[PATCH 6/8] libstdc++: Directly implement ranges::nth_element [PR100795]

[Jonathan Wakely]

On 26/06/25 22:25 -0400, Patrick Palka wrote:
>	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:

OK for trunk.


>---
> libstdc++-v3/include/bits/ranges_algo.h       | 47 +++++++++++++++++--
> .../25_algorithms/nth_element/constrained.cc  | 31 ++++++++++++
> 2 files changed, 73 insertions(+), 5 deletions(-)
>
>diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
>index a9924cd9c49e..b12da2af1263 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());
> }
>-- 
>2.50.0.131.gcf6f63ea6b
>
>