpointing<mfem::Vector, mfem::FileCheckpointStorage<mfem::Vector>>::Step;
+
+      auto dt_func_v = [&](StepV i)
+      {
+         const double dt = dt0 * (1.0 + 0.5 * std::sin(omega * double(i)));
+         MFEM_VERIFY(dt > 0.0, "dt_func produced non-positive dt.");
+         return dt;
+      };
+
+      auto make_snapshot = [](const mfem::Vector &u) { return u; };
+      auto restore_snapshot = [](const mfem::Vector &snap, mfem::Vector &state) { state = snap; };
+
+      auto primal_step = [&](mfem::Vector &u, StepV i)
+      {
+         const double dt = dt_func_v(i);
+         for (int j = 0; j < u.Size(); ++j)
+         {
+            u[j] = LogisticStep(u[j], alpha, dt);
+         }
+      };
+
+      auto adjoint_step = [&](mfem::Vector &lambda, const mfem::Vector &u_i, StepV i)
+      {
+         const double dt = dt_func_v(i);
+         MFEM_ASSERT(lambda.Size() == u_i.Size(), "Size mismatch.");
+         for (int j = 0; j < lambda.Size(); ++j)
+         {
+            lambda[j] *= LogisticJac(u_i[j], alpha, dt);
+         }
+      };
+
+      mfem::Vector u0v(n), ut(n);
+      for (int j = 0; j < n; ++j) { u0v[j] = 0.2 + 0.05 * std::cos(0.7*(j+1)); }
+      ut = target_v;
+
+      mfem::Vector u = u0v;
+      double t_phys = 0.0;
+      StepV i = 0;
+
+      while (t_phys < Tfinal)
+      {
+         ckpt.ForwardStep(i, u, primal_step, make_snapshot);
+         t_phys += dt_func_v(i);
+         ++i;
+      }
+      const StepV m = i;
+      const mfem::Vector u_m = u;
+
+      mfem::Vector diff(u_m);
+      diff -= ut;
+      const double J = 0.5 * mfem::InnerProduct(diff, diff);
+
+      // backward
+      mfem::Vector lambda = diff; // terminal adjoint
+      mfem::Vector u_work(n);
+
+      for (StepV j = m - 1; j >= 0; --j)
+      {
+         ckpt.BackwardStep(j, lambda, u_work,
+                           primal_step, adjoint_step,
+                           make_snapshot, restore_snapshot);
+         if (j == 0) { break; }
+      }
+      const mfem::Vector &grad_u0 = lambda;
+
+      // directional FD check
+      mfem::Vector v(n);
+      for (int j = 0; j < n; ++j) { v[j] = std::sin(0.3*(j+1)) + 0.1; }
+
+      auto forward_only_J = [&](const mfem::Vector &u_init)
+      {
+         mfem::Vector uu = u_init;
+         double tt = 0.0;
+         StepV k = 0;
+         while (tt < Tfinal)
+         {
+            primal_step(uu, k);
+            tt += dt_func_v(k);
+            ++k;
+         }
+         mfem::Vector dd(uu);
+         dd -= ut;
+         return 0.5 * mfem::InnerProduct(dd, dd);
+      };
+
+      mfem::Vector u_plus(u0v), u_minus(u0v);
+      u_plus.Add(eps, v);
+      u_minus.Add(-eps, v);
+
+      const double Jp = forward_only_J(u_plus);
+      const double Jm = forward_only_J(u_minus);
+      const double dJ_dir_fd = (Jp - Jm) / (2.0 * eps);
+      const double dJ_dir_adj = mfem::InnerProduct(grad_u0, v);
+
+      mfem::out << "\n[FileIO][Vector]\n";
+      mfem::out << "  m steps             = " << m << "\n";
+      mfem::out << "  J                   = " << J << "\n";
+      mfem::out << "  v·grad adjoint      = " << dJ_dir_adj << "\n";
+      mfem::out << "  FD directional      = " << dJ_dir_fd << "\n";
+      mfem::out << "  abs err             = " << std::abs(dJ_dir_adj - dJ_dir_fd) <<
+                "\n";
+      mfem::out << "  ||u_m||_2           = " << u_m.Norml2() << "\n";
+      mfem::out << "  ||u_m - u_target||2 = " << diff.Norml2() << "\n";
+   }
+
+   mfem::out << "\nDone.\n";
+   return 0;
+}
+

miniapps/mtop/chpt/ex_fixedslot_scalar_vector.cpp

@@ -0,0 +1,385 @@
+#include "mfem.hpp"
+
+
+#include <cmath>
+#include <iomanip>
+#include <string>
+
+using namespace mfem;
+
+// Logistic ODE (scalar or componentwise):
+//   du/dt = alpha*u*(1-u)
+// Explicit Euler:
+//   u_{i+1} = u_i + dt_i * alpha*u_i*(1-u_i)
+//
+// Objective:
+//   Scalar: J = 0.5*(u_m - target)^2
+//   Vector: J = 0.5*||u_m - u_target||^2
+//
+// Discrete adjoint:
+//   dF/du = 1 + dt_i*alpha*(1 - 2*u_i)
+//   lambda_i = (dF/du at u_i) * lambda_{i+1}
+
+static inline double LogisticStep(const double u, const double alpha,
+                                  const double dt)
+{
+   return u + dt * alpha * u * (1.0 - u);
+}
+
+static inline double LogisticJac(const double u, const double alpha,
+                                 const double dt)
+{
+   return 1.0 + dt * alpha * (1.0 - 2.0*u);
+}
+
+// --------------------------
+// Scalar run (double)
+// --------------------------
+template <typename Storage>
+static void RunScalarFixedSlot(int s,
+                               Storage &storage,
+                               double alpha,
+                               double dt0,
+                               double omega,
+                               double Tfinal,
+                               double u0,
+                               double target,
+                               double eps)
+{
+   using CKPT = mfem::DynamicCheckpointing<double, Storage>;
+   using Step = typename CKPT::Step;
+
+   CKPT ckpt(s, storage);
+
+   auto dt_func = [&](Step i)
+   {
+      const double dt = dt0 * (1.0 + 0.5 * std::sin(omega * double(i)));
+      MFEM_VERIFY(dt > 0.0, "dt_func produced non-positive dt.");
+      return dt;
+   };
+
+   auto make_snapshot = [](const double &u) { return u; };
+   auto restore_snapshot = [](const double &snap, double &state) { state = snap; };
+
+   auto primal_step = [&](double &u, Step i)
+   {
+      const double dt = dt_func(i);
+      u = LogisticStep(u, alpha, dt);
+   };
+
+   auto adjoint_step = [&](double &lambda, const double &u_i, Step i)
+   {
+      const double dt = dt_func(i);
+      lambda *= LogisticJac(u_i, alpha, dt);
+   };
+
+   // Forward sweep (unknown m; stop on accumulated physical time)
+   double u = u0;
+   double t_phys = 0.0;
+   Step i = 0;
+
+   while (t_phys < Tfinal)
+   {
+      ckpt.ForwardStep(i, u, primal_step, make_snapshot);
+      t_phys += dt_func(i);
+      ++i;
+   }
+
+   const Step m = i;
+   const double u_m = u;
+   const double J = 0.5 * (u_m - target) * (u_m - target);
+
+   // Backward sweep
+   MFEM_VERIFY(m > 0, "Scalar run produced m=0 steps.");
+
+   double lambda = (u_m - target); // terminal condition dJ/du_m
+   double u_work = 0.0;
+
+   for (Step j = m - 1; j >= 0; --j)
+   {
+      ckpt.BackwardStep(j, lambda, u_work,
+                        primal_step, adjoint_step,
+                        make_snapshot, restore_snapshot);
+      if (j == 0) { break; }
+   }
+   const double dJ_du0_adj = lambda;
+
+   // FD check
+   auto forward_only_J = [&](double u_init)
+   {
+      double uu = u_init;
+      double tt = 0.0;
+      Step k = 0;
+      while (tt < Tfinal)
+      {
+         uu = LogisticStep(uu, alpha, dt_func(k));
+         tt += dt_func(k);
+         ++k;
+      }
+      const double r = uu - target;
+      return 0.5 * r * r;
+   };
+
+   const double Jp = forward_only_J(u0 + eps);
+   const double Jm = forward_only_J(u0 - eps);
+   const double dJ_du0_fd = (Jp - Jm) / (2.0 * eps);
+
+   mfem::out << "\n[FixedSlot][Scalar]\n";
+   mfem::out << "  m steps        = " << m << "\n";
+   mfem::out << "  t_phys         = " << t_phys << "\n";
+   mfem::out << "  u_m            = " << u_m << "\n";
+   mfem::out << "  J              = " << J << "\n";
+   mfem::out << "  dJ/du0 adjoint  = " << dJ_du0_adj << "\n";
+   mfem::out << "  dJ/du0 FD       = " << dJ_du0_fd << "\n";
+   mfem::out << "  abs err         = " << std::abs(dJ_du0_adj - dJ_du0_fd) << "\n";
+}
+
+// --------------------------
+// Vector run (mfem::Vector)
+// --------------------------
+template <typename Storage>
+static void RunVectorFixedSlot(int s,
+                               Storage &storage,
+                               int n,
+                               double alpha,
+                               double dt0,
+                               double omega,
+                               double Tfinal,
+                               double target_val,
+                               double eps)
+{
+   using CKPT = mfem::DynamicCheckpointing<mfem::Vector, Storage>;
+   using Step = typename CKPT::Step;
+
+   CKPT ckpt(s, storage);
+
+   auto dt_func = [&](Step i)
+   {
+      const double dt = dt0 * (1.0 + 0.5 * std::sin(omega * double(i)));
+      MFEM_VERIFY(dt > 0.0, "dt_func produced non-positive dt.");
+      return dt;
+   };
+
+   auto make_snapshot = [](const mfem::Vector &u) { return u; };
+   auto restore_snapshot = [](const mfem::Vector &snap, mfem::Vector &state) { state = snap; };
+
+   auto primal_step = [&](mfem::Vector &u, Step i)
+   {
+      const double dt = dt_func(i);
+      for (int j = 0; j < u.Size(); ++j)
+      {
+         u[j] = LogisticStep(u[j], alpha, dt);
+      }
+   };
+
+   auto adjoint_step = [&](mfem::Vector &lambda, const mfem::Vector &u_i, Step i)
+   {
+
+      const double dt = dt_func(i);
+      MFEM_ASSERT(lambda.Size() == u_i.Size(), "lambda and u_i size mismatch.");
+      for (int j = 0; j < lambda.Size(); ++j)
+      {
+         lambda[j] *= LogisticJac(u_i[j], alpha, dt);
+      }
+   };
+
+   // Initial condition and target
+   mfem::Vector u0(n), u_target(n);
+   for (int j = 0; j < n; ++j)
+   {
+      u0[j] = 0.2 + 0.05 * std::cos(0.7 * (j + 1));
+   }
+   u_target = target_val;
+
+   // Forward sweep
+   mfem::Vector u = u0;
+   double t_phys = 0.0;
+   Step i = 0;
+
+   while (t_phys < Tfinal)
+   {
+      ckpt.ForwardStep(i, u, primal_step, make_snapshot);
+      t_phys += dt_func(i);
+      ++i;
+   }
+
+   const Step m = i;
+   const mfem::Vector u_m = u;
+
+   mfem::Vector diff(u_m);
+   diff -= u_target;
+   const double J = 0.5 * mfem::InnerProduct(diff, diff);
+
+   // Backward sweep
+   MFEM_VERIFY(m > 0, "Vector run produced m=0 steps.");
+
+   mfem::Vector lambda = diff; // terminal adjoint = dJ/du_m
+   mfem::Vector u_work(n);
+
+   for (Step j = m - 1; j >= 0; --j)
+   {
+      ckpt.BackwardStep(j, lambda, u_work,
+                        primal_step, adjoint_step,
+                        make_snapshot, restore_snapshot);
+      if (j == 0) { break; }
+   }
+   const mfem::Vector &grad_u0 = lambda;
+
+   // Directional FD check
+   mfem::Vector v(n);
+   for (int j = 0; j < n; ++j) { v[j] = std::sin(0.3 * (j + 1)) + 0.1; }
+
+   auto forward_only_J = [&](const mfem::Vector &u_init)
+   {
+      mfem::Vector uu = u_init;
+      double tt = 0.0;
+      Step k = 0;
+      while (tt < Tfinal)
+      {
+         primal_step(uu, k);
+         tt += dt_func(k);
+         ++k;
+      }
+      mfem::Vector dd(uu);
+      dd -= u_target;
+      return 0.5 * mfem::InnerProduct(dd, dd);
+   };
+
+   mfem::Vector u_plus(u0), u_minus(u0);
+   u_plus.Add(eps, v);
+   u_minus.Add(-eps, v);
+
+   const double Jp = forward_only_J(u_plus);
+   const double Jm = forward_only_J(u_minus);
+   const double dJ_dir_fd = (Jp - Jm) / (2.0 * eps);
+   const double dJ_dir_adj = mfem::InnerProduct(grad_u0, v);
+
+   mfem::out << "\n[FixedSlot][Vector]\n";
+   mfem::out << "  n              = " << n << "\n";
+   mfem::out << "  m steps         = " << m << "\n";
+   mfem::out << "  t_phys          = " << t_phys << "\n";
+   mfem::out << "  J               = " << J << "\n";
+   mfem::out << "  ||u_m||_2        = " << u_m.Norml2() << "\n";
+   mfem::out << "  ||u_m-u_target|| = " << diff.Norml2() << "\n";
+   mfem::out << "  v·grad adjoint   = " << dJ_dir_adj << "\n";
+   mfem::out << "  FD directional   = " << dJ_dir_fd << "\n";
+   mfem::out << "  abs err          = " << std::abs(dJ_dir_adj - dJ_dir_fd) <<
+             "\n";
+}
+
+int main(int argc, char *argv[])
+{
+   // Backend selection:
+   //   0 = fixed-slot memory (single RAM block)
+   //   1 = fixed-slot file   (single file with fixed offsets)
+   int backend = 0;
+
+   // Common parameters
+   int s = 8;                 // number of REAL stored checkpoints
+   double alpha = 2.0;
+   double dt0 = 0.02;
+   double omega = 0.2;
+   double Tfinal = 1.0;
+   double eps = 1e-7;
+
+   // Scalar parameters
+   double u0 = 0.2;
+   double target_s = 0.7;
+
+   // Vector parameters
+   int n = 64;                // must stay fixed for fixed-slot vector packing
+   double target_v = 0.7;
+
+   // File backend parameters
+   std::string scalar_file = "scalar_fixedslots.bin";
+   std::string vector_file = "vector_fixedslots.bin";
+   bool truncate_files = true;
+   bool flush_on_store = true; // safer for demo correctness with iostreams
+
+   OptionsParser args(argc, argv);
+   args.AddOption(&backend, "-b", "--backend",
+                  "Backend: 0=memory fixed-slots, 1=file fixed-slots (single file).");
+   args.AddOption(&s, "-s", "--checkpoints",
+                  "Checkpoint budget s (real checkpoints).");
+
+   args.AddOption(&alpha, "-a", "--alpha", "Logistic growth alpha.");
+   args.AddOption(&dt0, "-dt0", "--dt0",
+                  "Base dt for dt(i)=dt0*(1+0.5*sin(omega*i)).");
+   args.AddOption(&omega, "-om", "--omega",
+                  "Omega for dt(i)=dt0*(1+0.5*sin(omega*i)).");
+   args.AddOption(&Tfinal, "-T", "--tfinal",
+                  "Stop when accumulated time reaches Tfinal.");
+   args.AddOption(&eps, "-eps", "--fd-eps", "FD epsilon.");
+
+   args.AddOption(&u0, "-u0", "--u0", "Scalar initial condition u0.");
+   args.AddOption(&target_s, "-ts", "--target-scalar", "Scalar target.");
+
+   args.AddOption(&n, "-n", "--size", "Vector dimension (fixed).");
+   args.AddOption(&target_v, "-tv", "--target-vector",
+                  "Vector target value per component.");
+
+   args.AddOption(&scalar_file, "-sf", "--scalar-file",
+                  "File for scalar fixed-slot storage.");
+   args.AddOption(&vector_file, "-vf", "--vector-file",
+                  "File for vector fixed-slot storage.");
+   args.AddOption(&truncate_files, "-tr", "--truncate", "-ntr", "--no-truncate",
+                  "Truncate checkpoint files on startup.");
+   args.AddOption(&flush_on_store, "-fl", "--flush", "-nfl", "--no-flush",
+                  "Flush on each Store() (demo-safety; slower).");
+
+   args.Parse();
+   if (!args.Good())
+   {
+      args.PrintUsage(mfem::out);
+      return 1;
+   }
+   args.PrintOptions(mfem::out);
+
+   MFEM_VERIFY(s > 0, "Need s > 0.");
+   MFEM_VERIFY(n > 0, "Need n > 0.");
+   MFEM_VERIFY(dt0 > 0.0, "Need dt0 > 0.");
+   MFEM_VERIFY(Tfinal > 0.0, "Need Tfinal > 0.");
+
+   mfem::out << std::setprecision(15);
+
+   if (backend == 0)
+   {
+      mfem::out << "\nUsing fixed-slot MEMORY backend (single RAM block)\n";
+
+      // Scalar: fixed-size POD => trivial packer
+      mfem::FixedSlotMemoryCheckpointStorage<double> stor_s(s);
+      RunScalarFixedSlot(s, stor_s, alpha, dt0, omega, Tfinal, u0, target_s, eps);
+
+      // Vector: fixed-size packing (n must remain constant)
+      mfem::FixedVectorPacker packer(n);
+      mfem::FixedSlotMemoryCheckpointStorage<mfem::Vector, mfem::FixedVectorPacker>
+      stor_v(s, packer);
+      RunVectorFixedSlot(s, stor_v, n, alpha, dt0, omega, Tfinal, target_v, eps);
+   }
+   else if (backend == 1)
+   {
+      mfem::out <<
+                "\nUsing fixed-slot FILE backend (single file with fixed offsets)\n";
+
+      // Scalar file
+      mfem::FixedSlotFileCheckpointStorage<double> stor_s(scalar_file, s,
+                                                          mfem::TrivialFixedPacker<double>(),
+                                                          truncate_files,
+                                                          flush_on_store);
+      RunScalarFixedSlot(s, stor_s, alpha, dt0, omega, Tfinal, u0, target_s, eps);
+
+      // Vector file (fixed-size packing with n)
+      mfem::FixedVectorPacker packer(n);
+      mfem::FixedSlotFileCheckpointStorage<mfem::Vector, mfem::FixedVectorPacker>
+      stor_v(vector_file, s, packer, truncate_files, flush_on_store);
+      RunVectorFixedSlot(s, stor_v, n, alpha, dt0, omega, Tfinal, target_v, eps);
+   }
+   else
+   {
+      MFEM_ABORT("Unknown backend. Use -b 0 (memory) or -b 1 (file).");
+   }
+
+   mfem::out << "\nDone.\n";
+   return 0;
+}
+

