[libc++][ranges] Implement ranges::nth_element.

Differential Revision: https://reviews.llvm.org/D128149

libcxx/docs/Status/RangesAlgorithms.csv

@@ -76,7 +76,7 @@ Permutation,stable_partition,Not assigned,n/a,Not started
 Permutation,sort,Konstantin Varlamov,`D127557 <https://llvm.org/D127557>`_,✅
 Permutation,stable_sort,Konstantin Varlamov,`D127834 <https://llvm.org/D127834>`_,✅
 Permutation,partial_sort,Konstantin Varlamov,n/a,In progress
-Permutation,nth_element,Not assigned,n/a,Not started
+Permutation,nth_element,Konstantin Varlamov,`D128149 <https://llvm.org/D128149>`_,✅
 Permutation,inplace_merge,Not assigned,n/a,Not started
 Permutation,make_heap,Not assigned,n/a,Not started
 Permutation,push_heap,Not assigned,n/a,Not started

libcxx/include/CMakeLists.txt

@@ -103,6 +103,7 @@ set(files
   __algorithm/ranges_move.h
   __algorithm/ranges_move_backward.h
   __algorithm/ranges_none_of.h
+  __algorithm/ranges_nth_element.h
   __algorithm/ranges_remove.h
   __algorithm/ranges_remove_if.h
   __algorithm/ranges_replace.h

libcxx/include/__algorithm/nth_element.h

@@ -16,6 +16,7 @@
 #include <__debug>
 #include <__debug_utils/randomize_range.h>
 #include <__iterator/iterator_traits.h>
+#include <__utility/move.h>
 #include <__utility/swap.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -220,25 +221,35 @@ __nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _Rando
 }
 
 template <class _RandomAccessIterator, class _Compare>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-void
-nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp)
-{
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
+void __nth_element_impl(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last,
+                        _Compare& __comp) {
+  if (__nth == __last)
+    return;
+
   std::__debug_randomize_range(__first, __last);
-  typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
-  _VSTD::__nth_element<_Comp_ref>(__first, __nth, __last, __comp);
+
+  using _Comp_ref = typename __comp_ref_type<_Compare>::type;
+  std::__nth_element<_Comp_ref>(__first, __nth, __last, __comp);
+
   std::__debug_randomize_range(__first, __nth);
   if (__nth != __last) {
     std::__debug_randomize_range(++__nth, __last);
   }
 }
 
+template <class _RandomAccessIterator, class _Compare>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
+void nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last,
+                 _Compare __comp) {
+  std::__nth_element_impl(std::move(__first), std::move(__nth), std::move(__last), __comp);
+}
+
 template <class _RandomAccessIterator>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-void
-nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last)
-{
-    _VSTD::nth_element(__first, __nth, __last, __less<typename iterator_traits<_RandomAccessIterator>::value_type>());
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
+void nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last) {
+  std::nth_element(std::move(__first), std::move(__nth), std::move(__last), __less<typename
+      iterator_traits<_RandomAccessIterator>::value_type>());
 }
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/include/__algorithm/ranges_nth_element.h

@@ -0,0 +1,79 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ALGORITHM_RANGES_NTH_ELEMENT_H
+#define _LIBCPP___ALGORITHM_RANGES_NTH_ELEMENT_H
+
+#include <__algorithm/make_projected.h>
+#include <__algorithm/nth_element.h>
+#include <__config>
+#include <__functional/identity.h>
+#include <__functional/invoke.h>
+#include <__functional/ranges_operations.h>
+#include <__iterator/concepts.h>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/next.h>
+#include <__iterator/projected.h>
+#include <__iterator/sortable.h>
+#include <__ranges/access.h>
+#include <__ranges/concepts.h>
+#include <__ranges/dangling.h>
+#include <__utility/forward.h>
+#include <__utility/move.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace ranges {
+namespace __nth_element {
+
+struct __fn {
+  template <class _Iter, class _Sent, class _Comp, class _Proj>
+  _LIBCPP_HIDE_FROM_ABI constexpr static
+  _Iter __nth_element_fn_impl(_Iter __first, _Iter __nth, _Sent __last, _Comp& __comp, _Proj& __proj) {
+    auto __last_iter = ranges::next(__first, __last);
+
+    auto&& __projected_comp = ranges::__make_projected_comp(__comp, __proj);
+    std::__nth_element_impl(std::move(__first), std::move(__nth), __last_iter, __projected_comp);
+
+    return __last_iter;
+  }
+
+  template <random_access_iterator _Iter, sentinel_for<_Iter> _Sent, class _Comp = ranges::less, class _Proj = identity>
+    requires sortable<_Iter, _Comp, _Proj>
+  _LIBCPP_HIDE_FROM_ABI constexpr
+  _Iter operator()(_Iter __first, _Iter __nth, _Sent __last, _Comp __comp = {}, _Proj __proj = {}) const {
+    return __nth_element_fn_impl(std::move(__first), std::move(__nth), std::move(__last), __comp, __proj);
+  }
+
+  template <random_access_range _Range, class _Comp = ranges::less, class _Proj = identity>
+    requires sortable<iterator_t<_Range>, _Comp, _Proj>
+  _LIBCPP_HIDE_FROM_ABI constexpr
+  borrowed_iterator_t<_Range> operator()(_Range&& __r, iterator_t<_Range> __nth, _Comp __comp = {},
+                                         _Proj __proj = {}) const {
+    return __nth_element_fn_impl(ranges::begin(__r), std::move(__nth), ranges::end(__r), __comp, __proj);
+  }
+};
+
+} // namespace __nth_element
+
+inline namespace __cpo {
+  inline constexpr auto nth_element = __nth_element::__fn{};
+} // namespace __cpo
+} // namespace ranges
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+#endif // _LIBCPP___ALGORITHM_RANGES_NTH_ELEMENT_H

libcxx/include/algorithm

@@ -379,6 +379,17 @@ namespace ranges {
     constexpr borrowed_iterator_t<R>
       ranges::is_sorted_until(R&& r, Comp comp = {}, Proj proj = {});                       // since C++20
 
