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

[Zhikai Zeng via Phabricator via libcxx-commits]

Backl1ght created this revision.
Backl1ght added reviewers: philnik, ldionne.
Backl1ght added a project: libc++.
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for really small range, we don't need to sort the whole range, we only need to do partial sort, this can reduce some calculations.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D141614

Files:
  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 +26,20 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+// Assumes size > 0
+template <class _AlgPolicy, class _Compare, class _BidirectionalIterator>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void __selection_partial_sort(
+    _BidirectionalIterator __first, _BidirectionalIterator __nth, _BidirectionalIterator __last, _Compare __comp) {
+  _BidirectionalIterator __lm1 = __nth;
+  if (++__lm1 == __last)
+    --__lm1;
+  for (; __first != __lm1; ++__first) {
+    _BidirectionalIterator __i = std::__min_element<_Compare>(__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 +88,7 @@
         }
         if (__len <= __limit)
         {
-            std::__selection_sort<_AlgPolicy, _Compare>(__first, __last, __comp);
+            std::__selection_partial_sort<_AlgPolicy, _Compare>(__first, __nth, __last, __comp);
             return;
         }
         // __len > __limit >= 3


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