250 lines
8.6 KiB
C++
250 lines
8.6 KiB
C++
//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef _LIBCPP___ALGORITHM_NTH_ELEMENT_H
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#define _LIBCPP___ALGORITHM_NTH_ELEMENT_H
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#include <__algorithm/comp.h>
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#include <__algorithm/comp_ref_type.h>
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#include <__algorithm/sort.h>
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#include <__config>
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#include <__debug>
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#include <__iterator/iterator_traits.h>
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#include <__utility/swap.h>
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#if defined(_LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY)
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# include <__algorithm/shuffle.h>
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#endif
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
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#endif
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_LIBCPP_BEGIN_NAMESPACE_STD
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template<class _Compare, class _RandomAccessIterator>
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_LIBCPP_CONSTEXPR_AFTER_CXX11 bool
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__nth_element_find_guard(_RandomAccessIterator& __i, _RandomAccessIterator& __j,
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_RandomAccessIterator __m, _Compare __comp)
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{
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// manually guard downward moving __j against __i
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while (true) {
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if (__i == --__j) {
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return false;
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}
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if (__comp(*__j, *__m)) {
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return true; // found guard for downward moving __j, now use unguarded partition
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}
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}
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}
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template <class _Compare, class _RandomAccessIterator>
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_LIBCPP_CONSTEXPR_AFTER_CXX11 void
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__nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp)
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{
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// _Compare is known to be a reference type
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typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
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const difference_type __limit = 7;
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while (true)
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{
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if (__nth == __last)
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return;
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difference_type __len = __last - __first;
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switch (__len)
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{
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case 0:
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case 1:
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return;
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case 2:
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if (__comp(*--__last, *__first))
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swap(*__first, *__last);
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return;
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case 3:
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{
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_RandomAccessIterator __m = __first;
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_VSTD::__sort3<_Compare>(__first, ++__m, --__last, __comp);
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return;
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}
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}
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if (__len <= __limit)
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{
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_VSTD::__selection_sort<_Compare>(__first, __last, __comp);
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return;
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}
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// __len > __limit >= 3
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_RandomAccessIterator __m = __first + __len/2;
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_RandomAccessIterator __lm1 = __last;
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unsigned __n_swaps = _VSTD::__sort3<_Compare>(__first, __m, --__lm1, __comp);
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// *__m is median
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// partition [__first, __m) < *__m and *__m <= [__m, __last)
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// (this inhibits tossing elements equivalent to __m around unnecessarily)
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_RandomAccessIterator __i = __first;
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_RandomAccessIterator __j = __lm1;
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// j points beyond range to be tested, *__lm1 is known to be <= *__m
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// The search going up is known to be guarded but the search coming down isn't.
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// Prime the downward search with a guard.
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if (!__comp(*__i, *__m)) // if *__first == *__m
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{
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// *__first == *__m, *__first doesn't go in first part
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if (_VSTD::__nth_element_find_guard<_Compare>(__i, __j, __m, __comp)) {
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swap(*__i, *__j);
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++__n_swaps;
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} else {
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// *__first == *__m, *__m <= all other elements
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// Partition instead into [__first, __i) == *__first and *__first < [__i, __last)
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++__i; // __first + 1
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__j = __last;
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if (!__comp(*__first, *--__j)) { // we need a guard if *__first == *(__last-1)
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while (true) {
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if (__i == __j) {
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return; // [__first, __last) all equivalent elements
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} else if (__comp(*__first, *__i)) {
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swap(*__i, *__j);
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++__n_swaps;
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++__i;
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break;
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}
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++__i;
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}
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}
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// [__first, __i) == *__first and *__first < [__j, __last) and __j == __last - 1
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if (__i == __j) {
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return;
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}
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while (true) {
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while (!__comp(*__first, *__i))
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++__i;
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while (__comp(*__first, *--__j))
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;
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if (__i >= __j)
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break;
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swap(*__i, *__j);
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++__n_swaps;
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++__i;
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}
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// [__first, __i) == *__first and *__first < [__i, __last)
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// The first part is sorted,
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if (__nth < __i) {
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return;
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}
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// __nth_element the second part
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// _VSTD::__nth_element<_Compare>(__i, __nth, __last, __comp);
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__first = __i;
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continue;
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}
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}
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++__i;
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// j points beyond range to be tested, *__lm1 is known to be <= *__m
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// if not yet partitioned...
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if (__i < __j)
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{
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// known that *(__i - 1) < *__m
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while (true)
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{
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// __m still guards upward moving __i
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while (__comp(*__i, *__m))
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++__i;
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// It is now known that a guard exists for downward moving __j
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while (!__comp(*--__j, *__m))
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;
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if (__i >= __j)
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break;
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swap(*__i, *__j);
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++__n_swaps;
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// It is known that __m != __j
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// If __m just moved, follow it
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if (__m == __i)
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__m = __j;
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++__i;
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}
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}
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// [__first, __i) < *__m and *__m <= [__i, __last)
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if (__i != __m && __comp(*__m, *__i))
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{
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swap(*__i, *__m);
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++__n_swaps;
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}
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// [__first, __i) < *__i and *__i <= [__i+1, __last)
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if (__nth == __i)
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return;
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if (__n_swaps == 0)
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{
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// We were given a perfectly partitioned sequence. Coincidence?
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if (__nth < __i)
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{
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// Check for [__first, __i) already sorted
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__j = __m = __first;
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while (true) {
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if (++__j == __i) {
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// [__first, __i) sorted
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return;
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}
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if (__comp(*__j, *__m)) {
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// not yet sorted, so sort
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break;
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}
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__m = __j;
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}
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}
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else
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{
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// Check for [__i, __last) already sorted
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__j = __m = __i;
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while (true) {
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if (++__j == __last) {
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// [__i, __last) sorted
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return;
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}
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if (__comp(*__j, *__m)) {
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// not yet sorted, so sort
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break;
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}
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__m = __j;
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}
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}
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}
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// __nth_element on range containing __nth
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if (__nth < __i)
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{
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// _VSTD::__nth_element<_Compare>(__first, __nth, __i, __comp);
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__last = __i;
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}
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else
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{
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// _VSTD::__nth_element<_Compare>(__i+1, __nth, __last, __comp);
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__first = ++__i;
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}
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}
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}
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template <class _RandomAccessIterator, class _Compare>
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inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
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void
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nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp)
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{
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_LIBCPP_DEBUG_RANDOMIZE_RANGE(__first, __last);
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typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
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_VSTD::__nth_element<_Comp_ref>(__first, __nth, __last, __comp);
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_LIBCPP_DEBUG_RANDOMIZE_RANGE(__first, __nth);
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if (__nth != __last) {
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_LIBCPP_DEBUG_RANDOMIZE_RANGE(++__nth, __last);
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}
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}
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template <class _RandomAccessIterator>
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inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
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void
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nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last)
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{
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_VSTD::nth_element(__first, __nth, __last, __less<typename iterator_traits<_RandomAccessIterator>::value_type>());
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}
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___ALGORITHM_NTH_ELEMENT_H
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