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libstdc++: Fix division by zero in std::sample

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This fixes a division by zero in the selection-sampling std::__sample
overload when the input range is empty (and hence __unsampled_sz is 0).

libstdc++-v3/ChangeLog:

	* include/bits/stl_algo.h (__sample): Exit early when the
	input range is empty.
	* testsuite/25_algorithms/sample/3.cc: New test.
This commit is contained in:
Patrick Palka 2020-09-21 20:48:23 -04:00
parent f017952d31
commit 813ad9c4dd
2 changed files with 53 additions and 0 deletions
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3
libstdc++-v3/include/bits/stl_algo.h
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@ -5775,6 +5775,9 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
using _Gen = remove_reference_t<_UniformRandomBitGenerator>;
using __uc_type = common_type_t<typename _Gen::result_type, _USize>;
if (__first == __last)
return __out;
__distrib_type __d{};
_Size __unsampled_sz = std::distance(__first, __last);
__n = std::min(__n, __unsampled_sz);

50
libstdc++-v3/testsuite/25_algorithms/sample/3.cc Normal file
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@ -0,0 +1,50 @@
// Copyright (C) 2020 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library. This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// You should have received a copy of the GNU General Public License along
// with this library; see the file COPYING3. If not see
// <http://www.gnu.org/licenses/>.
// { dg-options "-std=gnu++17" }
// { dg-do run { target c++17 } }
// { dg-require-cstdint "" }
#include <algorithm>
#include <random>
#include <testsuite_hooks.h>
#include <testsuite_iterators.h>
std::mt19937 rng;
using std::sample;
using __gnu_test::test_container;
using __gnu_test::output_iterator_wrapper;
using __gnu_test::forward_iterator_wrapper;
void
test01()
{
const int in = 0;
test_container<const int, forward_iterator_wrapper> pop(&in, &in);
int out;
test_container<int, output_iterator_wrapper> samp(&out, &out + 1);
auto it = sample(pop.begin(), pop.end(), samp.begin(), 1, rng);
VERIFY( it.ptr == &out );
}
int
main()
{
test01();
}