miniapps/mtop/chpt/ex_revolve.cpp

@@ -0,0 +1,183 @@
+#include "mfem.hpp"
+
+using namespace mfem;
+
+// ============================================================
+// Mini example 1: scalar state (double)
+// u_{n+1} = factor * u_n
+// J = 0.5 (u_N - target)^2  => lambda_N = (u_N - target)
+// lambda_n = factor * lambda_{n+1}
+// ============================================================
+static void RunScalarExample(int Nsteps, int Ncheck, double factor)
+{
+   mfem::out << "\n--- Scalar REVOLVE example ---\n";
+   mfem::out << "Nsteps=" << Nsteps << ", Ncheckpoints=" << Ncheck << ", factor="
+             <<
+             factor << "\n";
+
+   const size_t snap_bytes = sizeof(double);
+   RevolveFixedSlotMemoryStorage storage(Ncheck, snap_bytes);
+   FixedStepRevolveCheckpointing<RevolveFixedSlotMemoryStorage> ckpt(Nsteps,
+                                                                     Ncheck,
+                                                                     snap_bytes, storage);
+
+   auto make_snapshot = [](const double &u, uint8_t *outb, size_t bytes)
+   {
+      MFEM_VERIFY(bytes == sizeof(double), "scalar snapshot size mismatch");
+      std::memcpy(outb, &u, sizeof(double));
+   };
+   auto restore_snapshot = [](double &u, const uint8_t *inb, size_t bytes)
+   {
+      MFEM_VERIFY(bytes == sizeof(double), "scalar snapshot size mismatch");
+      std::memcpy(&u, inb, sizeof(double));
+   };
+
+   auto primal_step = [factor](int /*step*/, double &u)
+   {
+      u *= factor;
+   };
+   auto adjoint_step = [factor](int /*step*/, const double & /*u_step*/,
+                                double &lambda)
+   {
+      lambda *= factor;
+   };
+
+   const double target = 2.0;
+
+   // Forward
+   double u = 1.0;
+   for (int i = 0; i < Nsteps; ++i)
+   {
+      ckpt.ForwardStep(i, u, primal_step, make_snapshot);
+   }
+   const double uN = u;
+
+   // Init adjoint at final state
+   double lambda = (uN - target);
+
+   // Reverse
+   double u_work = 0.0; // will be overwritten by restore_snapshot
+   for (int i = Nsteps - 1; i >= 0; --i)
+   {
+      ckpt.BackwardStep(i, lambda, u_work,
+                        primal_step, adjoint_step,
+                        make_snapshot, restore_snapshot);
+   }
+
+   // Analytic lambda_0 = factor^Nsteps * (uN - target)
+   double factorN = 1.0;
+   for (int k = 0; k < Nsteps; ++k) { factorN *= factor; }
+   const double lambda0_exact = factorN * (uN - target);
+
+   mfem::out << "uN = " << uN << "\n";
+   mfem::out << "lambda0 (computed) = " << lambda << "\n";
+   mfem::out << "lambda0 (exact)    = " << lambda0_exact << "\n";
+}
+
+// ============================================================
+// Mini example 2: mfem::Vector state
+// u_{n+1} = factor * u_n (elementwise scalar multiply)
+// J = 0.5 ||u_N - target||^2  => lambda_N = (u_N - target)
+// lambda_n = factor * lambda_{n+1}
+// ============================================================
+static void RunVectorExample(int Nsteps, int Ncheck, int dim, double factor)
+{
+   mfem::out << "\n--- mfem::Vector REVOLVE example ---\n";
+   mfem::out << "Nsteps=" << Nsteps << ", Ncheckpoints=" << Ncheck
+             << ", dim=" << dim << ", factor=" << factor << "\n";
+
+   MFEM_VERIFY(dim > 0, "dim must be > 0");
+
+   const size_t snap_bytes = sizeof(double) * size_t(dim);
+   RevolveFixedSlotMemoryStorage storage(Ncheck, snap_bytes);
+   FixedStepRevolveCheckpointing<RevolveFixedSlotMemoryStorage> ckpt(Nsteps,
+                                                                     Ncheck,
+                                                                     snap_bytes, storage);
+
+   auto make_snapshot = [](const mfem::Vector &u, uint8_t *outb, size_t bytes)
+   {
+      MFEM_VERIFY(bytes == sizeof(double) * size_t(u.Size()),
+                  "Vector snapshot size mismatch");
+      std::memcpy(outb, u.GetData(), bytes);
+   };
+   auto restore_snapshot = [](mfem::Vector &u, const uint8_t *inb, size_t bytes)
+   {
+      MFEM_VERIFY(bytes == sizeof(double) * size_t(u.Size()),
+                  "Vector snapshot size mismatch");
+      std::memcpy(u.GetData(), inb, bytes);
+   };
+
+   auto primal_step = [factor](int /*step*/, mfem::Vector &u)
+   {
+      u *= factor;
+   };
+   auto adjoint_step = [factor](int /*step*/, const mfem::Vector & /*u_step*/,
+                                mfem::Vector &lambda)
+   {
+      lambda *= factor;
+   };
+
+   mfem::Vector u(dim);
+   for (int k = 0; k < dim; ++k) { u[k] = 1.0 + 0.1*k; }
+
+   mfem::Vector target(dim);
+   target = 2.0;
+
+   // Forward
+   for (int i = 0; i < Nsteps; ++i)
+   {
+      ckpt.ForwardStep(i, u, primal_step, make_snapshot);
+   }
+   mfem::Vector uN(u); // copy for reporting
+
+   // Init adjoint: lambda_N = uN - target
+   mfem::Vector lambda(dim);
+   lambda = u;
+   lambda -= target;
+
+   // Reverse
+   mfem::Vector u_work(dim);
+   u_work = 0.0;
+   for (int i = Nsteps - 1; i >= 0; --i)
+   {
+      ckpt.BackwardStep(i, lambda, u_work,
+                        primal_step, adjoint_step,
+                        make_snapshot, restore_snapshot);
+   }
+
+   mfem::out << "||uN||_2      = " << uN.Norml2() << "\n";
+   mfem::out << "||lambda0||_2 = " << lambda.Norml2() << "\n";
+}
+
+int main(int argc, char *argv[])
+{
+   mfem::Device device("cpu");
+   device.Print();
+
+   int Nsteps = 20;
+   int Ncheck = 3;
+   int dim = 5;
+   double factor = 1.05;
+
+   mfem::OptionsParser args(argc, argv);
+   args.AddOption(&Nsteps, "-n", "--num-steps", "Number of primal steps.");
+   args.AddOption(&Ncheck, "-s", "--num-checkpoints",
+                  "Number of checkpoints (snaps).");
+   args.AddOption(&dim, "-d", "--dim",
+                  "Vector dimension for the mfem::Vector example.");
+   args.AddOption(&factor, "-f", "--factor",
+                  "Scalar factor in the toy update u_{n+1}=f*u_n.");
+   args.Parse();
+
+   if (!args.Good())
+   {
+      args.PrintUsage(mfem::out);
+      return 1;
+   }
+   args.PrintOptions(mfem::out);
+
+   RunScalarExample(Nsteps, Ncheck, factor);
+   RunVectorExample(Nsteps, Ncheck, dim, factor);
+
+   return 0;
+}

miniapps/mtop/chpt/ex_revolve_chpt_fixedslot.cpp

@@ -0,0 +1,174 @@
+#include "mfem.hpp"
+
+
+#include <cmath>
+#include <iomanip>
+#include <string>
+
+using namespace mfem;
+
+
+// the forward state consists of vector and
+// several additional values
+struct State
+{
+   mfem::real_t time = 0.0;
+   mfem::real_t obj  = 0.0;
+   mfem::Vector v;
+};
+
+/// class to manage the Snaphot Layout
+class StateSnapshotLayout
+{
+public:
+   // Constructor: n - size of the vector in the state
+   explicit StateSnapshotLayout(int n) : n_(n)
+   {
+      MFEM_VERIFY(n_ > 0, "StateSnapshotLayout: n must be > 0.");
+   }
+
+   // returns the total size of the snapshot in mfem::real_t
+   int N() const { return n_; }
+
+   // returns the total number of bytes necessary to store the state
+   int Bytes() const
+   {
+      return (2 + n_) * (int)sizeof(mfem::real_t);
+   }
+
+   // Pack State -> bytes (host bytes)
+   void Pack(const State &s, unsigned char *dst, int bytes) const
+   {
+      MFEM_VERIFY(dst != nullptr, "Pack: dst is null.");
+      MFEM_VERIFY(bytes == Bytes(), "Pack: snapshot byte size mismatch.");
+      MFEM_VERIFY(s.v.Size() == n_, "Pack: State.v size mismatch.");
+
+      // Ensure we read from host even if MFEM uses device memory.
+      const mfem::real_t *vh = s.v.HostRead();
+
+      std::memcpy(dst + 0*sizeof(mfem::real_t), &s.time, sizeof(mfem::real_t));
+      std::memcpy(dst + 1*sizeof(mfem::real_t), &s.obj,  sizeof(mfem::real_t));
+      std::memcpy(dst + 2*sizeof(mfem::real_t),
+                  vh, (std::size_t)n_ * sizeof(mfem::real_t));
+   }
+
+   // Unpack bytes -> State (host write)
+   void Unpack(const unsigned char *src, int bytes, State &state) const
+   {
+      MFEM_VERIFY(src != nullptr, "Unpack: src is null.");
+      MFEM_VERIFY(bytes == Bytes(), "Unpack: snapshot byte size mismatch.");
+
+      if (state.v.Size() != n_) { state.v.SetSize(n_); }
+      mfem::real_t *vh = state.v.HostWrite();
+
+      std::memcpy(&state.time, src + 0*sizeof(mfem::real_t), sizeof(mfem::real_t));
+      std::memcpy(&state.obj,  src + 1*sizeof(mfem::real_t), sizeof(mfem::real_t));
+      std::memcpy(vh,
+                  src + 2*sizeof(mfem::real_t),
+                  (std::size_t)n_ * sizeof(mfem::real_t));
+   }
+
+private:
+   int n_ = 0;
+};
+
+/* The REVOLVE manager (as provided earlier) uses this callback style:
+
+        * make_snapshot(const State&, uint8_t* out, size_t bytes)
+        * restore_snapshot(State&, const uint8_t* in, size_t bytes)
+        * primal_step(int step, State&)
+        * adjoint_step(int step, const State&, AdjointState&)
+
+        and a storage backend with:
+        * Save(slot, bytes)
+        * Load(slot, bytes)
+
+    REVOLVE manager will:
+        *request storing snapshots into checkpoint slots (takeshot)
+        *request restoring a checkpoint slot (restore)
+        *request recomputation forward (advance)
+        *then request the next adjoint step (firsturn / youturn)
+        all while using only (Ncheck) stored checkpoints.
+
+    Fixed-step REVOLVE manager:
+        * Snapshot is raw bytes packed/unpacked by your callbacks.
+        * Storage is a fixed indexed array (Save(slot) / Load(slot)),
+               because REVOLVE addresses checkpoints by slot index.
+
+    Best use when Nsteps is known in the begining of the simulation.
+*/
+
+int main(int argc, char *argv[])
+{
+   const int n = 100/* fixed State.v size */;
+   StateSnapshotLayout layout(n);
+   const std::size_t snapshot_bytes = (std::size_t)layout.Bytes();
+
+   const int Nsteps = 20 /* known number of time steps */;
+   const int Ncheck = 5 /* number of checkpoints (snaps) */;
+
+   // Fixed-slot file backend (single file)
+   RevolveFixedSlotFileStorage storage("revolve_ckpts.bin", Ncheck,
+                                       snapshot_bytes);
+   FixedStepRevolveCheckpointing<RevolveFixedSlotFileStorage>
+   cktp(Nsteps, Ncheck, snapshot_bytes, storage);
+
+   // Memory backend (single block) alternative:
+   // RevolveFixedSlotMemoryStorage storage(Ncheck, snapshot_bytes);
+   // FixedStepRevolveCheckpointing<RevolveFixedSlotMemoryStorage>
+   //                        cktp(Nsteps, Ncheck, snapshot_bytes, storage);
+
+
+   auto make_snapshot = [&](const State &s, uint8_t *outb, std::size_t bytes)
+   {
+      MFEM_VERIFY(bytes == snapshot_bytes, "make_snapshot: byte size mismatch");
+      layout.Pack(s, reinterpret_cast<unsigned char*>(outb), (int)bytes);
+   };
+
+   auto restore_snapshot = [&](State &s, const uint8_t *inb, std::size_t bytes)
+   {
+      MFEM_VERIFY(bytes == snapshot_bytes, "restore_snapshot: byte size mismatch");
+      layout.Unpack(reinterpret_cast<const unsigned char*>(inb), (int)bytes, s);
+   };
+
+   // set the work state
+   State u_work;
+   u_work.v.SetSize(n); u_work.v=0.0;
+
+   State u;
+   u.v.SetSize(n); u.v=0.0; u.obj=0.0; u.time=0.0;
+   // init u.time, u.obj, u.v ...
+
+   auto primal_step = [&](int step, State &u_inout)
+   {
+      (void)step;
+      // advance u_inout -> u_{step+1}
+   };
+
+   struct AdjointState { /* ... */ };
+   AdjointState lambda;
+
+   auto adjoint_step = [&](int step, const State &u_step,
+                           AdjointState &lambda_inout)
+   {
+      (void)step;
+      // update lambda_{step+1} -> lambda_step using u_step
+   };
+
+   // Forward: i = 0..Nsteps-1
+   for (int i = 0; i < Nsteps; ++i)
+   {
+      cktp.ForwardStep(i, u, primal_step, make_snapshot);
+   }
+
+   // Reverse: i = Nsteps-1..0
+   for (int i = Nsteps - 1; i >= 0; --i)
+   {
+      cktp.BackwardStep(i, lambda, u_work,
+                        primal_step, adjoint_step,
+                        make_snapshot, restore_snapshot);
+   }
+
+   mfem::out << "\nDone.\n";
+   return 0;
+};

miniapps/mtop/chpt/ex_scalar_adaptive.cpp

@@ -0,0 +1,212 @@
+#include "mfem.hpp"
+
+#include <cmath>
+#include <iomanip>
+
+using namespace mfem;
+
+// Scalar logistic ODE:
+//   du/dt = alpha*u*(1-u)
+// Explicit Euler with variable dt(i):
+//   u_{i+1} = u_i + dt_i*alpha*u_i*(1-u_i)
+// Objective:
+//   J = 0.5*(u_m - target)^2
+// Discrete adjoint (scalar):
+//   lambda_m = (u_m - target)
+//   lambda_i = (dF/du at u_i)^T * lambda_{i+1}
+//   dF/du = 1 + dt_i*alpha*(1 - 2*u_i)
+
+int main(int argc, char *argv[])
+{
+   int s = 3;
+   double alpha = 2.0;
+   double dt0 = 0.001;
+   double Tfinal = 1.0;
+   double u0 = 0.2;
+   double target = 0.7;
+   double eps = 1e-7;
+   double err = 1e-4;
+
+   OptionsParser args(argc, argv);
+   args.AddOption(&s,      "-s",   "--checkpoints",
+                  "Checkpoint budget s (real checkpoints).");
+   args.AddOption(&alpha,  "-a",   "--alpha",       "Logistic growth alpha.");
+   args.AddOption(&dt0,    "-dt0", "--dt0",         "Base for the time step dt.");
+   args.AddOption(&Tfinal, "-T",   "--tfinal",
+                  "Terminate when accumulated time reaches Tfinal.");
+   args.AddOption(&u0,     "-u0",  "--u0",          "Initial scalar state u0.");
+   args.AddOption(&target, "-ut",  "--target",
+                  "Target value in J=0.5*(u_m-target)^2.");
+   args.AddOption(&eps,    "-eps", "--fd-eps",      "Finite-difference epsilon.");
+   args.AddOption(&err,    "-err", "--time_err",
+                  "Allowed time integration error per time step.");
+   args.Parse();
+   if (!args.Good())
+   {
+      args.PrintUsage(mfem::out);
+      return 1;
+   }
+   args.PrintOptions(mfem::out);
+
+   MFEM_VERIFY(s > 0, "Need s > 0.");
+   MFEM_VERIFY(dt0 > 0.0, "Need dt0 > 0.");
+   MFEM_VERIFY(Tfinal > 0.0, "Need Tfinal > 0.");
+
+   using Step = mfem::DynamicCheckpointing<double>::Step;
+
+   struct my_state
+   {
+      my_state(double a, double b, double c, double d=0.0, double tp_=0.0)
+      {
+         t=a; dt=b; u=c; up=d; tp=tp_;
+      }
+
+      //t,dt,u
+      double t;
+      double dt;
+      double u;
+      double tp;
+      double up;
+   };
+
+   // Checkpoint manager:
+   //   State   = double
+   //   Snapshot= double
+   mfem::DynamicCheckpointing<my_state> ckpt(s);
+
+   auto make_snapshot = [](const my_state &u) -> my_state { return u; };
+   auto restore_snapshot = [](const my_state &snap, my_state &state) { state = snap; };
+
+   double tmax=Tfinal;
+
+   auto primal_step = [&](my_state &su, Step i)
+   {
+      su.up=su.u;
+      su.tp=su.t;
+
+      double t=su.t;
+      double dt = su.dt;
+      bool flag=true;
+
+      if ((tmax-t)<su.dt)
+      {
+         dt=tmax-t;
+         flag=false;
+      }
+
+      double u=su.u;
+
+      double s0=alpha * u * (1.0 - u);
+      double ue=u+dt*s0;
+      double s1=alpha * ue * (1.0 - ue);
+      double uh=u + 0.5 * dt * (s0 + s1);
+      double ee = std::abs(uh-ue);
+
+      if ((ee < 0.5*err) && (flag))
+      {
+         dt=1.25*dt;
+         ue=u+dt*s0;
+         s1=alpha * ue * (1.0 - ue);
+         uh=u + 0.5 * dt * (s0 + s1);
+         ee = std::abs(uh-ue);
+      }
+
+      while (ee > err)
+      {
+         dt=0.5*dt;
+         ue=u+dt*s0;
+         s1=alpha * ue * (1.0 - ue);
+         uh=u + 0.5 * dt * (s0 + s1);
+         ee = std::abs(uh-ue);
+      }
+
+      //mfem::out<<" t="<<t+dt<<" dt="<<dt<<" err="<<ee<<std::endl;
+
+      su.t = t+dt;
+      su.dt = dt;
+      su.u = uh;
+   };
+
+   auto adjoint_step = [&](double &lambda, const my_state &u_i, Step i)
+   {
+      const double dt = u_i.dt;
+      // const double dF_du = 1.0 + dt * alpha * (1.0 - 2.0 * u_i.u);
+      // lambda = dF_du * lambda;
+
+      const double s0=alpha * (1.0 -2.0 * u_i.u) *lambda;
+      const double le= lambda+dt*s0;
+      const double s1= alpha * (1.0 -2.0 * u_i.up) * le;
+      lambda = lambda + 0.5 *dt *(s0+s1);
+   };
+
+   // ---------------- Forward sweep (unknown m) ----------------
+   my_state u(0.0, dt0, u0);
+   double t_phys = 0.0;
+
+   Step i = 0;
+   while (t_phys < Tfinal)
+   {
+      ckpt.ForwardStep(i, u, primal_step, make_snapshot);
+      t_phys = u.t;
+      ++i;
+   }
+
+   const Step m = i;
+   const double u_m = u.u;
+   const double J = 0.5 * (u_m - target) * (u_m - target);
+
+   mfem::out << std::setprecision(15);
+   mfem::out << "\n[Scalar] Forward finished:\n";
+   mfem::out << "  m (steps) = " << m << "\n";
+   mfem::out << "  t_phys    = " << t_phys << "\n";
+   mfem::out << "  u_m       = " << u_m << "\n";
+   mfem::out << "  J         = " << J << "\n\n";
+
+   mfem::out <<
+             "[Scalar] Checkpoint set after forward sweep (step, level, stored):\n";
+   for (const auto &cp : ckpt.GetCheckpointInfo())
+   {
+      mfem::out << "  step=" << cp.step
+                << ", level=" << cp.level
+                << ", stored=" << (cp.stored ? "yes" : "no")
+                << (cp.stored ? "" : " (placeholder)")
+                << "\n";
+   }
+   mfem::out << "\n";
+
+   MFEM_VERIFY(m > 0, "Forward produced m=0 steps; nothing to do.");
+
+   // ---------------- Backward sweep (adjoint) ----------------
+   double lambda = (u_m - target); // terminal adjoint = dJ/du_m
+   my_state u_work (0.0,0.0,0.0);            // scratch primal state u_i
+
+   for (Step j = m - 1; j >= 0; --j)
+   {
+      ckpt.BackwardStep(j, lambda, u_work,
+                        primal_step, adjoint_step,
+                        make_snapshot, restore_snapshot);
+      if (j == 0) { break; } // avoid signed underflow
+   }
+
+   const double dJ_du0_adjoint = lambda;
+
+   // analytic solution
+   const double sol=u0*exp(alpha*Tfinal)/(1.0-u0+u0*exp(alpha*Tfinal));
+   const double grd=exp(alpha*Tfinal)/std::pow(1.0-u0+u0*exp(alpha*Tfinal),2.0);
+
+   mfem::out << "true sol = "<<sol<<" \n";
+
+
+   const double dJ_du0_fd = (sol - target) * grd;
+
+   const double abs_err = std::abs(dJ_du0_adjoint - dJ_du0_fd);
+   const double rel_err = abs_err / (std::abs(dJ_du0_fd) + 1e-30);
+
+   mfem::out << "[Scalar] Gradient check (dJ/du0):\n";
+   mfem::out << "  adjoint = " << dJ_du0_adjoint << "\n";
+   mfem::out << "  true grad = " << dJ_du0_fd << "\n";
+   mfem::out << "  abs err = " << abs_err << "\n";
+   mfem::out << "  rel err = " << rel_err << "\n\n";
+
+   return 0;
+}

