//===----------------------------------------------------------------------===//
//
// 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 <__algorithm/comp.h>
#include <__algorithm/comp_ref_type.h>
#include <__algorithm/iterator_operations.h>
#include <__algorithm/sort.h>
#include <__assert>
#include <__config>
#include <__debug_utils/randomize_range.h>
#include <__iterator/iterator_traits.h>
#include <__utility/move.h>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
#endif
_LIBCPP_PUSH_MACROS
#include <__undef_macros>
_LIBCPP_BEGIN_NAMESPACE_STD
template <class _Compare, class _RandomAccessIterator>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 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 _AlgPolicy, class _Compare, class _RandomAccessIterator>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void
// NOLINTNEXTLINE(readability-function-cognitive-complexity)
__nth_element(
_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp) {
using _Ops = _IterOps<_AlgPolicy>;
// _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))
_Ops::iter_swap(__first, __last);
return;
case 3: {
_RandomAccessIterator __m = __first;
std::__sort3<_AlgPolicy, _Compare>(__first, ++__m, --__last, __comp);
return;
}
}
if (__len <= __limit) {
std::__selection_sort<_AlgPolicy, _Compare>(__first, __last, __comp);
return;
}
// __len > __limit >= 3
_RandomAccessIterator __m = __first + __len / 2;
_RandomAccessIterator __lm1 = __last;
unsigned __n_swaps = std::__sort3<_AlgPolicy, _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 (std::__nth_element_find_guard<_Compare>(__i, __j, __m, __comp)) {
_Ops::iter_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)) {
_Ops::iter_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;
_LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
__i != __last,
"Would read out of bounds, does your comparator satisfy the strict-weak ordering requirement?");
}
do {
_LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
__j != __first,
"Would read out of bounds, does your comparator satisfy the strict-weak ordering requirement?");
--__j;
} while (__comp(*__first, *__j));
if (__i >= __j)
break;
_Ops::iter_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
// std::__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;
_LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
__i != __last,
"Would read out of bounds, does your comparator satisfy the strict-weak ordering requirement?");
}
// It is now known that a guard exists for downward moving __j
do {
_LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
__j != __first,
"Would read out of bounds, does your comparator satisfy the strict-weak ordering requirement?");
--__j;
} while (!__comp(*__j, *__m));
if (__i >= __j)
break;
_Ops::iter_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)) {
_Ops::iter_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) {
// std::__nth_element<_Compare>(__first, __nth, __i, __comp);
__last = __i;
} else {
// std::__nth_element<_Compare>(__i+1, __nth, __last, __comp);
__first = ++__i;
}
}
}
template <class _AlgPolicy, class _RandomAccessIterator, class _Compare>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __nth_element_impl(
_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare& __comp) {
if (__nth == __last)
return;
std::__debug_randomize_range<_AlgPolicy>(__first, __last);
std::__nth_element<_AlgPolicy, __comp_ref_type<_Compare> >(__first, __nth, __last, __comp);
std::__debug_randomize_range<_AlgPolicy>(__first, __nth);
if (__nth != __last) {
std::__debug_randomize_range<_AlgPolicy>(++__nth, __last);
}
}
template <class _RandomAccessIterator, class _Compare>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp) {
std::__nth_element_impl<_ClassicAlgPolicy>(std::move(__first), std::move(__nth), std::move(__last), __comp);
}
template <class _RandomAccessIterator>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last) {
std::nth_element(std::move(__first), std::move(__nth), std::move(__last), __less<>());
}
_LIBCPP_END_NAMESPACE_STD
_LIBCPP_POP_MACROS
#endif // _LIBCPP___ALGORITHM_NTH_ELEMENT_H
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