[libcxx-commits] [PATCH] D141614: [libc++] nth_element change to partial sort when range is really small

[Zhikai Zeng via Phabricator via libcxx-commits]

Backl1ght updated this revision to Diff 489235.
Backl1ght added a comment.

refine code


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D141614/new/

https://reviews.llvm.org/D141614

Files:
  libcxx/benchmarks/CMakeLists.txt
  libcxx/include/__algorithm/nth_element.h


Index: libcxx/include/__algorithm/nth_element.h
===================================================================
--- libcxx/include/__algorithm/nth_element.h
+++ libcxx/include/__algorithm/nth_element.h
@@ -12,6 +12,7 @@
 #include <__algorithm/comp.h>
 #include <__algorithm/comp_ref_type.h>
 #include <__algorithm/iterator_operations.h>
+#include <__algorithm/min_element.h>
 #include <__algorithm/sort.h>
 #include <__config>
 #include <__debug>
@@ -25,6 +26,25 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+// Implement partial sort base on selection sort. Rearranges elements such that the range [__first, __middle) contains
+// the sorted __middle − __first smallest elements in the range [first, last).
+// For selection sort, after each round, the position of element that __first represents is determined. That is, if we
+// end up when __first == __middle, [__first, __middle) contains the sorted __middle − __first smallest elements in the
+// range [first, last) already.
+template <class _AlgPolicy, class _Compare, class _BidirectionalIterator>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void __selection_partial_sort(
+    _BidirectionalIterator __first, _BidirectionalIterator __middle, _BidirectionalIterator __last, _Compare __comp) {
+  // if [__first, __last - 1) is determined, then [__first, __last) is determined.
+  if (__middle == __last)
+    --__middle;
+
+  for (; __first != __middle; ++__first) {
+    _BidirectionalIterator __i = std::__min_element(__first, __last, __comp);
+    if (__i != __first)
+      _IterOps<_AlgPolicy>::iter_swap(__first, __i);
+  }
+}
+
 template<class _Compare, class _RandomAccessIterator>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 bool
 __nth_element_find_guard(_RandomAccessIterator& __i, _RandomAccessIterator& __j,
@@ -73,7 +93,8 @@
         }
         if (__len <= __limit)
         {
-            std::__selection_sort<_AlgPolicy, _Compare>(__first, __last, __comp);
+            std::__selection_partial_sort<_AlgPolicy, _Compare>(__first, __nth + difference_type(1), __last, __comp);
+            // [__first, __nth) <= *__nth and *__nth <= (__nth, __last)
             return;
         }
         // __len > __limit >= 3
Index: libcxx/benchmarks/CMakeLists.txt
===================================================================
--- libcxx/benchmarks/CMakeLists.txt
+++ libcxx/benchmarks/CMakeLists.txt
@@ -162,6 +162,7 @@
     algorithms/make_heap.bench.cpp
     algorithms/make_heap_then_sort_heap.bench.cpp
     algorithms/min_max_element.bench.cpp
+    algorithms/nth_element.bench.cpp
     algorithms/pop_heap.bench.cpp
     algorithms/push_heap.bench.cpp
     algorithms/ranges_make_heap.bench.cpp


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