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Improve randomness of random_shuffle

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Ion Gaztañaga
2018-12-31 00:55:28 +01:00
parent df08df2d9d
commit 7dfabdf2d2
1 changed files with 15 additions and 1 deletions
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16
test/random_shuffle.hpp
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@@ -6,13 +6,27 @@
#include <boost/move/detail/iterator_traits.hpp> #include <boost/move/detail/iterator_traits.hpp>
#include <stdlib.h> #include <stdlib.h>
inline unsigned long long rand_15_bit()
{
//Many rand implementation only use 15 bits
//so make sure we have only 15 bits
return (unsigned long long)((std::rand()) & 0x7fffu);
}
inline unsigned long long ullrand()
{
return (rand_15_bit() << 54u) ^ (rand_15_bit() << 39u)
^ (rand_15_bit() << 26u) ^ (rand_15_bit() << 13u)
^ rand_15_bit();
}
template< class RandomIt > template< class RandomIt >
void random_shuffle( RandomIt first, RandomIt last ) void random_shuffle( RandomIt first, RandomIt last )
{ {
typedef typename boost::movelib::iterator_traits<RandomIt>::difference_type difference_type; typedef typename boost::movelib::iterator_traits<RandomIt>::difference_type difference_type;
difference_type n = last - first; difference_type n = last - first;
for (difference_type i = n-1; i > 0; --i) { for (difference_type i = n-1; i > 0; --i) {
difference_type j = std::rand() % (i+1); difference_type j = ullrand() % (i+1);
if(j != i) { if(j != i) {
boost::adl_move_swap(first[i], first[j]); boost::adl_move_swap(first[i], first[j]);
} }