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