+  template<random_access_iterator I, sentinel_for<I> S, class Comp = ranges::less,
+          class Proj = identity>
+    requires sortable<I, Comp, Proj>
+    constexpr I
+      ranges::nth_element(I first, I nth, S last, Comp comp = {}, Proj proj = {});            // since C++20
+
+  template<random_access_range R, class Comp = ranges::less, class Proj = identity>
+    requires sortable<iterator_t<R>, Comp, Proj>
+    constexpr borrowed_iterator_t<R>
+      ranges::nth_element(R&& r, iterator_t<R> nth, Comp comp = {}, Proj proj = {});          // since C++20
+
   template<forward_iterator I, sentinel_for<I> S, class T, class Proj = identity,
            indirect_strict_weak_order<const T*, projected<I, Proj>> Comp = ranges::less>
     constexpr I upper_bound(I first, S last, const T& value, Comp comp = {}, Proj proj = {}); // since C++20
@@ -1313,6 +1324,7 @@ template <class BidirectionalIterator, class Compare>
 #include <__algorithm/ranges_move.h>
 #include <__algorithm/ranges_move_backward.h>
 #include <__algorithm/ranges_none_of.h>
+#include <__algorithm/ranges_nth_element.h>
 #include <__algorithm/ranges_remove.h>
 #include <__algorithm/ranges_remove_if.h>
 #include <__algorithm/ranges_replace.h>

libcxx/include/module.modulemap.in

@@ -342,6 +342,7 @@ module std [system] {
       module ranges_move                     { private header "__algorithm/ranges_move.h" }
       module ranges_move_backward            { private header "__algorithm/ranges_move_backward.h" }
       module ranges_none_of                  { private header "__algorithm/ranges_none_of.h" }
+      module ranges_nth_element              { private header "__algorithm/ranges_nth_element.h" }
       module ranges_remove                   { private header "__algorithm/ranges_remove.h" }
       module ranges_remove_if                { private header "__algorithm/ranges_remove_if.h" }
       module ranges_replace                  { private header "__algorithm/ranges_replace.h" }

libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp

@@ -169,8 +169,8 @@ constexpr bool all_the_algorithms()
     //(void)std::ranges::next_permutation(a, Less(&copies)); assert(copies == 0);
     (void)std::ranges::none_of(first, last, UnaryTrue(&copies)); assert(copies == 0);
     (void)std::ranges::none_of(a, UnaryTrue(&copies)); assert(copies == 0);
-    //(void)std::ranges::nth_element(first, mid, last, Less(&copies)); assert(copies == 0);
-    //(void)std::ranges::nth_element(a, mid, Less(&copies)); assert(copies == 0);
+    (void)std::ranges::nth_element(first, mid, last, Less(&copies)); assert(copies == 0);
+    (void)std::ranges::nth_element(a, mid, Less(&copies)); assert(copies == 0);
     //(void)std::ranges::partial_sort(first, mid, last, Less(&copies)); assert(copies == 0);
     //(void)std::ranges::partial_sort(a, mid, Less(&copies)); assert(copies == 0);
     //(void)std::ranges::partial_sort_copy(first, last, first2, mid2, Less(&copies)); assert(copies == 0);

libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp

@@ -152,8 +152,8 @@ constexpr bool all_the_algorithms()
     //(void)std::ranges::next_permutation(a, Less(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::none_of(first, last, UnaryTrue(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::none_of(a, UnaryTrue(), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::nth_element(first, mid, last, Less(), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::nth_element(a, mid, Less(), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::nth_element(first, mid, last, Less(), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::nth_element(a, mid, Less(), Proj(&copies)); assert(copies == 0);
     //(void)std::ranges::partial_sort(first, mid, last, Less(), Proj(&copies)); assert(copies == 0);
     //(void)std::ranges::partial_sort(a, mid, Less(), Proj(&copies)); assert(copies == 0);
     //(void)std::ranges::partial_sort_copy(first, last, first2, mid2, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);

libcxx/test/libcxx/private_headers.verify.cpp

@@ -140,6 +140,7 @@ END-SCRIPT
 #include <__algorithm/ranges_move.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_move.h'}}
 #include <__algorithm/ranges_move_backward.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_move_backward.h'}}
 #include <__algorithm/ranges_none_of.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_none_of.h'}}
+#include <__algorithm/ranges_nth_element.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_nth_element.h'}}
 #include <__algorithm/ranges_remove.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_remove.h'}}
 #include <__algorithm/ranges_remove_if.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_remove_if.h'}}
 #include <__algorithm/ranges_replace.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_replace.h'}}