[PATCH 7/8] libstdc++: Directly implement ranges::sample [PR100795]

[Jonathan Wakely]

On 26/06/25 22:25 -0400, Patrick Palka wrote:
>	PR libstdc++/100795

OK for trunk (the FIXME can stay for now and be dealt with later).

>
>libstdc++-v3/ChangeLog:
>
>	* include/bits/ranges_algo.h (__sample_fn::operator()):
>	Reimplement the forward_iterator branch directly.
>	* testsuite/25_algorithms/sample/constrained.cc (test02):
>	New test.
>---
> libstdc++-v3/include/bits/ranges_algo.h       | 70 +++++++++++++++++--
> .../25_algorithms/sample/constrained.cc       | 28 ++++++++
> 2 files changed, 91 insertions(+), 7 deletions(-)
>
>diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
>index b12da2af1263..672a0ebce0de 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..150e2d2036e0 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 +60,36 @@ 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);
>+  using cat = std::iterator_traits<std::ranges::iterator_t<type>>::iterator_category;
>+  static_assert( std::same_as<cat, std::output_iterator_tag> );
>+  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();
> }
>-- 
>2.50.0.131.gcf6f63ea6b
>
>