miniapps/mtop/chpt/ex_scalar_field.cpp

@@ -0,0 +1,168 @@
+#include "mfem.hpp"
+
+#include <cmath>
+#include <iomanip>
+
+using namespace mfem;
+
+// Scalar logistic ODE:
+//   du/dt = alpha*u*(1-u)
+// Explicit Euler with variable dt(i):
+//   u_{i+1} = u_i + dt_i*alpha*u_i*(1-u_i)
+// Objective:
+//   J = 0.5*(u_m - target)^2
+// Discrete adjoint (scalar):
+//   lambda_m = (u_m - target)
+//   lambda_i = (dF/du at u_i)^T * lambda_{i+1}
+//   dF/du = 1 + dt_i*alpha*(1 - 2*u_i)
+
+int main(int argc, char *argv[])
+{
+   int s = 3;
+   double alpha = 2.0;
+   double dt0 = 0.02;
+   double omega = 0.2;
+   double Tfinal = 1.0;
+   double u0 = 0.2;
+   double target = 0.7;
+   double eps = 1e-7;
+
+   OptionsParser args(argc, argv);
+   args.AddOption(&s,      "-s",   "--checkpoints",
+                  "Checkpoint budget s (real checkpoints).");
+   args.AddOption(&alpha,  "-a",   "--alpha",       "Logistic growth alpha.");
+   args.AddOption(&dt0,    "-dt0", "--dt0",
+                  "Base dt for dt(i)=dt0*(1+0.5*sin(omega*i)).");
+   args.AddOption(&omega,  "-om",  "--omega",
+                  "Omega in dt(i)=dt0*(1+0.5*sin(omega*i)).");
+   args.AddOption(&Tfinal, "-T",   "--tfinal",
+                  "Terminate when accumulated time reaches Tfinal.");
+   args.AddOption(&u0,     "-u0",  "--u0",          "Initial scalar state u0.");
+   args.AddOption(&target, "-ut",  "--target",
+                  "Target value in J=0.5*(u_m-target)^2.");
+   args.AddOption(&eps,    "-eps", "--fd-eps",      "Finite-difference epsilon.");
+   args.Parse();
+   if (!args.Good())
+   {
+      args.PrintUsage(mfem::out);
+      return 1;
+   }
+   args.PrintOptions(mfem::out);
+
+   MFEM_VERIFY(s > 0, "Need s > 0.");
+   MFEM_VERIFY(dt0 > 0.0, "Need dt0 > 0.");
+   MFEM_VERIFY(Tfinal > 0.0, "Need Tfinal > 0.");
+
+   using Step = mfem::DynamicCheckpointing<double>::Step;
+
+   auto dt_func = [&](Step i)
+   {
+      const double dt = dt0 * (1.0 + 0.5 * std::sin(omega * double(i)));
+      MFEM_VERIFY(dt > 0.0, "dt_func produced non-positive dt.");
+      return dt;
+   };
+
+   // Checkpoint manager:
+   //   State   = double
+   //   Snapshot= double
+   mfem::DynamicCheckpointing<double> ckpt(s);
+
+   auto make_snapshot = [](const double &u) -> double { return u; };
+   auto restore_snapshot = [](const double &snap, double &state) { state = snap; };
+
+   auto primal_step = [&](double &u, Step i)
+   {
+      const double dt = dt_func(i);
+      u = u + dt * alpha * u * (1.0 - u);
+   };
+
+   auto adjoint_step = [&](double &lambda, const double &u_i, Step i)
+   {
+      const double dt = dt_func(i);
+      const double dF_du = 1.0 + dt * alpha * (1.0 - 2.0 * u_i);
+      lambda = dF_du * lambda;
+   };
+
+   // ---------------- Forward sweep (unknown m) ----------------
+   double u = u0;
+   double t_phys = 0.0;
+
+   Step i = 0;
+   while (t_phys < Tfinal)
+   {
+      ckpt.ForwardStep(i, u, primal_step, make_snapshot);
+      t_phys += dt_func(i);
+      ++i;
+   }
+
+   const Step m = i;
+   const double u_m = u;
+   const double J = 0.5 * (u_m - target) * (u_m - target);
+
+   mfem::out << std::setprecision(15);
+   mfem::out << "\n[Scalar] Forward finished:\n";
+   mfem::out << "  m (steps) = " << m << "\n";
+   mfem::out << "  t_phys    = " << t_phys << "\n";
+   mfem::out << "  u_m       = " << u_m << "\n";
+   mfem::out << "  J         = " << J << "\n\n";
+
+   mfem::out <<
+             "[Scalar] Checkpoint set after forward sweep (step, level, stored):\n";
+   for (const auto &cp : ckpt.GetCheckpointInfo())
+   {
+      mfem::out << "  step=" << cp.step
+                << ", level=" << cp.level
+                << ", stored=" << (cp.stored ? "yes" : "no")
+                << (cp.stored ? "" : " (placeholder)")
+                << "\n";
+   }
+   mfem::out << "\n";
+
+   MFEM_VERIFY(m > 0, "Forward produced m=0 steps; nothing to do.");
+
+   // ---------------- Backward sweep (adjoint) ----------------
+   double lambda = (u_m - target); // terminal adjoint = dJ/du_m
+   double u_work = 0.0;            // scratch primal state u_i
+
+   for (Step j = m - 1; j >= 0; --j)
+   {
+      ckpt.BackwardStep(j, lambda, u_work,
+                        primal_step, adjoint_step,
+                        make_snapshot, restore_snapshot);
+      if (j == 0) { break; } // avoid signed underflow
+   }
+
+   const double dJ_du0_adjoint = lambda;
+
+   // ---------------- Finite-difference gradient check ----------------
+   auto forward_only_J = [&](double u_init)
+   {
+      double uu = u_init;
+      double tt = 0.0;
+      Step k = 0;
+      while (tt < Tfinal)
+      {
+         primal_step(uu, k);
+         tt += dt_func(k);
+         ++k;
+      }
+      const double r = (uu - target);
+      return 0.5 * r * r;
+   };
+
+   const double Jp = forward_only_J(u0 + eps);
+   const double Jm = forward_only_J(u0 - eps);
+   const double dJ_du0_fd = (Jp - Jm) / (2.0 * eps);
+
+   const double abs_err = std::abs(dJ_du0_adjoint - dJ_du0_fd);
+   const double rel_err = abs_err / (std::abs(dJ_du0_fd) + 1e-30);
+
+   mfem::out << "[Scalar] Gradient check (dJ/du0):\n";
+   mfem::out << "  adjoint = " << dJ_du0_adjoint << "\n";
+   mfem::out << "  FD      = " << dJ_du0_fd << "\n";
+   mfem::out << "  abs err = " << abs_err << "\n";
+   mfem::out << "  rel err = " << rel_err << "\n\n";
+
+   return 0;
+}
+

miniapps/mtop/chpt/ex_vector_field.cpp

@@ -0,0 +1,199 @@
+#include "mfem.hpp"
+
+#include <cmath>
+#include <iomanip>
+
+using namespace mfem;
+
+// Vector logistic ODE (componentwise):
+//   du/dt = alpha*u*(1-u)   (applied to each component)
+// Explicit Euler:
+//   u_{i+1}[j] = u_i[j] + dt_i*alpha*u_i[j]*(1-u_i[j])
+// Objective:
+//   J = 0.5*||u_m - u_target||^2
+// Terminal adjoint:
+//   lambda_m = u_m - u_target
+// Adjoint step (componentwise):
+//   lambda_i[j] = (1 + dt_i*alpha*(1 - 2*u_i[j])) * lambda_{i+1}[j]
+
+int main(int argc, char *argv[])
+{
+   int s = 3;
+   int n = 16;
+   double alpha = 2.0;
+   double dt0 = 0.02;
+   double omega = 0.2;
+   double Tfinal = 1.0;
+   double target_val = 0.7;
+   double eps = 1e-7;
+
+   OptionsParser args(argc, argv);
+   args.AddOption(&s,         "-s",   "--checkpoints",
+                  "Checkpoint budget s (real checkpoints).");
+   args.AddOption(&n,         "-n",   "--size",        "Vector dimension n.");
+   args.AddOption(&alpha,     "-a",   "--alpha",       "Logistic growth alpha.");
+   args.AddOption(&dt0,       "-dt0", "--dt0",
+                  "Base dt for dt(i)=dt0*(1+0.5*sin(omega*i)).");
+   args.AddOption(&omega,     "-om",  "--omega",
+                  "Omega in dt(i)=dt0*(1+0.5*sin(omega*i)).");
+   args.AddOption(&Tfinal,    "-T",   "--tfinal",
+                  "Terminate when accumulated time reaches Tfinal.");
+   args.AddOption(&target_val,"-tv",  "--target",
+                  "Target value for each component.");
+   args.AddOption(&eps,       "-eps", "--fd-eps",
+                  "Finite-difference epsilon (directional).");
+   args.Parse();
+   if (!args.Good())
+   {
+      args.PrintUsage(mfem::out);
+      return 1;
+   }
+   args.PrintOptions(mfem::out);
+
+   MFEM_VERIFY(s > 0, "Need s > 0.");
+   MFEM_VERIFY(n > 0, "Need n > 0.");
+   MFEM_VERIFY(dt0 > 0.0, "Need dt0 > 0.");
+   MFEM_VERIFY(Tfinal > 0.0, "Need Tfinal > 0.");
+
+   using Step = mfem::DynamicCheckpointing<mfem::Vector>::Step;
+
+   auto dt_func = [&](Step i)
+   {
+      const double dt = dt0 * (1.0 + 0.5 * std::sin(omega * double(i)));
+      MFEM_VERIFY(dt > 0.0, "dt_func produced non-positive dt.");
+      return dt;
+   };
+
+   // Checkpoint manager:
+   //   State   = mfem::Vector
+   //   Snapshot= mfem::Vector
+   mfem::DynamicCheckpointing<mfem::Vector> ckpt(s);
+
+   auto make_snapshot = [](const mfem::Vector &u) -> mfem::Vector { return u; };
+   auto restore_snapshot = [](const mfem::Vector &snap, mfem::Vector &state) { state = snap; };
+
+   auto primal_step = [&](mfem::Vector &u, Step i)
+   {
+      const double dt = dt_func(i);
+      for (int j = 0; j < u.Size(); ++j)
+      {
+         const double uj = u[j];
+         u[j] = uj + dt * alpha * uj * (1.0 - uj);
+      }
+   };
+
+   auto adjoint_step = [&](mfem::Vector &lambda, const mfem::Vector &u_i, Step i)
+   {
+      const double dt = dt_func(i);
+      MFEM_ASSERT(lambda.Size() == u_i.Size(), "lambda and u_i size mismatch");
+      for (int j = 0; j < lambda.Size(); ++j)
+      {
+         const double dF_du = 1.0 + dt * alpha * (1.0 - 2.0 * u_i[j]);
+         lambda[j] *= dF_du;
+      }
+   };
+
+   // Initial condition and target
+   mfem::Vector u0(n), u_target(n);
+   for (int j = 0; j < n; ++j)
+   {
+      u0[j] = 0.2 + 0.05 * std::cos(0.7 * (j + 1));
+   }
+   u_target = target_val;
+
+   // ---------------- Forward sweep (unknown m) ----------------
+   mfem::Vector u = u0;
+   double t_phys = 0.0;
+
+   Step i = 0;
+   while (t_phys < Tfinal)
+   {
+      ckpt.ForwardStep(i, u, primal_step, make_snapshot);
+      t_phys += dt_func(i);
+      ++i;
+   }
+
+   const Step m = i;
+   const mfem::Vector u_m = u;
+
+   mfem::Vector diff(u_m);
+   diff -= u_target;
+   const double J = 0.5 * mfem::InnerProduct(diff, diff);
+
+   mfem::out << std::setprecision(15);
+   mfem::out << "\n[Vector] Forward finished:\n";
+   mfem::out << "  m (steps) = " << m << "\n";
+   mfem::out << "  t_phys    = " << t_phys << "\n";
+   mfem::out << "  J         = " << J << "\n";
+   mfem::out << "  ||u_m||   = " << u_m.Norml2() << "\n";
+   mfem::out << "  ||u_m-ut||= " << diff.Norml2() << "\n\n";
+
+   mfem::out <<
+             "[Vector] Checkpoint set after forward sweep (step, level, stored):\n";
+   for (const auto &cp : ckpt.GetCheckpointInfo())
+   {
+      mfem::out << "  step=" << cp.step
+                << ", level=" << cp.level
+                << ", stored=" << (cp.stored ? "yes" : "no")
+                << (cp.stored ? "" : " (placeholder)")
+                << "\n";
+   }
+   mfem::out << "\n";
+
+   MFEM_VERIFY(m > 0, "Forward produced m=0 steps; nothing to do.");
+
+   // ---------------- Backward sweep (adjoint) ----------------
+   mfem::Vector lambda = diff; // terminal = dJ/du_m
+   mfem::Vector u_work(n);     // scratch primal u_i
+
+   for (Step j = m - 1; j >= 0; --j)
+   {
+      ckpt.BackwardStep(j, lambda, u_work,
+                        primal_step, adjoint_step,
+                        make_snapshot, restore_snapshot);
+      if (j == 0) { break; }
+   }
+   const mfem::Vector &grad_u0 = lambda;
+
+   // ---------------- Directional FD check ----------------
+   mfem::Vector v(n);
+   for (int j = 0; j < n; ++j) { v[j] = std::sin(0.3 * (j + 1)) + 0.1; }
+
+   auto forward_only_J = [&](const mfem::Vector &u_init)
+   {
+      mfem::Vector uu = u_init;
+      double tt = 0.0;
+      Step k = 0;
+      while (tt < Tfinal)
+      {
+         primal_step(uu, k);
+         tt += dt_func(k);
+         ++k;
+      }
+      mfem::Vector dd(uu);
+      dd -= u_target;
+      return 0.5 * mfem::InnerProduct(dd, dd);
+   };
+
+   mfem::Vector u_plus(u0), u_minus(u0);
+   u_plus.Add(eps, v);
+   u_minus.Add(-eps, v);
+
+   const double Jp = forward_only_J(u_plus);
+   const double Jm = forward_only_J(u_minus);
+   const double dJ_dir_fd = (Jp - Jm) / (2.0 * eps);
+
+   const double dJ_dir_adj = mfem::InnerProduct(grad_u0, v);
+
+   const double abs_err = std::abs(dJ_dir_adj - dJ_dir_fd);
+   const double rel_err = abs_err / (std::abs(dJ_dir_fd) + 1e-30);
+
+   mfem::out << "[Vector] Directional derivative check:\n";
+   mfem::out << "  v·grad adjoint = " << dJ_dir_adj << "\n";
+   mfem::out << "  FD directional = " << dJ_dir_fd << "\n";
+   mfem::out << "  abs err        = " << abs_err << "\n";
+   mfem::out << "  rel err        = " << rel_err << "\n\n";
+
+   return 0;
+}
+

miniapps/mtop/chpt/makefile

@@ -0,0 +1,92 @@
+# Copyright (c) 2010-2025, Lawrence Livermore National Security, LLC. Produced
+# at the Lawrence Livermore National Laboratory. All Rights reserved. See files
+# LICENSE and NOTICE for details. LLNL-CODE-806117.
+#
+# This file is part of the MFEM library. For more information and source code
+# availability visit https://mfem.org.
+#
+# MFEM is free software; you can redistribute it and/or modify it under the
+# terms of the BSD-3 license. We welcome feedback and contributions, see file
+# CONTRIBUTING.md for details.
+
+# Use the MFEM build directory
+MFEM_DIR ?= ../../..
+MFEM_BUILD_DIR ?= ../../..
+MFEM_INSTALL_DIR ?= ../../../mfem
+SRC = $(if $(MFEM_DIR:../../..=),$(MFEM_DIR)/miniapps/mtop/chpt/,)
+CONFIG_MK = $(or $(wildcard $(MFEM_BUILD_DIR)/config/config.mk),\
+   $(wildcard $(MFEM_INSTALL_DIR)/share/mfem/config.mk))
+
+MFEM_LIB_FILE = mfem_is_not_built
+-include $(CONFIG_MK)
+
+SEQ_MINIAPPS = mtop_ex_scalar_chpt mtop_ex_adapt_chpt mtop_ex_vector_chpt \
+   mtop_ex_fixedslot mtop_ex_file mtop_ex_revolve mtop_ex_dyn \
+   mtop_ex_rev_fixedslot
+PAR_MINIAPPS =
+MINIAPPS_CLEAN := $(SEQ_MINIAPPS) $(PAR_MINIAPPS)
+
+ifeq ($(MFEM_USE_MPI),NO)
+   MINIAPPS = $(SEQ_MINIAPPS)
+else
+   MINIAPPS = $(SEQ_MINIAPPS) $(PAR_MINIAPPS)
+endif
+
+.SUFFIXES:
+.SUFFIXES: .o .cpp .mk
+.PHONY: all clean clean-build clean-exec
+.PRECIOUS: %.o
+
+# Remove built-in rules
+%: %.cpp
+%.o: %.cpp
+
+all: $(MINIAPPS)
+
+# Rules for building the miniapps
+mtop_ex_scalar_chpt: $(SRC)ex_scalar_field.cpp $(MFEM_LIB_FILE) $(CONFIG_MK)
+	$(MFEM_CXX) $(MFEM_FLAGS) $< -o $@ $(MFEM_LIBS)
+
+mtop_ex_adapt_chpt: $(SRC)ex_scalar_adaptive.cpp $(MFEM_LIB_FILE) $(CONFIG_MK)
+	$(MFEM_CXX) $(MFEM_FLAGS) $< -o $@ $(MFEM_LIBS)
+
+mtop_ex_vector_chpt: $(SRC)ex_vector_field.cpp $(MFEM_LIB_FILE) $(CONFIG_MK)
+	$(MFEM_CXX) $(MFEM_FLAGS) $< -o $@ $(MFEM_LIBS)
+
+mtop_ex_fixedslot: $(SRC)ex_fixedslot_scalar_vector.cpp $(MFEM_LIB_FILE) $(CONFIG_MK)
+	$(MFEM_CXX) $(MFEM_FLAGS) $< -o $@ $(MFEM_LIBS)
+
+mtop_ex_file: $(SRC)ex_fileio_scalar_vector_mfem.cpp $(MFEM_LIB_FILE) $(CONFIG_MK)
+	$(MFEM_CXX) $(MFEM_FLAGS) $< -o $@ $(MFEM_LIBS)
+
+mtop_ex_revolve: $(SRC)ex_revolve.cpp $(MFEM_LIB_FILE) $(CONFIG_MK)
+	$(MFEM_CXX) $(MFEM_FLAGS) $< -o $@ $(MFEM_LIBS)
+
+mtop_ex_dyn: $(SRC)ex_dynamic_chpt.cpp $(MFEM_LIB_FILE) $(CONFIG_MK)
+	$(MFEM_CXX) $(MFEM_FLAGS) $< -o $@ $(MFEM_LIBS)
+
+mtop_ex_rev_fixedslot: $(SRC)ex_revolve_chpt_fixedslot.cpp $(MFEM_LIB_FILE) $(CONFIG_MK)
+	$(MFEM_CXX) $(MFEM_FLAGS) $< -o $@ $(MFEM_LIBS)
+
+MFEM_TESTS = MINIAPPS
+include $(MFEM_TEST_MK)
+
+TEST_NAME := MTop checkpointing miniapp
+%-test-seq: %
+	@$(call mfem-test,$<,, $(TEST_NAME))
+
+# Testing: "test" target and mfem-test* variables are defined in config/test.mk
+
+# Generate an error message if the MFEM library is not built and exit
+$(MFEM_LIB_FILE):
+	$(error The MFEM library is not built)
+
+clean: clean-build clean-exec
+
+clean-build:
+	rm -f *.o *~ $(MINIAPPS_CLEAN)
+	rm -rf *.dSYM *.TVD.*breakpoints
+
+clean-exec:
+	rm -rf chk_scalar_files chk_vector_files dyn_ckpts
+	rm -f dynamic_ckpts.bin revolve_ckpts.bin

