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libstdc++: Fix std::random_shuffle for low RAND_MAX [PR88935]

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When RAND_MAX is small and the number of elements being shuffled is
close to it, we get very uneven distributions in std::random_shuffle.

This uses a simple xorshift generator seeded by std::rand if we can't
rely on std::rand itself.

libstdc++-v3/ChangeLog:

	PR libstdc++/88935
	* include/bits/stl_algo.h (random_shuffle) [RAND_MAX < INT_MAX]:
	Use xorshift instead of rand().
	* testsuite/25_algorithms/random_shuffle/88935.cc: New test.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Giovanni Bajo <rasky@develer.com>
This commit is contained in:
Giovanni Bajo 2024-07-31 20:03:40 +01:00 • committed by Jonathan Wakely
parent de1923f9f4
commit 125bab23ad
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2 changed files with 57 additions and 9 deletions
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42
libstdc++-v3/include/bits/stl_algo.h
View file

@ -4521,15 +4521,39 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
_RandomAccessIterator>)
__glibcxx_requires_valid_range(__first, __last);
if (__first != __last)
for (_RandomAccessIterator __i = __first + 1; __i != __last; ++__i)
{
// XXX rand() % N is not uniformly distributed
_RandomAccessIterator __j = __first
+ std::rand() % ((__i - __first) + 1);
if (__i != __j)
std::iter_swap(__i, __j);
}
if (__first == __last)
return;
#if RAND_MAX < __INT_MAX__
if (__builtin_expect((__last - __first) >= RAND_MAX / 4, 0))
{
// Use a xorshift implementation seeded by two calls to rand()
// instead of using rand() for all the random numbers needed.
unsigned __xss
= (unsigned)std::rand() ^ ((unsigned)std::rand() << 15);
for (_RandomAccessIterator __i = __first + 1; __i != __last; ++__i)
{
__xss += !__xss;
__xss ^= __xss << 13;
__xss ^= __xss >> 17;
__xss ^= __xss << 5;
_RandomAccessIterator __j = __first
+ (__xss % ((__i - __first) + 1));
if (__i != __j)
std::iter_swap(__i, __j);
}
return;
}
#endif
for (_RandomAccessIterator __i = __first + 1; __i != __last; ++__i)
{
// XXX rand() % N is not uniformly distributed
_RandomAccessIterator __j = __first
+ (std::rand() % ((__i - __first) + 1));
if (__i != __j)
std::iter_swap(__i, __j);
}
}
/**

24
libstdc++-v3/testsuite/25_algorithms/random_shuffle/88935.cc Normal file
View file

@ -0,0 +1,24 @@
// { dg-do run }
// { dg-options "-Wno-deprecated-declarations" }
// Bug 88935 std::random_shuffle does not work if the sequence
// is longer than RAND_MAX elements
#include <algorithm>
#include <vector>
#include <testsuite_hooks.h>
int main()
{
const std::size_t N = 30000;
std::vector<unsigned char> v(N, (unsigned char)0);
std::fill(v.begin() + (N / 5 * 4), v.end(), (unsigned char)1);
std::random_shuffle(v.begin(), v.end());
double sum = 0;
for (std::size_t i = 0; i < v.size(); ++i)
{
sum += v[i];
if (i > 0 && i % (N / 100) == 0)
VERIFY( (sum / i) < 0.3 );
}
}