diff --git a/fastlib/u/garryb/nbr/allnn.cc b/fastlib/u/garryb/nbr/allnn.cc
index 36ed23981b..13daac6704 100644
--- a/fastlib/u/garryb/nbr/allnn.cc
+++ b/fastlib/u/garryb/nbr/allnn.cc
@@ -218,7 +218,7 @@ class Allnn {
      */
     static double Heuristic(const Param& param,
         const QNode& q_node,  const RNode& r_node, const Delta& delta) {
-      return q_node.bound().MidDistanceSq(r_node.bound());
+      return r_node.bound().MinToMidSq(q_node.bound());
     }
   };
 };
diff --git a/fastlib/u/garryb/nbr/spbounds.h b/fastlib/u/garryb/nbr/spbounds.h
index 020aa7ac15..aeae062433 100644
--- a/fastlib/u/garryb/nbr/spbounds.h
+++ b/fastlib/u/garryb/nbr/spbounds.h
@@ -554,6 +554,40 @@ class SpHrectBound {
     return math::Pow<2, t_pow>(sumsq) / 4;
   }
 
+  /**
+   * Calculates closest-to-their-midpoint bounding box distance,
+   * i.e. calculates their midpoint and finds the minimum box-to-point
+   * distance.
+   *
+   * Equivalent to:
+   * <code>
+   * other.CalcMidpoint(&other_midpoint)
+   * return MinDistanceSqToPoint(other_midpoint)
+   * </code>
+   */
+  double MinToMidSq(const SpHrectBound& other) const {
+    double sumsq = 0;
+    const SpRange *a = this->bounds_;
+    const SpRange *b = other.bounds_;
+
+    DEBUG_ASSERT(dim_ == other.dim_);
+
+    for (index_t d = 0; d < dim_; d++) {
+      double v = b->mid();
+      double v1 = a->lo - v;
+      double v2 = v - a->hi;
+      
+      v = (v1 + fabs(v1)) + (v2 + fabs(v2));
+      
+      a++;
+      b++;
+      
+      sumsq += math::Pow<t_pow, 1>(v);
+    }
+
+    return math::Pow<2, t_pow>(sumsq) / 4;
+  }
+
   /**
    * Calculates maximum bound-to-point squared distance,
    * to the specified power.