miniapps/multiapp/CMakeLists.txt

@@ -1,52 +0,0 @@
-# Copyright (c) 2010-2025, Lawrence Livermore National Security, LLC. Produced
-# at the Lawrence Livermore National Laboratory. All Rights reserved. See files
-# LICENSE and NOTICE for details. LLNL-CODE-806117.
-#
-# This file is part of the MFEM library. For more information and source code
-# availability visit https://mfem.org.
-#
-# MFEM is free software; you can redistribute it and/or modify it under the
-# terms of the BSD-3 license. We welcome feedback and contributions, see file
-# CONTRIBUTING.md for details.
-
-set(MESH_FILES
-  backward-facing-step.msh
-  channel-cylinder.msh
-)
-
-# Add a target to copy the mesh files from the source directory; used by sample
-# runs.
-set(SRC_MESH_FILES)
-foreach(MESH_FILE ${MESH_FILES})
-  list(APPEND SRC_MESH_FILES ${CMAKE_CURRENT_SOURCE_DIR}/${MESH_FILE})
-endforeach()
-add_custom_command(OUTPUT data_is_copied
-  COMMAND ${CMAKE_COMMAND} -E copy_if_different ${SRC_MESH_FILES} .
-  COMMAND ${CMAKE_COMMAND} -E touch data_is_copied
-  COMMENT "Copying multiapp miniapps data files ...")
-add_custom_target(copy_miniapps_multiapp_data DEPENDS data_is_copied)
-
-
-list(APPEND MULTIAPP_COMMON_SOURCES
-	multiapp.cpp)
-
-list(APPEND MULTIAPP_COMMON_HEADERS
-	multiapp.hpp)
-
-set(MULTIAPP_COMMON_FILES
-	EXTRA_SOURCES ${MULTIAPP_COMMON_SOURCES}
-	EXTRA_HEADERS ${MULTIAPP_COMMON_HEADERS})
-
-
-# Parallel apps.
-if (MFEM_USE_MPI)
-  add_mfem_miniapp(coupled-diffusion
-    MAIN coupled-diffusion.cpp
-    ${MFEM_MINIAPPS_COMMON_HEADERS} ${MULTIAPP_COMMON_FILES}
-    LIBRARIES mfem-common)
-  add_dependencies(coupled-diffusion copy_miniapps_multiapp_data)
-
-# Add parallel tests.
-  # if (MFEM_ENABLE_TESTING)
-  # endif()
-endif()

