https://gcc.gnu.org/g:4fc387e2f6990986d72b023ee44b4e0030903247
commit r16-1746-g4fc387e2f6990986d72b023ee44b4e0030903247
Author: Patrick Palka <ppa...@redhat.com>
Date: Fri Jun 27 13:53:37 2025 -0400
libstdc++: Directly implement ranges::sample [PR100795]
PR libstdc++/100795
libstdc++-v3/ChangeLog:
* include/bits/ranges_algo.h (__sample_fn::operator()):
Reimplement the forward_iterator branch directly, based
on the stl_algo.h implementation. Add explicit cast to
_Out's difference_type in the !forward_iterator branch.
* testsuite/25_algorithms/sample/constrained.cc (test02):
New test.
Reviewed-by: Jonathan Wakely <jwak...@redhat.com>
Diff:
---
libstdc++-v3/include/bits/ranges_algo.h | 70 +++++++++++++++++++---
.../testsuite/25_algorithms/sample/constrained.cc | 26 ++++++++
2 files changed, 89 insertions(+), 7 deletions(-)
diff --git a/libstdc++-v3/include/bits/ranges_algo.h
b/libstdc++-v3/include/bits/ranges_algo.h
index 4d10eaa7f83d..972d18cbdf06 100644
--- a/libstdc++-v3/include/bits/ranges_algo.h
+++ b/libstdc++-v3/include/bits/ranges_algo.h
@@ -1839,14 +1839,70 @@ namespace ranges
operator()(_Iter __first, _Sent __last, _Out __out,
iter_difference_t<_Iter> __n, _Gen&& __g) const
{
+ // FIXME: Correctly handle integer-class difference types.
if constexpr (forward_iterator<_Iter>)
{
- // FIXME: Forwarding to std::sample here requires computing __lasti
- // which may take linear time.
- auto __lasti = ranges::next(__first, __last);
- return _GLIBCXX_STD_A::
- sample(std::move(__first), std::move(__lasti), std::move(__out),
- __n, std::forward<_Gen>(__g));
+ using _Size = iter_difference_t<_Iter>;
+ using __distrib_type = uniform_int_distribution<_Size>;
+ using __param_type = typename __distrib_type::param_type;
+ using _USize = __detail::__make_unsigned_like_t<_Size>;
+ using __uc_type
+ = common_type_t<typename remove_reference_t<_Gen>::result_type,
_USize>;
+
+ if (__first == __last)
+ return __out;
+
+ __distrib_type __d{};
+ _Size __unsampled_sz = ranges::distance(__first, __last);
+ __n = std::min(__n, __unsampled_sz);
+
+ // If possible, we use __gen_two_uniform_ints to efficiently produce
+ // two random numbers using a single distribution invocation:
+
+ const __uc_type __urngrange = __g.max() - __g.min();
+ if (__urngrange / __uc_type(__unsampled_sz) >=
__uc_type(__unsampled_sz))
+ // I.e. (__urngrange >= __unsampled_sz * __unsampled_sz) but
without
+ // wrapping issues.
+ {
+ while (__n != 0 && __unsampled_sz >= 2)
+ {
+ const pair<_Size, _Size> __p =
+ __gen_two_uniform_ints(__unsampled_sz, __unsampled_sz -
1, __g);
+
+ --__unsampled_sz;
+ if (__p.first < __n)
+ {
+ *__out = *__first;
+ ++__out;
+ --__n;
+ }
+
+ ++__first;
+
+ if (__n == 0) break;
+
+ --__unsampled_sz;
+ if (__p.second < __n)
+ {
+ *__out = *__first;
+ ++__out;
+ --__n;
+ }
+
+ ++__first;
+ }
+ }
+
+ // The loop above is otherwise equivalent to this one-at-a-time
version:
+
+ for (; __n != 0; ++__first)
+ if (__d(__g, __param_type{0, --__unsampled_sz}) < __n)
+ {
+ *__out = *__first;
+ ++__out;
+ --__n;
+ }
+ return __out;
}
else
{
@@ -1867,7 +1923,7 @@ namespace ranges
if (__k < __n)
__out[__k] = *__first;
}
- return __out + __sample_sz;
+ return __out + iter_difference_t<_Out>(__sample_sz);
}
}
diff --git a/libstdc++-v3/testsuite/25_algorithms/sample/constrained.cc
b/libstdc++-v3/testsuite/25_algorithms/sample/constrained.cc
index b9945b164903..0ec3e0bdddd0 100644
--- a/libstdc++-v3/testsuite/25_algorithms/sample/constrained.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/sample/constrained.cc
@@ -20,6 +20,7 @@
#include <algorithm>
#include <random>
+#include <ranges>
#include <testsuite_hooks.h>
#include <testsuite_iterators.h>
@@ -59,9 +60,34 @@ test01()
}
}
+void
+test02()
+{
+ // PR libstdc++/100795 - ranges::sample should not use std::sample
+#if 0 // FIXME: ranges::sample rejects integer-class difference types.
+#if __SIZEOF_INT128__
+ auto v = std::views::iota(__int128(0), __int128(20));
+#else
+ auto v = std::views::iota(0ll, 20ll);
+#endif
+#else
+ auto v = std::views::iota(0, 20);
+#endif
+
+ int storage[20] = {2,5,4,3,1,6,7,9,10,8,11,14,12,13,15,16,18,0,19,17};
+ auto w = v | std::views::transform([&](auto i) -> int& { return storage[i];
});
+ using type = decltype(w);
+ static_assert( std::ranges::random_access_range<type> );
+
+ ranges::sample(v, w.begin(), 20, rng);
+ ranges::sort(w);
+ VERIFY( ranges::equal(w, v) );
+}
+
int
main()
{
test01<forward_iterator_wrapper, output_iterator_wrapper>();
test01<input_iterator_wrapper, random_access_iterator_wrapper>();
+ test02();
}
