From: Patrick Palka <ppalka@redhat.com>
Date: Fri, 27 Jun 2025 17:53:34 +0000 (-0400)
Subject: libstdc++: Directly implement ranges::nth_element [PR100795]
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=d06373fa3483a1ccb182c2da7e6cf2330f48bac1;p=thirdparty%2Fgcc.git

libstdc++: Directly implement ranges::nth_element [PR100795]

	PR libstdc++/100795

libstdc++-v3/ChangeLog:

	* include/bits/ranges_algo.h (__detail::__introselect): New,
	based on the stl_algo.h implementation.
	(nth_element_fn::operator()): Reimplement in terms of the above.
	* testsuite/25_algorithms/nth_element/constrained.cc:

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
---

diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
index a214e03d7b88..4d10eaa7f83d 100644
--- a/libstdc++-v3/include/bits/ranges_algo.h
+++ b/libstdc++-v3/include/bits/ranges_algo.h
@@ -2805,6 +2805,33 @@ namespace ranges
 
   inline constexpr __is_sorted_fn is_sorted{};
 
+  namespace __detail
+  {
+    template<typename _Iter, typename _Comp>
+      constexpr void
+      __introselect(_Iter __first, _Iter __nth, _Iter __last,
+		    iter_difference_t<_Iter> __depth_limit, _Comp __comp)
+      {
+	while (__last - __first > 3)
+	  {
+	    if (__depth_limit == 0)
+	      {
+		__detail::__heap_select(__first, __nth + 1, __last, __comp);
+		// Place the nth largest element in its final position.
+		ranges::iter_swap(__first, __nth);
+		return;
+	      }
+	    --__depth_limit;
+	    _Iter __cut = __detail::__unguarded_partition_pivot(__first, __last, __comp);
+	    if (__cut <= __nth)
+	      __first = __cut;
+	    else
+	      __last = __cut;
+	  }
+	__detail::__insertion_sort(__first, __last, __comp);
+      }
+  } // namespace __detail
+
   struct __nth_element_fn
   {
     template<random_access_iterator _Iter, sentinel_for<_Iter> _Sent,
@@ -2814,11 +2841,21 @@ namespace ranges
       operator()(_Iter __first, _Iter __nth, _Sent __last,
 		 _Comp __comp = {}, _Proj __proj = {}) const
       {
-	auto __lasti = ranges::next(__first, __last);
-	_GLIBCXX_STD_A::nth_element(std::move(__first), std::move(__nth),
-				    __lasti,
-				    __detail::__make_comp_proj(__comp, __proj));
-	return __lasti;
+	if constexpr (!same_as<_Iter, _Sent>)
+	  return (*this)(__first, __nth, ranges::next(__first, __last),
+			 std::move(__comp), std::move(__proj));
+	else
+	  {
+	    if (__first == __last || __nth == __last)
+	      return __last;
+
+	    auto __comp_proj = __detail::__make_comp_proj(__comp, __proj);
+	    auto __n = __detail::__to_unsigned_like(__last - __first);
+	    __detail::__introselect(__first, __nth, __last,
+				    std::__bit_width(__n) * 2,
+				    __comp_proj);
+	    return __last;
+	  }
       }
 
     template<random_access_range _Range,
diff --git a/libstdc++-v3/testsuite/25_algorithms/nth_element/constrained.cc b/libstdc++-v3/testsuite/25_algorithms/nth_element/constrained.cc
index 3189b225610e..8cb1625dd4d5 100644
--- a/libstdc++-v3/testsuite/25_algorithms/nth_element/constrained.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/nth_element/constrained.cc
@@ -20,6 +20,7 @@
 
 #include <algorithm>
 #include <random>
+#include <ranges>
 #include <testsuite_hooks.h>
 #include <testsuite_iterators.h>
 
@@ -67,9 +68,39 @@ test02()
   return x[3] == 4;
 }
 
+constexpr bool
+test03()
+{
+  // PR libstdc++/100795 - ranges::sort should not use std::sort directly
+#if __SIZEOF_INT128__
+  auto v = std::views::iota(__int128(0), __int128(20));
+#else
+  auto v = std::views::iota(0ll, 20ll);
+#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::nth_element(w, w.begin() + 10);
+  VERIFY( w[10] == 10 );
+
+  ranges::nth_element(w, w.begin() + 5, std::ranges::greater{});
+  VERIFY( w[5] == 19 - 5 );
+
+  ranges::nth_element(w, w.begin() + 15, std::ranges::greater{}, std::negate{});
+  VERIFY( w[15] == 15 );
+
+  return true;
+}
+
 int
 main()
 {
   test01();
   static_assert(test02());
+  static_assert(test03());
 }