[gcc r16-1746] libstdc++: Directly implement ranges::sample [PR100795]

[Patrick Palka]

https://gcc.gnu.org/g:4fc387e2f6990986d72b023ee44b4e0030903247

commit r16-1746-g4fc387e2f6990986d72b023ee44b4e0030903247
Author: Patrick Palka <ppalka@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 <jwakely@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();
 }