miniapps/multiapp/coupled-diffusion.cpp

@@ -1,895 +0,0 @@
-#include "mfem.hpp"
-#include "multiapp.hpp"
-#include <fstream>
-using namespace mfem;
-using namespace std;
-
-struct CaseContext
-{
-   int ser_ref = 1;         // Serial mesh refinement
-   int order = 3;           // Finite element order
-   bool visualization = true;// Visualization on/off
-   int grad_mode = 1;       // Gradient mode for the coupled operator - 0: finite difference,
-                            //                                          1: back/forward propagation
-   bool coupled = true;     // Coupled (true) vs. uncoupled (false) solves
-   int nl_iter = 50;        // Maximum number of nonlinear iterations
-   int lin_iter = 2000;     // Maximum number of linear iterations
-
-#if defined(MFEM_USE_DOUBLE)
-   real_t tol_nsolve = 1e-4;
-   real_t tol_lsolve = 1e-6;
-#elif defined(MFEM_USE_SINGLE)
-   real_t tol_nsolve = 1e-3;
-   real_t tol_lsolve = 1e-3;
-#else
-#error "Only single and double precision are supported!"
-   real_t tol_nsolve = 0;
-   real_t tol_lsolve = 0;
-#endif
-} ctx;
-
-void SetSolverParameters(IterativeSolver *solver, real_t rtol, real_t atol , int max_it,
-                         int print_level, bool iterative_mode);
-
-
-/// A functional diffusion coefficient (i.e., k(T))
-class FunctionalCoefficient : public Coefficient
-{
-public:
-   enum Mode { FUNC = 0, GRAD = 1};
-
-protected:
-   ParGridFunction *T_gf = nullptr;
-   real_t kref = 1.0;
-   real_t a0 = 0.0, a1 = 0.0, a2 = 0.0;
-   int findex = 0;
-   Mode mode = Mode::FUNC; // otherwise, grad
-
-public:
-   FunctionalCoefficient(ParGridFunction *T_gf, real_t kref): 
-                         T_gf(T_gf), kref(kref) { }
-
-   FunctionalCoefficient(ParGridFunction *T_gf, real_t kref, real_t a0):
-                         T_gf(T_gf), kref(kref), a0(a0) { findex = 1; }
-
-   FunctionalCoefficient(ParGridFunction *T_gf, real_t kref,
-                         real_t a0, real_t a1, real_t a2): T_gf(T_gf),
-                         kref(kref), a0(a0), a1(a1), a2(a2) { findex = 2; }
-
-    real_t Exponential(real_t x, bool eval_f) const
-    { 
-      real_t f = kref*exp(a0*x);
-      return (eval_f ? f : a0*f);
-    }
-    real_t Polynomial(real_t x, bool eval_f) const
-    { 
-      return (eval_f ? kref*(a0 + a1*x + a2*x*x) : kref*(a1 + 2*a2*x));
-    }
-
-    void SetMode(Mode mode) { this->mode = mode; }
-    Mode GetMode() const { return mode; }
-    
-    void UpdateGridFunction(ParGridFunction *gf) { T_gf = gf; }
-
-    real_t Eval(real_t x, bool eval_f) const
-    {
-        switch (findex)
-        {
-        case 1:
-         return Exponential(x, eval_f);
-        case 2:
-         return Polynomial(x, eval_f);
-        default:
-         return kref;
-        }
-    }
-
-    real_t Eval(ElementTransformation &Tr,
-              const IntegrationPoint &ip) override
-    {
-        real_t T = T_gf ? T_gf->GetValue(Tr, ip) : 0.0;
-        bool eval_f = (mode == Mode::FUNC);
-        return Eval(T, eval_f);
-    }
-};
-
-
-/// A coefficient defined by the product of grid functions, e.g. k(T) = prod_i x_i
-class GridFunctionProductCoefficient : public Coefficient
-{
-protected:
-   std::vector<ParGridFunction*> &x;
-
-public:
-   GridFunctionProductCoefficient(std::vector<ParGridFunction*> &x) : x(x) { }
-
-   real_t Eval(ElementTransformation &Tr, const IntegrationPoint &ip) override
-   {
-      real_t prod = 1.0;
-      for(size_t i = 0; i < x.size(); i++)
-      {
-         real_t val = x[i]->GetValue(Tr, ip);
-         prod *= val;
-      }
-      return prod;
-   }
-};
-
-class CoefficientIntegrator : public NonlinearFormIntegrator
-{
-protected:
-   FunctionalCoefficient *func = nullptr;
-   Vector shape;
-
-public:
-   CoefficientIntegrator(FunctionalCoefficient *func) : func(func) { }
-
-
-   void SetCoefficient(FunctionalCoefficient *f) { func = f; }
-
-   void AssembleElementVector(const FiniteElement &el,
-                              ElementTransformation &Tr,
-                              const Vector &elfun, Vector &elvect)
-   {
-      int dof = el.GetDof();
-      shape.SetSize(dof);
-      elvect.SetSize(dof);
-      elvect = 0.0;
-
-      const IntegrationRule *ir = &el.GetNodes();
-
-      for (int i = 0; i < ir->GetNPoints(); i++)
-      {
-         const IntegrationPoint &ip = ir->IntPoint(i);
-         el.CalcShape(ip, shape);
-         Tr.SetIntPoint(&ip);
-         real_t x = elfun * shape; // Evaluate the function at the integration point
-         real_t fval = func->Eval(x, true);
-         for (int j = 0; j < dof; j++)
-         {
-            elvect(j) += fval * shape(j);
-         }
-      }
-   }
-
-   void AssembleElementGrad(const FiniteElement &el, ElementTransformation &Tr,
-                            const Vector &elfun, DenseMatrix &elmat)
-   {
-      int dof = el.GetDof();
-      shape.SetSize(dof);
-      elmat.SetSize(dof);
-      elmat = 0.0;
-
-      const IntegrationRule *ir = &el.GetNodes();
-
-      for (int i = 0; i < ir->GetNPoints(); i++)
-      {
-         const IntegrationPoint &ip = ir->IntPoint(i);
-         el.CalcShape(ip, shape);
-         Tr.SetIntPoint(&ip);
-         real_t x = elfun * shape; // Evaluate the function at the integration point
-         real_t dfdx = func->Eval(x, false); // Evaluate the derivative of the function at the integration point
-         for (int j = 0; j < dof; j++)
-         {
-            elmat(j,j) += dfdx * shape(j); // Diagonal contribution to the Jacobian
-         }
-      }
-   }
-};
-
-
-class NonlinearDiffusionIntegrator : public NonlinearFormIntegrator
-{
-protected:
-   Coefficient *k;
-   Coefficient *dk;
-
-   Vector u, vec, shape;
-   DenseMatrix dshape, dshapedxt, adjJ;
-public:
-   NonlinearDiffusionIntegrator(Coefficient *kappa, Coefficient *dkappa) :
-                                k(kappa), dk(dkappa) { }
-
-   virtual void AssembleElementVector(const FiniteElement &el,
-                                      ElementTransformation &Tr,
-                                      const Vector &elfun, Vector &elvec)
-   {
-      int dim = el.GetDim();
-      int dof = el.GetDof();
-      real_t w;
-
-      elvec.SetSize(dof);
-      elvec = 0.0;
-
-      const IntegrationRule *ir = IntRule ? IntRule : &IntRules.Get(el.GetGeomType(), 2*el.GetOrder());
-      u.SetSize(dim);
-      vec.SetSize(dim);
-      dshape.SetSize(dof, dim);
-      adjJ.SetSize(dim, dim);
-
-      for (int i = 0; i < ir->GetNPoints(); i++)
-      {
-         const IntegrationPoint &ip = ir->IntPoint(i);
-         el.CalcDShape(ip, dshape);
-
-         Tr.SetIntPoint(&ip);
-         CalcAdjugate(Tr.Jacobian(), adjJ);
-         w = ip.weight / Tr.Weight();
-
-         dshape.MultTranspose(elfun, u);
-         adjJ.MultTranspose(u, vec);
-         if(k)
-         {
-            w *= k->Eval(Tr, ip);
-         }
-
-         vec *= w;
-         adjJ.Mult(vec, u);
-         dshape.AddMult(u, elvec);
-      }
-   }
-
-   void AssembleElementGrad(const FiniteElement &el, ElementTransformation &Tr,
-                            const Vector &elfun, DenseMatrix &elmat)
-   {
-      int dim = el.GetDim();
-      int dof = el.GetDof();
-      real_t w, k0 = 0.0, dk0 = 0.0;
-
-      elmat.SetSize(dof);
-      elmat = 0.0;
-
-      const IntegrationRule *ir = IntRule ? IntRule : &IntRules.Get(el.GetGeomType(), 2*el.GetOrder());
-      u.SetSize(dim);
-      shape.SetSize(dof);
-      vec.SetSize(dof);
-      dshape.SetSize(dof, dim);
-      dshapedxt.SetSize(dof, dim);
-
-      // f = grad(psi) * k(u) * grad(T)
-      // df/dT = grad(psi) ( k(u0) * grad(T) + k'(u0) * grad(u0) * T )
-      for (int i = 0; i < ir->GetNPoints(); i++)
-      {
-         const IntegrationPoint &ip = ir->IntPoint(i);
-         el.CalcShape(ip, shape);
-         el.CalcDShape(ip, dshape);
-
-         Tr.SetIntPoint(&ip);
-         w = ip.weight / Tr.Weight();
-
-         Mult(dshape, Tr.AdjugateJacobian(), dshapedxt);
-
-         k0  = k ? k->Eval(Tr, ip) : 0.0;
-         dk0 = dk ? dk->Eval(Tr, ip) : 0.0;
-
-         if(k0 != 0.0) // grad(psi) * k(u0) * grad(T)
-         {
-            real_t kdT = w*k0;
-            AddMult_a_AAt(kdT, dshapedxt, elmat);
-         }
-
-         if(dk0 != 0.0) // grad(psi) * (k'(T0) * grad(T0)) * T
-         {
-            dk0 = w*dk->Eval(Tr, ip);
-            dshapedxt.MultTranspose(elfun, u); // grad(T0) in physical space
-            u *= dk0; // k'(T0) * grad(T0)
-            dshapedxt.Mult(u, vec); // grad(psi) * k'(T0) * grad(T0)
-            AddMultVWt(vec, shape, elmat); // grad(psi) * k'(T0) * grad(T0) * T
-         }
-      }
-   }
-};
-
-/// An application that takes an input field T, and computes an output field k(T)
-// represented by the FunctionalCoefficient class.
-class DiffusionCoefficient : public GraphNode
-{
-public:
-   using Mode = FunctionalCoefficient::Mode;
-
-protected:
-   ParFiniteElementSpace &fes;
-   mutable ParGridFunction T, k;
-   mutable FunctionalCoefficient *kc;
-   // mutable Vector tdof, kdof, dk_dof, dT_dof;
-   mutable Mode mode = Mode::FUNC;
-
-   mutable ParNonlinearForm Nform;
-   mutable Operator *J = nullptr; // Jacobian for the nonlinear form
-
-   CoefficientIntegrator *coeff_integrator = nullptr;
-
-public:
-   DiffusionCoefficient(ParFiniteElementSpace &fes) :
-                        GraphNode(fes.GetTrueVSize()), fes(fes), T(&fes), k(&fes),
-                        kc(new FunctionalCoefficient(&T, 1.0, 5.0e-2)),
-                        Nform(&fes),
-                        coeff_integrator(new CoefficientIntegrator(kc))
-   {
-      k = 0.0;
-      T = 0.0;
-      k.ProjectCoefficient(*kc);
-
-      // Testing with the nonlinear form framework to compute k(T) and dk/dT
-      Nform.AddDomainIntegrator(coeff_integrator); // Transfer ownership
-      Nform.SetGradientType(Operator::Type::Hypre_ParCSR);
-      Nform.Setup();
-
-      SetInputOffsets(Array<int>({0, fes.GetTrueVSize()}));
-      SetOutputOffsets(Array<int>({0, fes.GetTrueVSize()}));
-   }
-
-   void SetMode(Mode mode) { this->mode = mode; }
-
-   FunctionalCoefficient* GetCoefficient() { return kc; }
-
-   void SetCoefficient(FunctionalCoefficient *fc)
-   {
-      if(kc) delete kc;
-      kc = fc;
-      kc->SetMode(mode);
-      kc->UpdateGridFunction(&T);
-      coeff_integrator->SetCoefficient(kc);
-   }
-
-   void Mult(const Vector &x, Vector &y) const override
-   {
-      BlockVector xb(x.GetData(), InputOffsets());
-      BlockVector yb(y.GetData(), OutputOffsets());
-
-      MultiVector xmv(1), ymv(1);
-      xmv.MakeRef(0, xb.GetBlock(0));
-      ymv.MakeRef(0, yb.GetBlock(0));
-
-      const_cast<DiffusionCoefficient*>(this)->Mult(xmv, ymv);
-   }
-
-   void Mult(const MultiVector &x, MultiVector &y) override
-   {
-      const Vector &tdof = x[0];
-      Vector &kdof = y[0];
-
-      Nform.Mult(tdof, kdof);
-      if(exec_mode == GraphNode::GRADIENT_MODE)
-      {
-         J = &Nform.GetGradient(tdof); // Store jacobian for JVP
-      }
-      else
-      {
-         J = nullptr; // Clear the Jacobian if not in gradient mode
-      }
-   }
-
-   // TODO: Possibly delete and only support MultiVector version of GradientMult
-   void GradientMult(const Vector &x, const Vector &dx, Vector &dy) const override
-   {
-      BlockVector xb(x.GetData(), InputOffsets());
-      BlockVector dxb(dx.GetData(), InputOffsets());
-      BlockVector dyb(dy.GetData(), OutputOffsets());
-
-      MultiVector xmv(1), dxmv(1), dymv(1);
-      xmv.MakeRef(0, xb.GetBlock(0));
-      dxmv.MakeRef(0, dxb.GetBlock(0));
-      dymv.MakeRef(0, dyb.GetBlock(0));
-      const_cast<DiffusionCoefficient*>(this)->GradientMult(xmv, dxmv, dymv);
-   }
-
-   void GradientMult(const MultiVector &x, const MultiVector &dx, MultiVector &dy) const override
-   {
-      const Vector &tdof = x[0];
-      const Vector &xadj = dx[0];
-      Vector &yadj = dy[0];
-
-      if(J)
-      {
-         J->Mult(xadj, yadj);
-      }
-      else
-      {
-         J = &Nform.GetGradient(tdof); // Store jacobian for JVP
-         J->Mult(xadj, yadj);
-      }
-   }
-
-   ~DiffusionCoefficient() override
-   {
-      if(kc) delete kc;
-   }
-};
-
-/// An application that takes n input fields x_i, and computes an output 
-/// field prod(x) := y = prod_i x_i.
-/// Also provides the derivative dy/dx_i = prod_{j!=i} x_j * dx_i/dx for i = 0,...,n-1.
-class ProductGridFunctions : public GraphNode
-{
-protected:
-
-   ParFiniteElementSpace &fes;
-   mutable std::vector<ParGridFunction*> x_gf;
-   mutable Vector dfdx;
-   mutable ParGridFunction y_gf;
-   mutable GridFunctionProductCoefficient prod_coeff;
-
-public:
-   ProductGridFunctions(ParFiniteElementSpace &fes, int n) :
-                     //   GraphNode(fes.GetTrueVSize()),
-                       GraphNode(fes.GetTrueVSize(), fes.GetTrueVSize() * n),
-                       fes(fes), x_gf(n),
-                       y_gf(&fes), prod_coeff(x_gf)
-   {
-      Array<int> offsets(n+1);
-      offsets[0] = 0;
-      for (int i = 0; i < n; i++)
-      {
-         x_gf[i]  = new ParGridFunction(&fes);
-         *x_gf[i] = 0.0;
-         offsets[i+1] = offsets[i] + fes.GetTrueVSize();
-      }
-      y_gf = 0.0;
-      y_gf.ProjectCoefficient(prod_coeff);
-
-      SetInputOffsets(offsets);
-      SetOutputOffsets(Array<int>({0, fes.GetTrueVSize()}));
-   }
-
-   void Mult(const Vector &x, Vector &y) const override
-   {
-      BlockVector xb(x.GetData(), InputOffsets());
-      BlockVector yb(y.GetData(), OutputOffsets());
-
-      MultiVector xmv(x_gf.size()), ymv(1);
-      for (size_t i = 0; i < x_gf.size(); i++)
-      {
-         xmv.MakeRef(i, xb.GetBlock(i));
-      }
-      ymv.MakeRef(0, yb.GetBlock(0));
-      const_cast<ProductGridFunctions*>(this)->Mult(xmv, ymv);
-   }
-
-   void Mult(const MultiVector &x, MultiVector &y) override
-   {
-      for (size_t i = 0; i < x_gf.size(); i++)
-      {
-         const Vector &x_dof = x[i];
-         x_gf[i]->SetFromTrueDofs(x_dof);
-      }
-
-      Field *out_field = OutputField(0);
-      Vector &y_dof = y[0];
-      y_gf.ProjectCoefficient(prod_coeff);
