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368faacac7
This commit reverts 5aaefa51 (and also partly 7f285f48e77 and b6d75682f9, which were related to the original commit). As landed, 5aaefa51 had unintended consequences on some downstream bots and didn't have proper coverage upstream due to a few subtle things. Implementing this is something we should do in libc++, however we'll first need to address a few issues listed in https://reviews.llvm.org/D106124#3349710. Differential Revision: https://reviews.llvm.org/D120683
79 lines
2.5 KiB
C++
79 lines
2.5 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_SIFT_DOWN_H
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#define _LIBCPP___ALGORITHM_SIFT_DOWN_H
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#include <__config>
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#include <__iterator/iterator_traits.h>
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#include <__utility/move.h>
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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 void
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__sift_down(_RandomAccessIterator __first, _Compare __comp,
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typename iterator_traits<_RandomAccessIterator>::difference_type __len,
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_RandomAccessIterator __start)
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{
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typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
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typedef typename iterator_traits<_RandomAccessIterator>::value_type value_type;
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// left-child of __start is at 2 * __start + 1
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// right-child of __start is at 2 * __start + 2
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difference_type __child = __start - __first;
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if (__len < 2 || (__len - 2) / 2 < __child)
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return;
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__child = 2 * __child + 1;
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_RandomAccessIterator __child_i = __first + __child;
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if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + difference_type(1)))) {
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// right-child exists and is greater than left-child
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++__child_i;
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++__child;
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}
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// check if we are in heap-order
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if (__comp(*__child_i, *__start))
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// we are, __start is larger than its largest child
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return;
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value_type __top(_VSTD::move(*__start));
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do
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{
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// we are not in heap-order, swap the parent with its largest child
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*__start = _VSTD::move(*__child_i);
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__start = __child_i;
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if ((__len - 2) / 2 < __child)
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break;
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// recompute the child based off of the updated parent
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__child = 2 * __child + 1;
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__child_i = __first + __child;
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if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + difference_type(1)))) {
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// right-child exists and is greater than left-child
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++__child_i;
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++__child;
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}
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// check if we are in heap-order
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} while (!__comp(*__child_i, __top));
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*__start = _VSTD::move(__top);
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}
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___ALGORITHM_SIFT_DOWN_H
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