76 lines
1.4 KiB
C++
76 lines
1.4 KiB
C++
// Copyright 2007 Georgia Institute of Technology. All rights reserved.
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// ABSOLUTELY NOT FOR DISTRIBUTION
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/**
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* @file discrete.cc
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*
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* Helpers for discrete math (implementation).
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*/
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#include "discrete.h"
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#include <stdlib.h>
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double math::BinomialCoefficient(int n, int k) {
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int n_k = n - k;
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double nchsk = 1;
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int i;
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if(k > n || k < 0) {
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return 0;
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}
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if(k < n_k) {
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k = n_k;
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n_k = n - k;
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}
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for(i = 1; i <= n_k; i++) {
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nchsk *= (++k);
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nchsk /= i;
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}
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return nchsk;
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}
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double math::Factorial(int d) {
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double v = 1;
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DEBUG_ASSERT(d >= 0);
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for (int i = 2; i <= d; i++) {
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v *= i;
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}
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return v;
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}
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void math::MakeIdentityPermutation(index_t size, index_t *array) {
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for (index_t i = 0; i < size; i++) {
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array[i] = i;
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}
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}
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void math::MakeRandomPermutation(index_t size, index_t *array) {
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// Regular permutation algorithm.
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// This is cache inefficient for large sizes; large caches might
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// warrant a more sophisticated blocked algorithm.
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if (unlikely(size == 0)) {
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return;
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}
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array[0] = 0;
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for (index_t i = 1; i < size; i++) {
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index_t victim = rand() % i;
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array[i] = array[victim];
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array[victim] = i;
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}
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}
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void math::MakeInversePermutation(index_t size,
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const index_t *original, index_t *reverse) {
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for (index_t i = 0; i < size; i++) {
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reverse[original[i]] = i;
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}
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}
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