-      y_gf.GetTrueDofs(y_dof);
-   }
-
-   // TODO: Possibly delete and only support MultiVector version of GradientMult
-   void GradientMult(const Vector &x, const Vector &dx, Vector &dy) const override
-   {
-      BlockVector xb(x.GetData(), InputOffsets());
-      BlockVector dxb(dx.GetData(), InputOffsets());
-      BlockVector dyb(dy.GetData(), OutputOffsets());
-
-      MultiVector xmv(x_gf.size()), dxmv(x_gf.size()), dymv(1);
-      for (size_t i = 0; i < x_gf.size(); i++)
-      {
-         xmv.MakeRef(i, xb.GetBlock(i));
-         dxmv.MakeRef(i, dxb.GetBlock(i));
-      }
-      dymv.MakeRef(0, dyb.GetBlock(0));
-      const_cast<ProductGridFunctions*>(this)->GradientMult(xmv, dxmv, dymv);
-   }
-
-   void GradientMult(const MultiVector &x, const MultiVector &dx, MultiVector &dy) const override
-   {
-      // Jacobian vector product for y = prod_i x_i is:
-      // dy/dx = sum_i (prod_{j!=i} x_j * dx_i/dx)
-      for (size_t i = 0; i < x_gf.size(); i++)
-      {
-         const Vector &x_dof = x[i];
-         x_gf[i]->SetFromTrueDofs(x_dof); // Set all x_i
-      }
-
-      Vector &jvp = dy[0];
-      jvp = 0.0;
-      for (size_t i = 0; i < x_gf.size(); i++)
-      {
-         const Vector &x_dof = x[i];
-         const Vector &dx_dof = dx[i]; // Get dx_i/dx
-
-         x_gf[i]->SetFromTrueDofs(dx_dof); // Set x_i = dx_i/dx for i-th term in the product
-         y_gf.ProjectCoefficient(prod_coeff); // Recompute product with x_i replaced by dx_i/dx
-         y_gf.GetTrueDofs(dfdx); // Get prod_{j!=i} x_j * dx_i/dx for i-th term
-         jvp += dfdx; // Accumulate contribution from i-th term
-         x_gf[i]->SetFromTrueDofs(x_dof); // reset to original value for next iteration
-      }
-   }
-
-   ~ProductGridFunctions() override
-   {
-      for (size_t i = 0; i < x_gf.size(); i++)
-      {
-         if(x_gf[i]) delete x_gf[i];
-      }
-   }
-};
-
-
-/// An application that represents the nonlinear diffusion operator: f(T) = -Div(k(u) grad(T)) 
-/// with input field T and k, and output field f(T).
-class DiffusionOperator : public GraphNode
-{
-public:
-
-   // Mesh and finite element space
-   ParMesh &mesh;
-   ParFiniteElementSpace &fes;
-
-   /// Essential dof array.
-   Array<int> ess_tdofs;
-
-   /// Grid functions for the temperature and heat flux
-   mutable ParGridFunction T, k, dk;
-   mutable GridFunctionCoefficient k_gfc, dk_gfc;
-   mutable ParNonlinearForm Nform;
-   mutable ParLinearForm bform;
-   mutable Vector b;
-
-   ConstantCoefficient zero_coeff, one_coeff;
-
-   mutable FunctionalCoefficient *kc = nullptr;
-   mutable HypreParMatrix *dfdk_mat = nullptr, *dfdT_mat = nullptr;
-
-public:
-
-   DiffusionOperator(ParFiniteElementSpace &fes_) :
-                     // GraphNode(fes_.GetTrueVSize()),
-                     GraphNode(fes_.GetTrueVSize(),2*fes_.GetTrueVSize()),
-                     mesh(*fes_.GetParMesh()), fes(fes_),
-                     T(&fes), k(&fes), dk(&fes),
-                     k_gfc(&k), dk_gfc(&dk),
-                     Nform(&fes), bform(&fes),
-                     zero_coeff(0.0), one_coeff(1.0)
-   {
-      fes.GetBoundaryTrueDofs(ess_tdofs);
-      T = 0.0;
-      k = 0.0;
-      dk = 0.0;
-
-      bform.AddDomainIntegrator(new DomainLFIntegrator(one_coeff));
-      Nform.AddDomainIntegrator(new NonlinearDiffusionIntegrator(&k_gfc, &dk_gfc));
-      Nform.SetGradientType(Operator::Type::Hypre_ParCSR);
-
-      b.SetSize(fes.GetTrueVSize()); b = 0.0;
-      Assemble();
-
-      SetInputOffsets(Array<int>({0, fes.GetTrueVSize(), 2*fes.GetTrueVSize()}));
-      SetOutputOffsets(Array<int>({0, fes.GetTrueVSize()}));
-   }
-
-   void SetCoefficient(FunctionalCoefficient *fc) { kc = fc; }
-
-   void Assemble()
-   {
-      AssembleLinearForms();
-      AssembleBilinearForms();
-      AssembleNonlinearForms();
-   }
-
-   void AssembleBilinearForms()
-   {}
-
-   void AssembleNonlinearForms()
-   {
-      Nform.SetEssentialTrueDofs(ess_tdofs);
-      Nform.Setup();
-   }
-
-   void AssembleLinearForms()
-   {
-      bform.Assemble();
-      bform.ParallelAssemble(b);
-   }
-
-   void Mult(const Vector &x, Vector &y) const override
-   {
-      BlockVector xb(x.GetData(), InputOffsets());
-      BlockVector yb(y.GetData(), OutputOffsets());
-
-      MultiVector xmv(2), ymv(1);
-      xmv.MakeRef(0, xb.GetBlock(0));
-      xmv.MakeRef(1, xb.GetBlock(1));
-      ymv.MakeRef(0, yb.GetBlock(0));
-
-      const_cast<DiffusionOperator*>(this)->Mult(xmv, ymv);
-   }
-
-   void Mult(const MultiVector &x, MultiVector &y) override
-   {
-      const Vector &tdofs = x[0];
-      const Vector &kdofs = x[1];
-      Vector &fdofs = y[0];
-
-      k.SetFromTrueDofs(kdofs); // update for use in k_gfc
-
-      if(exec_mode == GraphNode::GRADIENT_MODE)
-      {
-         if(dfdT_mat) delete dfdT_mat;
-         if(dfdk_mat) delete dfdk_mat;
-
-         dk = 0.0;
-         k.SetFromTrueDofs(kdofs);
-         Operator* grad = &Nform.GetGradient(tdofs);
-         dfdT_mat = new HypreParMatrix(dynamic_cast<const HypreParMatrix&>(*grad)); // deep copy
-
-         dk = 1.0;
-         k  = 0.0;
-         grad = &Nform.GetGradient(tdofs);
-         dfdk_mat = new HypreParMatrix(dynamic_cast<const HypreParMatrix&>(*grad)); // deep copy
-      }
-      else
-      {
-         if(dfdT_mat) { delete dfdT_mat; dfdT_mat = nullptr; }
-         if(dfdk_mat) { delete dfdk_mat; dfdk_mat = nullptr; }
-      }
-
-      Nform.Mult(tdofs, fdofs);
-      fdofs.SetSubVector(ess_tdofs, 0.0);
-   }
-
-   // Exact block jacobian [df/dT, df/dk]
-   Operator& GetGradient(const Vector &x) const override
-   {
-      MFEM_ABORT("GetGradient not implemented for DiffusionOperator");
-   }
-
-   // TODO: Possibly delete and only support MultiVector version of GradientMult
-   void GradientMult(const Vector &x, const Vector &dx, Vector &dy) const override
-   {
-      BlockVector xb(x.GetData(), InputOffsets());
-      BlockVector dxb(dx.GetData(), InputOffsets());
-      BlockVector dyb(dy.GetData(), OutputOffsets());
-
-      Vector &Tadj = dxb.GetBlock(0);
-      Vector &kadj = dxb.GetBlock(1);
-      Vector &yadj = dyb.GetBlock(0);
-
-      Vector &tdofs = xb.GetBlock(0);
-      Vector &kdofs = xb.GetBlock(1);
-
-      dfdT_mat->Mult(Tadj, yadj);
-      dfdk_mat->AddMult(kadj, yadj);
-   }
-
-   void GradientMult(const MultiVector &x, const MultiVector &dx, MultiVector &dy) const override
-   {
-      const Vector &Tadj = dx[0];
-      const Vector &kadj = dx[1];
-      Vector &yadj = dy[0];
-
-      const Vector &tdofs = x[0];
-      const Vector &kdofs = x[1];
-
-      dfdT_mat->Mult(Tadj, yadj);
-      dfdk_mat->AddMult(kadj, yadj);
-   }
-
-   /// @brief Destroy the DiffusionOperator object
-   ~DiffusionOperator() override
-   {
-      if(dfdT_mat) delete dfdT_mat;
-      if(dfdk_mat) delete dfdk_mat;
-   }
-};
-
-
-int main(int argc, char *argv[])
-{
-   Mpi::Init();
-   Hypre::Init();
-
-   using GradMode = DAGraph::GradMode;
-
-   OptionsParser args(argc, argv);
-   args.AddOption(&ctx.order, "-o", "--order",
-                  "Finite element order (polynomial degree).");
-   args.AddOption(&ctx.visualization, "-vis", "--visualization", "-no-vis",
-                  "--no-visualization",
-                  "Enable or disable GLVis visualization.");
-   args.AddOption(&ctx.ser_ref, "-rs", "--serial-refine",
-                  "Number of times to refine the mesh in serial.");
-
-   args.AddOption(&ctx.grad_mode, "-gm", "--grad-mode",
-                  "Gradient mode for the coupled operator (0: exact, 1: finite difference, 2: algorithmic differentiation)");
-   args.AddOption(&ctx.coupled, "-cp", "--coupled", "-ucp", "--uncoupled",
-                  "Coupled (true) vs. uncoupled (false) solves.");
-   args.ParseCheck();
-
-
-   int order = ctx.order;
-   std::string mesh_file = "../../data/star.mesh";
-   Mesh *serial_mesh = new Mesh(mesh_file);
-   int dim = serial_mesh->Dimension();
-
-   for (int i = 0; i < ctx.ser_ref; ++i) { serial_mesh->UniformRefinement(); }
-   serial_mesh->SetCurvature(order, false, dim, Ordering::byNODES);
-
-   ParMesh pmesh(MPI_COMM_WORLD, *serial_mesh);
-   delete serial_mesh;
-   pmesh.UniformRefinement();
-
-   // Finite element spaces
-   H1_FECollection fec(order, dim);
-   ParFiniteElementSpace fes(&pmesh, &fec);
-
-   // Build all operator nodes
-   DiffusionCoefficient diff_coeff_1(fes);
-   diff_coeff_1.SetName("k(T1)");
-   diff_coeff_1.SetCoefficient(new FunctionalCoefficient(nullptr, 1.0, 3.5e-2));
-   // diff_coeff_1.SetCoefficient(new FunctionalCoefficient(nullptr, 1.0, 1.0, 0.1, 0.0));
-
-   DiffusionCoefficient diff_coeff_2(fes);
-   diff_coeff_2.SetName("k(T2)");
-   diff_coeff_2.SetCoefficient(new FunctionalCoefficient(nullptr, 1.0, 1.0, 2.0, 0.0));
-   // diff_coeff_2.SetCoefficient(new FunctionalCoefficient(nullptr, 1.5, 2.5e-2));
-
-   ProductGridFunctions prod_coeff(fes, 2);
-   prod_coeff.SetName("k(T1,T2)");
-
-   DiffusionOperator diff_op1(fes);
-   diff_op1.SetName("Div(k(T1,T2) grad(T1))");
-   diff_op1.SetCoefficient(diff_coeff_1.GetCoefficient());
-
-   DiffusionOperator diff_op2(fes);
-   diff_op2.SetName("Div(k(T1,T2) grad(T2))");
-   diff_op2.SetCoefficient(diff_coeff_2.GetCoefficient());
-
-
-   // Build the DAG in any order, and then sort it to ensure the correct execution order
-   DAGraph dag(5);
-   dag.AddOperator(&diff_coeff_1);
-   dag.AddOperator(&diff_op1, fes.GetTrueVSize());
-   dag.AddOperator(&diff_op2, fes.GetTrueVSize());
-   dag.AddOperator(&diff_coeff_2);
-   dag.AddOperator(&prod_coeff);
-
-   Vector k1vec(fes.GetTrueVSize()); k1vec = 0.0;
-   Vector k2vec(fes.GetTrueVSize()); k2vec = 0.0;
-   Vector kpvec(fes.GetTrueVSize()); kpvec = 0.0;
-
-   Vector k1adj(fes.GetTrueVSize()); k1adj = 0.0;
-   Vector k2adj(fes.GetTrueVSize()); k2adj = 0.0;
-   // Vector kpadj(fes.GetTrueVSize()); kpadj = 0.0;
-
-   // Input fields get data from 'x' in DAGraph::Mult(x, y)
-   Field T1_field(nullptr, nullptr);
-   Field T2_field(nullptr, nullptr);
-
-   // Write space for data and adjoint only needed
-   // for the intermediate fields k1, k2, and k_prod
-   Field k1_field(&k1vec, &k1adj);
-   Field k2_field(&k2vec, &k2adj);
-   Field kp_field(&kpvec, &kpvec); // can use same space for data & adjoint
-
-   // Output fields get data from 'y' in DAGraph::Mult(x, y)
-   Field f1_field(nullptr, nullptr);
-   Field f2_field(nullptr, nullptr);
-
-
-   // Add input and output to the DAG
-   int sz = fes.GetTrueVSize();
-   dag.AddInput(&T1_field, sz);
-   dag.AddInput(&T2_field, sz);
-   dag.AddOutput(&f1_field, sz);
-   dag.AddOutput(&f2_field, sz);
-
-   // Form connections between the nodes in the DAG
-   diff_coeff_1.AddInput(&T1_field);
-   diff_coeff_1.AddOutput(&k1_field);
-
-   diff_coeff_2.AddInput(&T2_field);
-   diff_coeff_2.AddOutput(&k2_field);
-
-   prod_coeff.AddInputs(&k1_field, &k2_field);
-   prod_coeff.AddOutput(&kp_field);
-
-   diff_op1.AddInput(&T1_field);
-   diff_op1.AddOutput(&f1_field);
-
-   diff_op2.AddInput(&T2_field);
-   diff_op2.AddOutput(&f2_field);
-
-   if(ctx.coupled)
-   {
-      diff_op1.AddInput(&kp_field); // kp_field
-      diff_op2.AddInput(prod_coeff.OutputField(0)); // Can also use kp_field directly
-   }
-   else
-   {
-      diff_op1.AddInput(&k1_field); // Can also use diff_coeff_1.OutputField(0)
-      diff_op2.AddInput(&k2_field); // Can also use diff_coeff_2.OutputField(0)
-   }
-
-   // Assemble DAG: topological sort, validate nodes, etc.
-   dag.Assemble();
-
-   std::string output_prefix = ctx.coupled ? "Coupled_Diffusion" : "Uncoupled_Diffusion";
-
-   if(Mpi::Root())
-   {
-      std::ofstream fout(output_prefix+"-dag.txt");
-      fout << "{\n";
-      dag.Save(fout);
-      fout << "}\n";
-      fout << std::flush;
-      fout.close();
-   }
-
-   // Set initial guess and boundary conditions for T1 and T2
-   Array<int> ess_tdofs;
-   fes.GetBoundaryTrueDofs(ess_tdofs);
-
-   int T1_idx = 0;
-   int T2_idx = 1;
-
-   BlockVector xb(dag.InputOffsets());
-   BlockVector yb(dag.OutputOffsets());
-
-   xb.GetBlock(T1_idx).Randomize();
-   xb.GetBlock(T2_idx).Randomize();
-   xb.GetBlock(T1_idx).SetSubVector(ess_tdofs, 0.0);
-   xb.GetBlock(T2_idx).SetSubVector(ess_tdofs, 0.0);
-
-   // Build the nonlinear solver and linear solver for the DAG
-   NewtonSolver newton_solver(pmesh.GetComm());
-   GMRESSolver linear_solver(pmesh.GetComm());
-   linear_solver.SetKDim(500);
-   SetSolverParameters(&newton_solver, ctx.tol_nsolve, 0.0, ctx.nl_iter, 1, true);
-   SetSolverParameters(&linear_solver, ctx.tol_lsolve, 0.0, ctx.lin_iter, 1, false);
-
-   newton_solver.SetPreconditioner(linear_solver);
-   linear_solver.SetPrintLevel(1);
-
-   // Set the gradient mode for the DAG and solve the coupled system
-   GradMode gm = static_cast<GradMode>(ctx.grad_mode);
-   dag.SetGradientMode(gm);
-   newton_solver.SetOperator(dag);
-   newton_solver.Mult(xb, yb);
-
-   ParaViewDataCollection *pv = nullptr;
-   if (ctx.visualization)
-   {
-      std::string pv_prefix;
-      switch (ctx.grad_mode)
-      {
-         case 0: pv_prefix = "FD"; break;
-         case 1: pv_prefix = "MF"; break;
-         default: pv_prefix = "Unknown"; break;
-      }
-
-      pv = new ParaViewDataCollection(output_prefix+"-"+pv_prefix, &pmesh);
-      pv->SetLevelsOfDetail(order);
-      pv->SetDataFormat(VTKFormat::BINARY);
-      pv->SetHighOrderOutput(true);
-
-      ParGridFunction T1_gf(&fes);
-      ParGridFunction T2_gf(&fes);
-      T1_gf.SetFromTrueDofs(yb.GetBlock(T1_idx));
-      T2_gf.SetFromTrueDofs(yb.GetBlock(T2_idx));
-
-      pv->RegisterField("T1", &T1_gf);
-      pv->RegisterField("T2", &T2_gf);
-      pv->Save();
-      delete pv;
-   }
-
-   std::cout << "Finished solving the coupled diffusion problem." << std::endl;
-   return 0;
-}
-
-void SetSolverParameters(IterativeSolver *solver, real_t rtol, real_t atol,
-                         int max_it, int print_level, bool iterative_mode)
-{
-    solver->SetRelTol(rtol);
-    solver->SetAbsTol(atol);
-    solver->SetMaxIter(max_it);
-    solver->SetPrintLevel(print_level);
-    solver->iterative_mode = iterative_mode;
-}

miniapps/multiapp/multiapp.cpp

@@ -1,873 +0,0 @@
-// Copyright (c) 2010-2025, Lawrence Livermore National Security, LLC. Produced
-// at the Lawrence Livermore National Laboratory. All Rights reserved. See files
-// LICENSE and NOTICE for details. LLNL-CODE-806117.
-//
-// This file is part of the MFEM library. For more information and source code
-// availability visit https://mfem.org.
-//
-// MFEM is free software; you can redistribute it and/or modify it under the
-// terms of the BSD-3 license. We welcome feedback and contributions, see file
-// CONTRIBUTING.md for details.
-
-#include "multiapp.hpp"
-
-namespace mfem
-{
-
-DAGraph::~DAGraph()
-{
-    for(int i=0; i < nnodes; i++)
-    {
-        if(node_owned[i] && nodes[i]) delete nodes[i];
-    }
-    if(grad) delete grad;
-}
-
-void DAGraph::Assemble()
-{
-    // Sort graph nodes topologically to ensure correct execution order
-    // Ordering is not unique, hence, id->index maps are needed
-    TopologicalSort();
-
-    // Collect all fields from the nodes into the field map
-    CollectFieldMaps();
-
-    // Compute depth of the graph nodes
-    ComputeDepth();
-
-    // Validate each node
-    for (auto &node : nodes)
-    {
-        ValidateNode(*node);
-    }
-
-    // Update width and height of the DAG from offsets
-    // Check that the input and output offsets are consistent
-    ValidateOffsets();
-    width  = input_offsets.Last();
-    height = output_offsets.Last();
-
-    // Delete any existing gradient operator as node ordering may have changed
-    if (grad) delete grad;
-
-    assembled = true;
-}
-
-void DAGraph::ValidateOffsets()
-{
-    // Check that the input and output offsets are consistent
-    // with the number of inputs and outputs
-    if(InputFields().Size() > 1)
-    {
-        MFEM_ASSERT(input_offsets.Size() == InputFields().Size() + 1,
-                    "Input offsets size inconsistent with number of input fields");
-    }
-    else
-    {
-        input_offsets = Array<int>({0, nodes[0]->Width()});
-    }
-
-    if(OutputFields().Size() > 1)
-    {
-        MFEM_ASSERT(output_offsets.Size() == OutputFields().Size() + 1,
-                    "Output offsets size inconsistent with number of output fields");
-    }
-    else
-    {
-        output_offsets = Array<int>({0, nodes.Last()->Height()});
-    }
-}
-
-void DAGraph::ValidateNode(GraphNode &node)
-{
-    // Validate that the node's input and output fields are consistent with the graph's field map
-    auto inputs = node.InputFields();
-    auto outputs = node.OutputFields();
-
-    // Check offsets match width and height of the node
-    MFEM_ASSERT(node.InputOffsets().Last() == node.Width(),
-                "Node ID: " << node.ID() << " input offsets do not match node width.");
-    MFEM_ASSERT(node.OutputOffsets().Last() == node.Height(),
-                "Node ID: " << node.ID() << " output offsets do not match node height.");
-
-    // Check number of input and output fields match the offsets
-    MFEM_ASSERT(node.InputOffsets().Size() == inputs.Size() + 1,
-                "Node input offsets size inconsistent with number of input fields");
-    MFEM_ASSERT(node.OutputOffsets().Size() == outputs.Size() + 1,
-                "Node output offsets size inconsistent with number of output fields");
-
-    // Check that all input and output fields are registered in the graph's field map
-    for(auto input_field : inputs)
-    {
-        MFEM_ASSERT(fid_to_index.Has(input_field->ID()),
-                    "Input field ID " << input_field->ID() << " not found in graph's field map");
-    }
-    for(auto output_field : outputs)
-    {
-        MFEM_ASSERT(fid_to_index.Has(output_field->ID()),
-                    "Output field ID " << output_field->ID() << " not found in graph's field map");
-    }
-}
-
-void DAGraph::TopologicalSort()
-{
-    Array<int> sorted_indices;
-    sorted_indices.Reserve(nnodes);
-
-    Array<bool> visited(nnodes);
-    visited = false; // Initialize all nodes as unvisited
-
-    // Perform a depth-first search to sort the nodes topologically
-    std::function<void(int)> DepthFirstSearch = [&](int node_index)
-    {
-        if(visited[node_index]) return;
-        visited[node_index] = true;
-        auto node = nodes[node_index];
-        // Visit all nodes that this node depends on
-        for(auto input_field : node->InputFields())
-        {
-            for(int j=0; j < nnodes; j++)
-            {
-                auto other_node = nodes[j];
-                if(other_node == node) continue;
-                for(auto output_field : other_node->OutputFields())
-                {
-                    if(input_field->ID() == output_field->ID()) // Compare by unique ID
-                    {
-                        DepthFirstSearch(j);
-                    }
-                }
-            }
-        }
-        sorted_indices.push_back(node_index);
-    };
-
-    for(int i=0; i < nnodes; i++)
-    {
-        DepthFirstSearch(i);
-    }
-
-    nodes.Permute(sorted_indices);
-    node_owned.Permute(sorted_indices);
-
-    // Update the node indices after sorting
-    for(int i=0; i < nnodes; i++)
-    {
-        nodes[i]->SetNodeIndex(i);
-    }
-
-    sorted = true;
-}
-
-void DAGraph::ComputeDepth()
-{
-    // Compute depth of ordered nodes
-    node_depth.SetSize(nnodes);
-    node_depth = 0;
-    for(int i=0; i < nnodes; i++)
-    {
-        int max_depth = 0;
-        auto node = nodes[i];
-        for(auto input_field : node->InputFields())
-        {
-            for(int j=0; j < i; j++)
-            {
-                auto other_node = nodes[j];
-                if(other_node == node) continue;
-                for(auto output_field : other_node->OutputFields())
-                {
-                    if(input_field->ID() == output_field->ID()) // Compare by unique ID
-                    {
-                        max_depth = std::max(max_depth, node_depth[j] + 1);
-                    }
-                }
-            }
-        }
-        node_depth[i] = max_depth;
-    }
-}
-
-void DAGraph::CollectFieldMaps()
-{
-    MFEM_ASSERT(sorted, "DAGraph must be topologically sorted before collecting fields");
-
-    fid_to_index.clear();
-    fid_to_field.clear();
-
-    int nfields = 0;
-    for (auto f : InputFields())
-    {
-        fid_to_index.Register(f->ID(), nfields++);
-        fid_to_field.Register(f->ID(), f);
-    }
-
-    for (auto &node : nodes)
-    {
-        for (auto f : node->OutputFields())
-        {
-            if (!fid_to_index.Has(f->ID()))
-            {
-                fid_to_index.Register(f->ID(), nfields++);
-            }
-            if (!fid_to_field.Has(f->ID()))
-            {
-                fid_to_field.Register(f->ID(), f);
-            }
-        }
-    }
-    // TODO: Possibly add all intermediate fields from nodes to the graph's FieldCollection
-}
-
-void DAGraph::Mult(const Vector &x, Vector &y) const
-{
-    MFEM_ASSERT(width == x.Size(), "Input vector size (" << x.Size()
-                << ") must match matrix width (" << width << ")");
-
-    MFEM_ASSERT(height == y.Size(), "Output vector size (" << y.Size()
-                << ") must match matrix height (" << height << ")");
-
-    auto inputs  = InputFields();
-    auto outputs = OutputFields();
-
-    BlockVector xb(x.GetData(), input_offsets);
-    BlockVector yb(y.GetData(), output_offsets);
-    MultiVector xmv(inputs.Size()), ymv(outputs.Size());
-
-    // Set the data pointers of the input and output fields
-    // of the graph to point to the corresponding blocks of
-    // the input and output vectors
-    for(int i=0; i < inputs.Size(); i++)
-    {
-        xmv.MakeRef(i, xb.GetBlock(i));
-    }
-
-    for(int i=0; i < outputs.Size(); i++)
-    {
-        ymv.MakeRef(i, yb.GetBlock(i));
-    }
-
-    const_cast<DAGraph*>(this)->Mult(xmv, ymv);
-}
-
-void DAGraph::Mult(const MultiVector &x, MultiVector &y)
-{
-    auto inputs  = InputFields();
-    auto outputs = OutputFields();
-
-    MFEM_ASSERT(inputs.Size() == x.NumBlocks(), "Number of input blocks (" << x.NumBlocks()
-                << ") must match number of input fields (" << inputs.Size() << ")");
-
-    MFEM_ASSERT(outputs.Size() == y.NumBlocks(), "Number of output blocks (" << y.NumBlocks()
-                << ") must match number of output fields (" << outputs.Size() << ")");
-
-    for(int i=0; i < inputs.Size(); i++)
-    {
-        inputs[i]->SetData(const_cast<Vector*>(&x[i]));
-    }
-    for (int i=0; i < outputs.Size(); i++)
-    {
-        outputs[i]->SetData(&y[i]);
-    }
-
-    auto index_map = GetFieldIdToIndexMap();
-    auto fld_map = GetFieldIdToFieldMap();
-    int nfields = index_map.NumFields();
-    MultiVector ymv(nfields); // TODO: Should this be a member function?
-
-    // Assemble the multivector from the individual fields based on their IDs
-    // This multivector contains all input, output, and intermediate fields in the graph
-    for (auto const& [id, idx] : index_map)
-    {
-        if (fld_map.Has(id))
-        {
-            auto field = fld_map.Get(id);
-            ymv.MakeRef(idx, *field->Data());
-        }
-        else
-        {
-            MFEM_ABORT("Field ID " << id << " not found in field map");
-        }
-    }
-    
-    Execute(x, ymv);
-
-    for(auto &f : inputs)
-    {
-        f->SetData(nullptr);
-    }
-    for(auto &f : outputs)
-    {
-        f->SetData(nullptr);
-    }
-}
-
-void DAGraph::Execute(const MultiVector &x, MultiVector &y) const
-{
-    MFEM_ASSERT(assembled, "DAGraph must be assembled before calling Execute()");
-
-    MFEM_ASSERT(x.NumBlocks() == InputFields().Size(),
-                "Number of input blocks (" << x.NumBlocks()
-                << ") must match number of input fields (" << InputFields().Size() << ")");
-    
-    auto index_map = GetFieldIdToIndexMap();
-    
-    MFEM_ASSERT(y.NumBlocks() == index_map.NumFields(),
-                "Number of output blocks (" << y.NumBlocks()
-                << ") must match number of fields (" << index_map.NumFields() << ")");
-
-    auto inputs  = InputFields();
-    for(int i=0; i < inputs.Size(); i++)
-    {
-        int idx = index_map.Get(inputs[i]->ID());
-        if(&y[idx] != &x[i]) // copy data, if address is different
-        {
-            y[idx] = x[i];
-        }
-    }
-
-    if(input_type == InputType::VECTOR)
-    {
-        x_node.SetSize(MaxWidth());
-        y_node.SetSize(MaxHeight());
-
-        for (auto node : nodes)
-        {
-            x_node.SetSize(node->Width());
-            y_node.SetSize(node->Height());
-
-            // Assemble input fields into a single vector for the node
-            auto node_inputs = node->InputFields();
-            auto ioffsets = node->InputOffsets();
-            for (int i=0; i < node_inputs.Size(); i++)
-            {
-                auto in_field = node_inputs[i];
-                int idx = index_map.Get(in_field->ID());
-                x_node.SetVector(y[idx],ioffsets[i]);
-            }
-
-            node->Mult(x_node, y_node);
-
-            // Disassemble output vector back
-            auto node_outputs = node->OutputFields();
-            BlockVector ynb(y_node.GetData(), node->OutputOffsets());
-            for (int i=0; i < node_outputs.Size(); i++)
-            {
-                auto out_field = node_outputs[i];
-                int idx = index_map.Get(out_field->ID());
-                y[idx] = ynb.GetBlock(i);
-            }
-        }
-    }
-    else if(input_type == InputType::MULTIVECTOR)
-    {
-        for (auto node : nodes)
-        {
-            auto node_inputs = node->InputFields();
-            auto node_outputs = node->OutputFields();
-            xmv_node.SetNumBlocks(node_inputs.Size());
-            ymv_node.SetNumBlocks(node_outputs.Size());
-
-            for (int i=0; i < node_inputs.Size(); i++)
-            {
-                int idx = index_map.Get(node_inputs[i]->ID());
-                xmv_node.MakeRef(i, y[idx]);
-            }
-            for (int i=0; i < node_outputs.Size(); i++)
-            {
-                int idx = index_map.Get(node_outputs[i]->ID());
-                ymv_node.MakeRef(i, y[idx]);
-            }
-            node->Mult(xmv_node, ymv_node);
-        }
-    }
-    else if(input_type == InputType::NONE)
-    {
-        Vector x_unused, y_unused;
-        for (auto node : nodes)
-        {
-            node->Mult(x_unused, y_unused);
-        }
-    }
-    else
-    {
-        MFEM_ABORT("DAGraph::Execute() not implemented for input type: "
-                    << static_cast<int>(input_type));
-    }
-}
-
-Operator& DAGraph::GetGradient(const Vector &x) const
-{
-    // TODO: Should/could be removed
-    if(grad_mode == GradMode::FINITE_DIFF)
-    {
-        if(!grad)
-        {
-            grad = new future::FDJacobian(*this, x, 1e-6);
-        }
-        else
-        {
-            grad->GetGradient(x); // Update the FDJacobian with new point x
-        }
-        return *grad;
-    }
-
-    MFEM_ASSERT(static_cast<int>(grad_mode) < static_cast<int>(GradMode::NONE),
-                "DAGraph::GetGradient() called with invalid grad_mode: "
-                << static_cast<int>(grad_mode));
-
-    if(!grad)
-    {
-        grad = new GraphGradient(const_cast<DAGraph&>(*this));
-    }
-
-    if(grad_mode == GradMode::ASSEMBLED)
-    {
-        return grad->GetGradient(x); // Assemble the Jacobian matrix
-    }
-    else // GradMode::MATRIX_FREE
-    {
-        dynamic_cast<GraphGradient*>(grad)->Update(x); // Update the GraphGradient with new point x
-    }
-
-    return *grad;
-}
-
-GraphGradient::GraphGradient(DAGraph &dag) : Operator(dag.Height(), dag.Width()),
-                                             graph(&dag)
-{
-    MFEM_ASSERT(graph->IsAssembled(), "GraphGradient requires an assembled DAGraph.");
-    MFEM_ASSERT(graph->IsSorted(), "GraphGradient requires a topologically sorted DAGraph.");
-
-    auto index_map = graph->GetFieldIdToIndexMap();
-    auto field_map = graph->GetFieldIdToFieldMap();
-
-    MFEM_ASSERT(index_map.NumFields() == field_map.NumFields(),
-                "Mismatch in number of fields between index_map and field_map");
-
-    x_work.DeleteAll(); // Clear any existing pointers
-    x_work.SetSize(index_map.NumFields());
-    x_work = nullptr; // Initialize all pointers to nullptr
-    xlin.SetNumBlocks(index_map.NumFields());
-
-    for (auto const& [id, idx] : index_map)
-    {
-        MFEM_ASSERT(idx >= 0 && idx < x_work.Size(), "Index out of bounds for field ID: " << id);
-        MFEM_ASSERT(field_map.Has(id), "Field ID not found in field_map: " << id);
-
-        if(x_work[idx] == nullptr)
-        {
-            x_work[idx] = new Vector(); // Allocate a new Vector for this field
-        }
-        xlin.MakeRef(idx, *x_work[idx]); // Make xlin refer to the allocated Vector
-    }
-
-}
-
-void GraphGradient::Update(const Vector &x)
-{
-    MFEM_ASSERT(graph != nullptr, "GraphGradient operator requires a non-null DAGraph pointer.");
-
-    auto set_exec_mode = [&](DAGraph::ExecutionMode mode)
-    {
-        for (auto &node : graph->Nodes())
-        {
-            node->SetExecutionMode(mode);
-        }
-    };
-
-    auto inputs = graph->InputFields();
-    BlockVector xb(x.GetData(), graph->InputOffsets());
-    MultiVector xmv(inputs.Size());
-    for(int i=0; i < inputs.Size(); i++)
-    {
-        xmv.MakeRef(i, xb.GetBlock(i));
-    }
-
-    set_exec_mode(DAGraph::ExecutionMode::GRADIENT_MODE);
-    graph->Execute(xmv, xlin); // Forward pass to populate fields for gradient computations
-    set_exec_mode(DAGraph::ExecutionMode::DEFAULT_MODE); // Reset execution mode for forward pass
-}
-
-void GraphGradient::Mult(const Vector &x, Vector &y) const
-{
-    MFEM_ASSERT(x.Size() == graph->Width(), "Input vector size (" << x.Size()
-                << ") must match graph width (" << graph->Width() << ")");
-
-    MFEM_ASSERT(y.Size() == graph->Height(), "Output vector size (" << y.Size()
-                << ") must match graph height (" << graph->Height() << ")");
-
-    auto in_offsets  = graph->InputOffsets();
-    auto out_offsets = graph->OutputOffsets();
-
-    auto inputs  = graph->InputFields();
-    auto outputs = graph->OutputFields();
-
-    BlockVector xb(x.GetData(), in_offsets);
-    BlockVector yb(y.GetData(), out_offsets);
-    MultiVector xmv(inputs.Size()), ymv(outputs.Size());
-
-    for(int i=0; i < inputs.Size(); i++)
-    {
-        xmv.MakeRef(i, xb.GetBlock(i));
-    }
-
-    for(int i=0; i < outputs.Size(); i++)
-    {
-        ymv.MakeRef(i, yb.GetBlock(i));
-    }
-
-    const_cast<GraphGradient*>(this)->Mult(xmv, ymv); // Forward mode: compute JVP, y = J(z) * x
-}
-
-void GraphGradient::Mult(const MultiVector &x, MultiVector &y)
-{
-    auto inputs  = graph->InputFields();
-    auto outputs = graph->OutputFields();
-
-    MFEM_ASSERT(inputs.Size() == x.NumBlocks(), "Number of input blocks (" << x.NumBlocks()
-                << ") must match number of input fields (" << inputs.Size() << ")");
-
-    MFEM_ASSERT(outputs.Size() == y.NumBlocks(), "Number of output blocks (" << y.NumBlocks()
-                << ") must match number of output fields (" << outputs.Size() << ")");
-
-    for(int i=0; i < inputs.Size(); i++)
-    {
-        inputs[i]->SetAdjoint(const_cast<Vector*>(&x[i]));
-    }
-    for (int i=0; i < outputs.Size(); i++)
-    {
-        outputs[i]->SetAdjoint(&y[i]);
-    }
-
-    auto index_map = graph->GetFieldIdToIndexMap();
-    auto fld_map = graph->GetFieldIdToFieldMap();
-    int nfields = index_map.NumFields();
-    MultiVector ymv(nfields); // TODO: Should this be a member function?
-
-    // Assemble the multivector from the individual fields based on their IDs
-    // This multivector contains all input, output, and intermediate fields in the graph
-    for (auto const& [id, idx] : index_map)
-    {
-        if (fld_map.Has(id))
-        {
-            auto field = fld_map.Get(id);
-            ymv.MakeRef(idx, *field->Adjoint());
-        }
-        else
-        {
-            MFEM_ABORT("Field ID " << id << " not found in field map");
-        }
-    }
-
-    Forward(x, ymv); // Forward mode: compute JVP, y = J(z) * x
-
-    for (auto &f : inputs)
-    {
-        f->SetAdjoint(nullptr);
-    }
-    for (auto &f : outputs)
-    {
-        f->SetAdjoint(nullptr);
-    }
-}
-
-void GraphGradient::MultTranspose(const Vector &x, Vector &y) const
-{
-    MFEM_ASSERT(x.Size() == graph->Height(), "Input vector size (" << x.Size()
-                << ") must match graph height (" << graph->Height() << ")");
-    MFEM_ASSERT(y.Size() == graph->Width(), "Output vector size (" << y.Size()
-                << ") must match graph width (" << graph->Width() << ")");
-
-    auto in_offsets  = graph->InputOffsets();
-    auto out_offsets = graph->OutputOffsets();
-
-    auto inputs  = graph->InputFields();
-    auto outputs = graph->OutputFields();
-
-    BlockVector xb(x.GetData(), out_offsets);
-    BlockVector yb(y.GetData(), in_offsets);
-    MultiVector xmv(outputs.Size()), ymv(inputs.Size());
-
-    for(int i=0; i < inputs.Size(); i++)
-    {
-        xmv.MakeRef(i, xb.GetBlock(i));
-    }
-
-    for(int i=0; i < outputs.Size(); i++)
-    {
-        ymv.MakeRef(i, yb.GetBlock(i));
-    }
-    const_cast<GraphGradient*>(this)->MultTranspose(xmv, ymv); // Reverse mode: compute VJP, y = J(z)^T * x
-}
-
-void GraphGradient::MultTranspose(const MultiVector &x, MultiVector &y)
-{
-    auto inputs  = graph->InputFields();
-    auto outputs = graph->OutputFields();
-
-    MFEM_ASSERT(outputs.Size() == x.NumBlocks(), "Number of input blocks (" << x.NumBlocks()
-                << ") must match number of output fields (" << outputs.Size() << ")");
-
-    MFEM_ASSERT(inputs.Size() == y.NumBlocks(), "Number of output blocks (" << y.NumBlocks()
-                << ") must match number of input fields (" << inputs.Size() << ")");
-
-    for(int i=0; i < outputs.Size(); i++)
-    {
-        outputs[i]->SetAdjoint(const_cast<Vector*>(&x[i]));
-    }
-    for (int i=0; i < inputs.Size(); i++)
-    {
-        inputs[i]->SetAdjoint(&y[i]);
-    }
-
-    auto index_map = graph->GetFieldIdToIndexMap();
-    auto fld_map = graph->GetFieldIdToFieldMap();
-    int nfields = index_map.NumFields();
-    MultiVector ymv(nfields); // TODO: Should this be a member function?
-
-    for(auto const& [id, idx] : index_map)
-    {
-        if (fld_map.Has(id))
-        {
-            auto field = fld_map.Get(id);
-            ymv.MakeRef(idx, *field->Adjoint());
-        }
-        else
-        {
-            MFEM_ABORT("Field ID " << id << " not found in field map");
-        }
-    }
-
-    Reverse(x, ymv); // Reverse mode: compute VJP, y = J(z)^T * x
-
-    for (auto &f : outputs)
-    {
-        f->SetAdjoint(nullptr);
-    }
-    for (auto &f : inputs)
-    {
-        f->SetAdjoint(nullptr);
-    }
-}
-
-void GraphGradient::Forward(const MultiVector &x, MultiVector &y) const
-{
-    MFEM_ASSERT(x.NumBlocks() == graph->InputFields().Size(),
-                "Number of input blocks (" << x.NumBlocks()
-                << ") must match number of input fields (" << graph->InputFields().Size() << ")");
-    
-    auto in_type = graph->GetInputType();
-    auto index_map  = graph->GetFieldIdToIndexMap();
-    auto field_map = graph->GetFieldIdToFieldMap();
-
-    MFEM_ASSERT(y.NumBlocks() == index_map.NumFields(),
-                "Number of output blocks (" << y.NumBlocks()
-                << ") must match number of fields (" << index_map.NumFields() << ")");
-
-    auto inputs  = graph->InputFields();
-    for(int i=0; i < inputs.Size(); i++)
-    {
-        int idx = index_map.Get(inputs[i]->ID());
-        if(&y[idx] != &x[i]) // copy data, if address is different
-        {
-            y[idx] = x[i];
-        }
-    }
-
-    if(in_type == InputType::VECTOR)
-    {
-        x0.SetSize(graph->MaxWidth());
-        dx.SetSize(graph->MaxWidth());
-        dy.SetSize(graph->MaxHeight());
-
-        auto nodes = graph->Nodes();
-        for (auto node : nodes)
-        {
-            x0.SetSize(node->Width());
-            dx.SetSize(node->Width());
-            dy.SetSize(node->Height());
-
-            // Assemble input fields into a single vector for the node
-            auto node_inputs = node->InputFields();
-            auto ioffsets = node->InputOffsets();
-            for(int i=0; i < node_inputs.Size(); i++)
-            {
-                auto in_field = node_inputs[i];
-                MFEM_ASSERT(index_map.Has(in_field->ID()), "Input field ID not found in index_map");
-                int idx = index_map.Get(in_field->ID());
-                x0.SetVector(xlin[idx], ioffsets[i]);
-                dx.SetVector(y[idx], ioffsets[i]);
-            }
-
-            node->GradientMult(x0, dx, dy); // Compute JVP for the node
-
-            // Disassemble output vector back
-            auto node_outputs = node->OutputFields();
-            BlockVector ynb(dy.GetData(), node->OutputOffsets());
-            for(int i=0; i < node_outputs.Size(); i++)
-            {
-                auto out_field = node_outputs[i];
-                MFEM_ASSERT(index_map.Has(out_field->ID()), "Output field ID not found in index_map");
-                int idx = index_map.Get(out_field->ID());
-                y[idx] = ynb.GetBlock(i);
-            }
-        }
-    }
-    else if(in_type == InputType::MULTIVECTOR)
-    {
-        auto nodes = graph->Nodes();
-        for (auto node : nodes)
-        {
-            auto node_inputs = node->InputFields();
-            auto node_outputs = node->OutputFields();
-            x0_mv.SetNumBlocks(node_inputs.Size());
-            dx_mv.SetNumBlocks(node_inputs.Size());
-            dy_mv.SetNumBlocks(node_outputs.Size());
-
-            for(int i=0; i < node_inputs.Size(); i++)
-            {
-                int idx = index_map.Get(node_inputs[i]->ID());
-                x0_mv.MakeRef(i, xlin[idx]);
-                dx_mv.MakeRef(i, y[idx]);
-            }
-            for(int i=0; i < node_outputs.Size(); i++)
-            {
-                int idx = index_map.Get(node_outputs[i]->ID());
-                dy_mv.MakeRef(i, y[idx]);
-            }
-            node->GradientMult(x0_mv, dx_mv, dy_mv); // Compute JVP for the node
-        }
-    }
-    else if(in_type == InputType::NONE)
-    {
-        Vector x_unused, dx_unused, dy_unused;
-        auto nodes = graph->Nodes();
-        for (auto node : nodes)
-        {
-            node->GradientMult(x_unused, dx_unused, dy_unused);
-        }
-    }
-    else
-    {
-        MFEM_ABORT("GraphGradient::Forward() not implemented for input type: "
-                    << static_cast<int>(in_type));
-    }
-}
-
-void GraphGradient::Reverse(const MultiVector &x, MultiVector &y) const
-{
-    MFEM_ASSERT(x.NumBlocks() == graph->OutputFields().Size(),
-                "Number of input blocks (" << x.NumBlocks()
-                << ") must match number of output fields (" << graph->OutputFields().Size() << ")");
-
-    auto in_type = graph->GetInputType();
-    auto index_map  = graph->GetFieldIdToIndexMap();
-    auto field_map = graph->GetFieldIdToFieldMap();
-    int nnodes  = graph->Size();
-
-    MFEM_ASSERT(y.NumBlocks() == index_map.NumFields(),
-                "Number of output blocks (" << y.NumBlocks()
-                << ") must match number of fields (" << index_map.NumFields() << ")");
-
-    auto outputs  = graph->OutputFields();
-    for(int i=0; i < outputs.Size(); i++)
-    {
-        int idx = index_map.Get(outputs[i]->ID());
-        if(&y[idx] != &x[i]) // copy data, if address is different
-        {
-            y[idx] = x[i];
-        }
-    }
-
-    if(in_type == InputType::VECTOR)
-    {
-        x0.SetSize(graph->MaxWidth());
-        dx.SetSize(graph->MaxHeight());
-        dy.SetSize(graph->MaxWidth());
-
-        for (int i=nnodes-1; i >= 0; i--)
-        {
-            auto node = graph->GetNode(i);
-            x0.SetSize(node->Width());
-            dx.SetSize(node->Height());
-            dy.SetSize(node->Width());
-
-            auto node_inputs = node->InputFields();
-            auto ioffsets = node->InputOffsets();
-            for(int i=0; i < node_inputs.Size(); i++)
-            {
-                auto in_field = node_inputs[i];
-                MFEM_ASSERT(index_map.Has(in_field->ID()), "Input field ID not found in index_map");
-                int idx = index_map.Get(in_field->ID());
-                x0.SetVector(xlin[idx], ioffsets[i]);
-            }
-
-            auto node_outputs = node->OutputFields();
-            auto ooffsets = node->OutputOffsets();
-            for(int i=0; i < node_outputs.Size(); i++)
-            {
-                auto out_field = node_outputs[i];
-                MFEM_ASSERT(index_map.Has(out_field->ID()), "Output field ID not found in index_map");
-                int idx = index_map.Get(out_field->ID());
-                dx.SetVector(y[idx], ooffsets[i]);
-            }
-
-            node->GradientMultTranspose(x0, dx, dy); // Compute JVP for the node
-
-            BlockVector dynb(dy.GetData(), node->InputOffsets());
-            for(int i=0; i < node_inputs.Size(); i++)
-            {
-                int idx = index_map.Get(node_inputs[i]->ID());
-                y[idx] = dynb.GetBlock(i);
-            }
-        }
-    }
-    else if(in_type == InputType::MULTIVECTOR)
-    {
-        for (int i=nnodes-1; i >= 0; i--)
-        {
-            auto node = graph->GetNode(i);
-            auto node_inputs = node->InputFields();
-            auto node_outputs = node->OutputFields();
-            x0_mv.SetNumBlocks(node_inputs.Size());
-            dx_mv.SetNumBlocks(node_outputs.Size());
-            dy_mv.SetNumBlocks(node_inputs.Size());
-
-            for(int i=0; i < node_inputs.Size(); i++)
-            {
-                int idx = index_map.Get(node_inputs[i]->ID());
-                x0_mv.MakeRef(i, xlin[idx]);
-                dy_mv.MakeRef(i, y[idx]);
-            }
-            for(int i=0; i < node_outputs.Size(); i++)
-            {
-                int idx = index_map.Get(node_outputs[i]->ID());
-                dx_mv.MakeRef(i, y[idx]);
-            }
-            node->GradientMultTranspose(x0_mv, dx_mv, dy_mv); // Compute JVP for the node
-        }
-    }
-    else if(in_type == InputType::NONE)
-    {
-        Vector x_unused, dx_unused, dy_unused;
-        for (int i=nnodes-1; i >= 0; i--)
-        {
-            auto node = graph->GetNode(i);
-            node->GradientMultTranspose(x_unused, dx_unused, dy_unused); // Compute VJP for the node
-        }
-    }
-    else
-    {
-        MFEM_ABORT("GraphGradient::Reverse() not implemented for input type: "
-                    << static_cast<int>(in_type));
-    }
-}
-
-Operator& GraphGradient::GetGradient(const Vector &x) const
-{
-    // Used to build Jacobian matrix
-    MFEM_ABORT("GraphGradient::GetGradient() not implemented");
-}
-
-
-} // namespace mfem

miniapps/multiapp/multiapp.hpp

@@ -1,838 +0,0 @@
-// Copyright (c) 2010-2025, Lawrence Livermore National Security, LLC. Produced
-// at the Lawrence Livermore National Laboratory. All Rights reserved. See files
-// LICENSE and NOTICE for details. LLNL-CODE-806117.
-//
-// This file is part of the MFEM library. For more information and source code
-// availability visit https://mfem.org.
-//
-// MFEM is free software; you can redistribute it and/or modify it under the
-// terms of the BSD-3 license. We welcome feedback and contributions, see file
-// CONTRIBUTING.md for details.
-
-
-#ifndef MFEM_MULTIAPP_HPP
-#define MFEM_MULTIAPP_HPP
-
-#include "mfem.hpp"
-
-namespace mfem
-{
-
-/// Forward declarations needed below
-class Field;
-class FieldCollection;
-class GraphNode;
-class DAGraph;
-class GraphGradient;
-
-
-/// @brief Base class for storing data (Vector) and distinguishing
-/// fields variables
-class Field
-{
-public:
-    enum Type ///< Not used for now, but could be used to distinguish between input/output fields
-    {
-        INPUT , ///< Input field
-        OUTPUT, ///< Output field
-        DEFAULT ///< Any field
-    };
-
-    friend class GraphNode;
-
-private:
-    Type type = Type::DEFAULT;
-    inline static int next_id = 0;
-
-protected:
-    Vector *data = nullptr;
-    Vector *adjoint = nullptr; // For storing derivative info
-    int id = -1; // initialized to invalid id
-
-    std::string name; // Optional name for the field
-    Operator *oper  = nullptr; // Operator that outputs this field
-
-    int GetValidID(int id_, int lb=0, int ub = std::numeric_limits<int>::max())
-    {
-        return (id_ >= lb && id_ <= ub) ? id_ : next_id++;
-    }
-
-public:
-
-    ///@brief Constructor for a Field of type Type with optional ID
-    Field(Vector *field, Vector *adjoint, Type type, int id_ = -1) :
-          type(type), data(field), adjoint(adjoint), id(GetValidID(id_)),
-          name("Field_" + std::to_string(id)) { }
-
-    ///@brief Constructor for a Field of Default type with optional ID
-    Field(Vector *field, Vector *adjoint, int id_ = -1) : 
-          Field(field, adjoint, Type::DEFAULT, id_) { }
-
-    ///@brief Constructor for an input field
-    Field(Vector *field, int id_ = -1) :
-          Field(field, nullptr, Type::DEFAULT, id_) { }
-
-    ///@brief Constructor for a Field of type Type
-    Field(Vector *field, Type type, int id_ = -1) :
-          Field(field, nullptr, type, id_) { }
-
-    ///@brief Get the stored internally stored data pointer
-    Vector* Data() const { return data; }
-    Vector* Adjoint() const { return adjoint; }
-    Operator* GetOperator() const { return oper; }
-
-    ///@brief Set the internally stored data pointer
-    virtual void SetData(Vector *field) { data = field; }
-    virtual void SetAdjoint(Vector *adj) { adjoint = adj; }
-    virtual void SetOperator(Operator *op) { oper = op; }
-
-    virtual void GetData(Vector &field) const { field = *data; }
-    virtual void GetAdjoint(Vector &adj) const { adj = *adjoint; }
-
-    std::string Name() const { return name; }
-    void SetName(const std::string &n) { name = n; }
-    int ID() const { return id; }
-
-    void SetID(int i)
-    {
-        MFEM_ASSERT(i >= 0, "ID must be non-negative.");
-        id = i;
-    }
-
-    bool IsInput() const {return (type == Type::INPUT);}
-    bool IsOutput() const {return (type == Type::OUTPUT);}
-    bool IsDefault() const {return (type == Type::DEFAULT);}
-
-    virtual ~Field() = default;
-
-protected:
-
-    ///@brief Set the type of the field (prevents changing type of input/output fields)
-    void SetType(Type t)
-    {
-        type = t;
-    }
-};
-
-/// @brief A collection of Fields, each identified by a name
-class FieldCollection
-{
-public:
-    using FieldMap = GenericFieldMap<std::string, Field*>;
-    using IndexMap = GenericFieldMap<std::string, int>;
-
-private:
-    std::string name; /// Name of the collection
-    Operator *oper = nullptr; /// Operator associated with this collection (not owned)
-    FieldMap fields; /// Map from field name to Field pointer
-    IndexMap index_map; /// Map from field name to index in input/output vectors
-
-    Array<Field*> input_fields;  // Input fields for this node
-    Array<Field*> output_fields; // Output fields for this node
-
-public:
-
-    FieldCollection() = default;
-
-    /// @brief Constructor with collection name and optional associated operator
-    FieldCollection(std::string collection_name, Operator *op = nullptr):
-                    name(collection_name), oper(op) {}
-
-    /// @brief Constructor with associated operator and default collection name
-    FieldCollection(Operator *op) : name("FieldCollection"), oper(op) {}
-
-    /// @brief Get the number of fields in the collection
-    int Size() const { return fields.NumFields(); }
-
-    /// @brief Set the name of the collection
-    void SetName(const std::string &collection_name) { name = collection_name;}
-
-    /// @brief Get the name of the collection
-    std::string Name() const { return name; }
-
-    /// @brief Set the operator associated with this collection
-    void SetOperator(Operator *op){ oper = op; }
-
-    /// @brief Get the operator associated with this collection
-    const Operator* GetOperator() const { return oper; }
-
-    /// @brief Get the field associated with the given name, or nullptr if not found
-    Field* GetField(const std::string &field_name) const
-    {
-        return fields.Get(field_name);
-    }
-
-    /// @brief Add a field to the collection with a given name and ownership flag
-    void AddField(const std::string &field_name, Field *field, bool own = false)
-    {
-        if(fields.Has(field_name))
-        {
-            MFEM_WARNING("FieldCollection::AddField: Field with name "
-                         << field_name << " already exists. Replacing existing field.");
-        }
-        fields.Register(field_name, field, own);
-    }
-
-    void AddInput(const std::string &field_name,
-                  Field *field, bool own = false)
-    {
-        bool has_field = fields.Has(field_name);
-        bool has_index = index_map.Has(field_name);
-        if(has_field && has_index)
-        {
-            int i = index_map.Get(field_name);
-            input_fields[i] = field;
-        }
-        else
-        {
-            input_fields.push_back(field);
-            index_map.Register(field_name, input_fields.Size() - 1);
-        }
-        AddField(field_name, field, own);
-    }
-
-    void AddOutput(const std::string &field_name,
-                   Field *field, bool own = false)
-    {
-        bool has_field = fields.Has(field_name);
-        bool has_index = index_map.Has(field_name);
-        if(has_field && has_index)
-        {
-            int i = index_map.Get(field_name);
-            output_fields[i] = field;
-        }
-        else
-        {
-            output_fields.push_back(field);
-            index_map.Register(field_name, output_fields.Size() - 1);
-        }
-
-        AddField(field_name, field, own);
-        if(field->GetOperator() == nullptr)
-        {
-            field->SetOperator(oper);
-        }
-    }
-
-    Array<Field*>& InputFields() { return input_fields; }
-    Array<Field*>& OutputFields() { return output_fields; }
-
-    Field* InputField(int i) const { return input_fields[i]; }
-    Field *InputField(const std::string &field_name) const
-    {
-        bool has_index = index_map.Has(field_name);
-        if(!has_index)
-        {
-            MFEM_WARNING("FieldCollection::InputField: Field with name "
-                         << field_name << " does not exist in the collection.");
-            return nullptr;
-        }
-
-        int index = index_map.Get(field_name);
-        MFEM_VERIFY(index >= 0 && index < input_fields.Size(),
-                    "FieldCollection::InputField: Invalid index for field name: "
-                    << field_name << ".");
-        return input_fields[index];
-    }
-
-    Field* OutputField(int i) const { return output_fields[i]; }
-    Field *OutputField(const std::string &field_name) const
-    {
-        bool has_index = index_map.Has(field_name);
-        if(!has_index)
-        {
-            MFEM_WARNING("FieldCollection::OutputField: Field with name "
-                         << field_name << " does not exist in the collection.");
-            return nullptr;
-        }
-
-        int index = index_map.Get(field_name);
-        MFEM_VERIFY(index >= 0 && index < output_fields.Size(),
-                    "FieldCollection::OutputField: Invalid index for field name: "
-                    << field_name << ".");
-        return output_fields[index];
-    }
-
-    FieldMap &Fields() { return fields; }
-    FieldMap Fields() const { return fields; }
-
-    virtual void Save (std::ostream &out) const
-    {
-        out << "\"Fields\":\n";
-        out << "{\n";
-        for (auto f = fields.begin(); f != fields.end(); ++f)
-        {
-            std::string f_name = f->first;
-            Field *f_obj = f->second;
-            // out << "  " << f_name << ": ID " << f_obj->ID() << ",\n";
-            // out << f_obj->ID() << ": " << f_name << ",\n";
-            out << '\"' << f_obj->ID() << "\": \"" << f_name << "\"";
-            if(f != std::prev(fields.end())) out << ",";
-            out << "\n";
-        }
-        out << "},\n";
-
-        out << "\"Inputs\":\n";
-        out << "{\n";
-        for (int i = 0; i < input_fields.Size(); ++i)
-        {
-            Field *f_obj = input_fields[i];
-            out << '\"' << f_obj->ID() << "\": \"" << f_obj->Name() << "\"";
-            if(i != input_fields.Size() - 1) out << ",";
-            out << "\n";
-        }
-        out << "},\n";
-
-        out << "\"Outputs\":\n";
-        out << "{\n";
-        for (int i = 0; i < output_fields.Size(); ++i)
-        {
-            Field *f_obj = output_fields[i];
-            out << '\"' << f_obj->ID() << "\": \"" << f_obj->Name() << "\"";
-            if(i != output_fields.Size() - 1) out << ",";
-            out << "\n";
-        }
-        out << "}\n";
-    }
-
-    Field* HasField(const Field &field) const
-    {
-        for (auto f = fields.begin(); f != fields.end(); ++f)
-        {
-            if(f->second == &field)
-            {
-                return f->second;
-            }
-        }
-        return nullptr;
-    }
-
-    Field* HasField(const std::string &field_name) const
-    {
-        return fields.Get(field_name);
-    }
-
-    Field* HasField(const int id) const
-    {
-        for (auto f = fields.begin(); f != fields.end(); ++f)
-        {
-            if(f->second->ID() == id)
-            {
-                return f->second;
-            }
-        }
-        return nullptr;
-    }
-
-    ~FieldCollection(){}
-
-};
-
-
-class GraphNode : public Operator
-{
-public:
-    enum ExecutionMode
-    {
-        GRADIENT_MODE, ///< Node is being executed as part of a gradient evaluation
-        DEFAULT_MODE   ///< Node is being executed as default, operator evaluation
-    };
-
-private:
-    inline static int next_id = 0;
-
-protected:
-    int id = -1;
-    int node_index = -1;
-    mutable ExecutionMode exec_mode = DEFAULT_MODE;
-
-    std::string name;
-    mutable FieldCollection field_collection; ///< Collection of fields associated with this node
-
-    // Offsets to be used for operation on BlockVector
-    Array<int> input_offsets;  ///< Offsets for input fields
-    Array<int> output_offsets; ///< Offsets for output fields
-
-    int GetValidID(int id_, int lb=0, int ub = std::numeric_limits<int>::max())
-    {
-        return (id_ >= lb && id_ <= ub) ? id_ : next_id++;
-    }
-
-public:
-
-    GraphNode(int h, int w) : Operator(h,w), id(GetValidID(-1)),
-                              name("Node_" + std::to_string(id)),
-                              field_collection(this) { }
-
-    GraphNode(int s = 0) : GraphNode(s, s) { }
-
-    void SetNodeIndex(int index){ node_index = index; }
-    int GetNodeIndex() const { return node_index; }
-
-    void SetExecutionMode(ExecutionMode mode) { exec_mode = mode; }
-    ExecutionMode GetExecutionMode() const { return exec_mode; }
-
-    void SetName(const std::string &name_) { name = name_; }
-    std::string Name() const { return name; }
-
-    void SetID(int id_) { id = id_; }
-    int ID() const { return id; }
-
-    FieldCollection::FieldMap& Fields() { return field_collection.Fields(); }
-    Field* Fields(const std::string &f) { return field_collection.GetField(f); }
-
-    FieldCollection::FieldMap Fields() const { return field_collection.Fields(); }
-    Field* Fields(const std::string &f) const { return field_collection.GetField(f); }
-
-    Array<Field*>& InputFields() const { return field_collection.InputFields(); }
-    Array<Field*>& OutputFields() const { return field_collection.OutputFields(); }
-    Field* InputField(int i) const { return field_collection.InputField(i); }
-    Field* OutputField(int i) const { return field_collection.OutputField(i); }
-
-
-    virtual void AddInput(const std::string &field_name,
-                          Field *field, bool own = false)
-    { field_collection.AddInput(field_name, field, own); }
-
-    virtual void AddInput(Field *field, bool own = false)
-    { AddInput(field->Name(), field, own); }
-
-    template<bool OwnInputs = false,
-             typename... Args,
-             bool AreFields = std::conjunction<std::is_base_of<Field, std::remove_pointer_t<Args>> ...>::value,
-             typename std::enable_if<AreFields, bool>::type = true >
-    void AddInputs(Args... args)
-    {
-        ((AddInput(std::forward<Args>(args), OwnInputs)), ...);
-    }
-
-    virtual void AddOutput(const std::string &field_name,
-                           Field *field, bool own = false)
-    { field_collection.AddOutput(field_name, field, own); }
-
-    virtual void AddOutput(Field *field, bool own = false)
-    { AddOutput(field->Name(), field, own); }
-
-    template<bool OwnOutputs = false,
-             typename... Args,
-             bool AreFields = std::conjunction<std::is_base_of<Field, std::remove_pointer_t<Args>> ...>::value,
-             typename std::enable_if<AreFields, bool>::type = true >
-    void AddOutputs(Args... args)
-    {
-        ((AddOutput(std::forward<Args>(args), OwnOutputs)), ...);
-    }
-
-    virtual void Save (std::ostream &out) const
-    {
-        out << "\"Node-" << id << "\" : " << std::endl;
-        out << "{\n";
-        out << "\"Name\": \"" << name << "\",\n";
-        field_collection.Save(out);
-        out << "}";
-    }
-
-    virtual void Mult(const Vector &x, Vector &y) const override
-    {
-        MFEM_ABORT("GraphNode::Mult() not implemented");
-    }
-
-    virtual void Mult(const MultiVector &x, MultiVector &y) override
-    {
-        MFEM_ABORT("GraphNode::Mult(MultiVector) not implemented");
-    }
-
-    using Operator::GetGradient;
-
-    // TODO: Possibly remove this and only support MultiVector version of GradientMult
-    virtual void GradientMult(const Vector &x, const Vector &dx, Vector &dy) const
-    {
-        MFEM_ABORT("GraphNode::GradientMult() not implemented");
-        GetGradient(x).Mult(dx, dy);
-    }
-
-    virtual void GradientMult(const MultiVector &x, const MultiVector &dx, MultiVector &dy) const
-    {
-        MFEM_ABORT("GraphNode::GradientMult() not implemented");
-        GetGradient(x).Mult(dx, dy);
-    }
-
-    // TODO: Possibly remove this and only support MultiVector version of GradientMultTranspose
-    virtual void GradientMultTranspose(const Vector &x, const Vector &dx, Vector &dy) const
-    {
-        MFEM_ABORT("GraphNode::GradientMultTranspose() not implemented");
-        GetGradient(x).MultTranspose(dx, dy);
-    }
-
-    virtual void GradientMultTranspose(const MultiVector &x, const MultiVector &dx, MultiVector &dy) const
-    {
-        MFEM_ABORT("GraphNode::GradientMultTranspose() not implemented");
-        // GetGradient(x).MultTranspose(dx, dy); // Not yet implemented
-    }
-
-    /// @brief Return the input offsets for block starts.
-    Array<int>& InputOffsets() { return input_offsets; }
-
-    /// @brief Read only access to the input offsets for block starts.
-    const Array<int>& InputOffsets() const { return input_offsets; }
-
-    void SetInputOffsets(const Array<int> &offsets) { input_offsets = offsets; }
-
-    /// @brief Return the output offsets for block starts.
-    Array<int>& OutputOffsets() { return output_offsets; }
-
-    /// @brief Read only access to the output offsets for block starts.
-    const Array<int>& OutputOffsets() const { return output_offsets; }
-
-    void SetOutputOffsets(const Array<int> &offsets) { output_offsets = offsets; }
-
-    virtual ~GraphNode() = default;
-};
-
-
-/**
-   @brief An abstract, type-erased class to define the interface for 
-   operators, not inherited from @a GraphNode. It performs SFINAE 
-   checks for stored operator's member functions and override the Mult
-   to call the stored object's functions.
- */
-template <typename OpType>
-class AbstractOperator : public GraphNode
-{
-protected:
-    /// Define a template class 'check' to test for the existence of member functions
-    template <typename C>
-    class CheckMember{
-        private:
-
-        /// @brief A type trait to check if the erased class has the function Mult
-        /// with the needed signatures.
-        template<class T>
-        using Mult = decltype(std::declval<T&>().Mult(std::declval<const Vector&>(),
-                                                      std::declval<Vector&>()));
-
-        template<class T>
-        using MultPtr = decltype(std::declval<T&>().Mult(std::declval<const int>(),
-                                                         std::declval<const real_t*>(),
-                                                         std::declval<const int>(),
-                                                         std::declval<real_t*>()));
-        // ---------------------------------------------------------------------
-        
-        template <typename T, template<typename> typename Func, typename R>
-        static constexpr auto Check(T*) -> typename std::is_same< Func<T>, R>::type;
-
-        template <typename, template<typename> typename, typename >
-        static constexpr std::false_type Check(...);
-
-        // --- Check for the existence of the member functions
-        typedef decltype(Check<C,Mult,void>(0)) Has_Mult;
-        typedef decltype(Check<C,MultPtr,void>(0)) Has_MultPtr;
-    public:
-        static constexpr bool HasMult  = Has_Mult::value;
-        static constexpr bool HasMultPtr  = Has_MultPtr::value;
-    };
-
-    OpType *op;  ///< Pointer to the operator
-
-public:
-
-    constexpr bool HasExecute(){return CheckMember<OpType>::HasStep;}
-    constexpr bool HasMult(){return CheckMember<OpType>::HasMult;}
-
-
-    /// @brief Constructor for the type-erased AbstractOperator class
-    AbstractOperator(OpType *op_, int h, int w) : GraphNode(h,w), op(op_)
-    { }
-
-    /// @brief Constructor for the type-erased AbstractOperator class.
-    AbstractOperator(OpType *op_, int s = 0) : AbstractOperator(op_,s,s) {}
-
-    /**
-       @brief Perform Mult operation with the stored operator, if it exists.
-     */
-    void Mult(const Vector &x, Vector &y) const override
-    {
-        if constexpr (CheckMember<OpType>::HasMult)
-        {
-            op->Mult(x,y);
-        }
-        else if constexpr (CheckMember<OpType>::HasMultPtr)
-        {
-            op->Mult(x.Size(), x.GetData(), y.Size(), y.GetData());
-        }
-        else
-        {
-            MFEM_ABORT("The AbstractOperator does not have the function, "
-                       "Mult(const Vector&, Vector&) or "
-                       "Mult(int, double*, int, double*).");
-        }
-    }
-};
-
-
-/**
-   @brief A class to store and coupled multiple operators together.
- */
-class DAGraph : public GraphNode
-{
-public:
-
-    using IntToIntMap = GenericFieldMap<int, int>;
-    using IntToFieldMap = GenericFieldMap<int, Field*>;
-
-    enum class GradMode
-    {
-        FINITE_DIFF = 0,  ///< Finite difference Jacobian
-        MATRIX_FREE = 1,  ///< Matrix-free Jacobian
-        ASSEMBLED = 2,    ///< Assembled Jacobian
-        NONE = 3          ///< Not implemented
-    };
-
-    enum InputType
-    {
-        VECTOR,      ///< Asemble the input blockvector from individual fields
-        MULTIVECTOR, ///< Asemble the multivector from individual fields
-        NONE         ///< No input
-    };
-
-protected:
-    Array<GraphNode*> nodes; ///< Vector of individual operators
-    Array<bool> node_owned; ///< Whether the operators are owned
-    Array<int> node_depth; ///< Depth of each operator in the graph
-
-    int max_width  = 0;     ///< Largest operator width
-    int max_height = 0;     ///< Largest operator height
-    int nnodes     = 0;     ///< The number of nodes
-    bool sorted    = false; ///< True if the nodes are topologically sorted
-    bool assembled = false; ///< True if the graph is assembled
-
-    GradMode grad_mode = GradMode::MATRIX_FREE; ///< Gradient mode for the graph
-    mutable Operator *grad = nullptr; ///< Gradient operator
-
-    InputType input_type = InputType::MULTIVECTOR; ///< Input type for the graph
-    mutable Vector x_node, y_node; ///< Temporary vectors for evaluating nodes
-    mutable MultiVector xmv_node, ymv_node; ///< Temporary multivectors for evaluating nodes
-
-    IntToFieldMap fid_to_field; ///< Map from Field ID to Field pointer
-    IntToIntMap fid_to_index; ///< Map from ID to index in an array; needed since ordering is not unique
-
-    friend class GraphGradient;
-
-public:
-    /**
-       @brief Construct a new CoupledOperator object.
-       @param nop Total number of operators to couple
-     */
-    DAGraph(const int nop) : GraphNode()
-    {
-        nodes.Reserve(nop);
-        node_owned.Reserve(nop);
-    }
-
-    /**
-       @brief Construct a new CoupledOperator object for an 
-       abstract non/mfem operator.
-     */
-    template <class OpType>
-    DAGraph(const OpType &op) : DAGraph(1)
-    {
-        AddOperator(op);
-    }
-
-    /**
-       @brief Add an operator to the list of coupled operator and
-       return pointer to it. Not owned unless it's not derived from GraphNode.
-     */
-    template <class OpType>
-    GraphNode* AddOperator(OpType *op_, int h, int w)
-    {
-        // Add operator to list of operators
-        if constexpr(std::is_base_of<GraphNode, OpType>::value)
-        {
-            nodes.push_back(op_);
-            node_owned.Append(false);
-        } 
-        else
-        {
-            nodes.push_back(new AbstractOperator<OpType>(op_,h,w));
-            node_owned.Append(true);
-        }
-        nnodes++;
-
-        // Update size of the coupled operator and the block offsets
-        GraphNode* op = nodes.Last();
-        op->SetNodeIndex(nnodes-1); // Set the index of the operator
-
-        int ht = op->Height();
-        int wt = op->Width();
-
-        max_width = std::max(max_width, wt);
-        max_height = std::max(max_height, ht);
-        sorted = false;
-
-        return op;
-    }
-
-    /// @brief Add an operator to the list of coupled operator and return pointer to it.
-    template <class OpType>
-    GraphNode* AddOperator(OpType *op_, int s = 0) { return AddOperator(op_,s,s);}
-
-    /// @brief Get the number of coupled operators
-    int Size(){return nnodes;}
-
-    /// @brief Get the size of the largest operator
-    int MaxWidth() const {return max_width;}
-    int MaxHeight() const {return max_height;}
-
-    IntToIntMap &GetFieldIdToIndexMap() { return fid_to_index; }
-    IntToIntMap GetFieldIdToIndexMap() const { return fid_to_index; }
-
-    IntToFieldMap &GetFieldIdToFieldMap() { return fid_to_field; }
-    IntToFieldMap GetFieldIdToFieldMap() const { return fid_to_field; }
-
-    /// @brief Get the operator at index @a i
-    GraphNode* GetNode(const int i)
-    {
-        MFEM_ASSERT(i >= 0 && i < nnodes,
-               "index [" << i << "] is out of range [0," << nnodes << ")");
-        return nodes[i];
-    }
-
-    Array<GraphNode*>& Nodes() { return nodes; }
-
-    /// @brief Specify whether the operator at index @a i is owned.
-    void OwnNode(const int i, bool own = true)
-    {
-        MFEM_ASSERT(i >= 0 && i < nnodes,
-               "index [" << i << "] is out of range [0," << nnodes << ")");
-        node_owned[i] = own;
-    }
-
-    void Assemble();
-    bool IsAssembled() const { return assembled; }
-
-    void TopologicalSort();
-    bool IsSorted() const { return sorted; }
-
-    void ComputeDepth();
-
-    void ValidateOffsets();
-
-    void ValidateNode(GraphNode &node);
-
-    void CollectFieldMaps();
-
-    using GraphNode::AddInput;
-    void AddInput(Field *field, int sz, bool own = false)
-    {
-        if(input_offsets.Size() == 0)
-        {   // First entry
-            input_offsets.Append(0);
-        }
-        input_offsets.Append(input_offsets.Last() + sz);
-        AddInput(field, own);
-    }
-
-    using GraphNode::AddOutput;
-    void AddOutput(Field *field, int sz, bool own = false)
-    {
-        if(output_offsets.Size() == 0)
-        {   // First entry
-            output_offsets.Append(0);
-        }
-        output_offsets.Append(output_offsets.Last() + sz);
-        AddOutput(field, own);
-    }
-
-    /// @brief Set the gradient mode for the coupled operator
-    void SetGradientMode(GradMode mode)
-    {
-        if(mode != grad_mode)
-        {
-            if(grad) { delete grad; grad = nullptr; }
-            grad_mode = mode;
-        }
-    }
-
-    void SetInputType(InputType type) { input_type = type; }
-    InputType GetInputType() const { return input_type; }
-
-    /**
-       @brief Apply the operator to the vector @a x 
-       and return the result in @a y.
-     */
-    virtual void Mult(const Vector &x, Vector &y) const override;
-
-    virtual void Mult(const MultiVector &x, MultiVector &y) override;
-
-    virtual void Execute(const MultiVector &x, MultiVector &y) const;
-
-    virtual void Save (std::ostream &out) const
-    {
-        out << "\"DAGraph\":\n";
-        out << "{\n";
-        // out << "\"nodes\" : " << nnodes << ",\n";
-        out << "\"Nodes\":\n";
-        out << "{\n";
-        for (int i = 0; i < nodes.Size(); i++)
-        {
-            nodes[i]->Save(out);
-            if(i != nodes.Size()-1) out << ",";
-            out << "\n";
-        }
-        out << "},\n"; // End of Nodes
-        field_collection.Save(out);
-        out << "}\n";
-    }
-
-    Operator& GetGradient(const Vector &x) const override;
-
-    /// @brief Destroy the Coupled Application object
-    ~DAGraph();
-};
-
-
-
-class GraphGradient : public Operator
-{
-public:
-    using InputType = DAGraph::InputType;
-
-protected:
-    mutable DAGraph *graph = nullptr; ///< Pointer to the DAGraph for which this is the gradient operator
-    Array<Vector*> x_work; ///< Array to store linearization point (intermediate fields)
-    mutable MultiVector xlin;
-    mutable Vector x0, dx, dy;
-    mutable MultiVector x0_mv, dx_mv, dy_mv;
-
-public:
-    GraphGradient(DAGraph &dag);
-
-    void Update(const Vector &x);
-
-    void Mult(const Vector &x, Vector &y) const override;
-
-    void Mult(const MultiVector &x, MultiVector &y) override;
-
-    void MultTranspose(const Vector &x, Vector &y) const override;
-
-    void MultTranspose(const MultiVector &x, MultiVector &y);
-
-    Operator &GetGradient(const Vector &x) const override;
-
-    void Forward(const MultiVector &x, MultiVector &y) const;
-
-    void Reverse(const MultiVector &x, MultiVector &y) const;
-
-    ~GraphGradient()
-    {
-        for (auto &v : x_work)
-        {
-            if(v) { delete v; v = nullptr; }
-        }
-        x_work.DeleteAll();
-    }
-};
-
-} //mfem namespace
-
-#endif