sift_down.h revision 1.1.1.1 
1//===----------------------------------------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#ifndef _LIBCPP___ALGORITHM_SIFT_DOWN_H
10#define _LIBCPP___ALGORITHM_SIFT_DOWN_H
11
12#include <__config>
13#include <__iterator/iterator_traits.h>
14#include <__utility/move.h>
15
16#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
17#pragma GCC system_header
18#endif
19
20_LIBCPP_PUSH_MACROS
21#include <__undef_macros>
22
23_LIBCPP_BEGIN_NAMESPACE_STD
24
25template <class _Compare, class _RandomAccessIterator>
26_LIBCPP_CONSTEXPR_AFTER_CXX11 void
27__sift_down(_RandomAccessIterator __first, _RandomAccessIterator /*__last*/,
28            _Compare __comp,
29            typename iterator_traits<_RandomAccessIterator>::difference_type __len,
30            _RandomAccessIterator __start)
31{
32    typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
33    typedef typename iterator_traits<_RandomAccessIterator>::value_type value_type;
34    // left-child of __start is at 2 * __start + 1
35    // right-child of __start is at 2 * __start + 2
36    difference_type __child = __start - __first;
37
38    if (__len < 2 || (__len - 2) / 2 < __child)
39        return;
40
41    __child = 2 * __child + 1;
42    _RandomAccessIterator __child_i = __first + __child;
43
44    if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + 1))) {
45        // right-child exists and is greater than left-child
46        ++__child_i;
47        ++__child;
48    }
49
50    // check if we are in heap-order
51    if (__comp(*__child_i, *__start))
52        // we are, __start is larger than it's largest child
53        return;
54
55    value_type __top(_VSTD::move(*__start));
56    do
57    {
58        // we are not in heap-order, swap the parent with its largest child
59        *__start = _VSTD::move(*__child_i);
60        __start = __child_i;
61
62        if ((__len - 2) / 2 < __child)
63            break;
64
65        // recompute the child based off of the updated parent
66        __child = 2 * __child + 1;
67        __child_i = __first + __child;
68
69        if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + 1))) {
70            // right-child exists and is greater than left-child
71            ++__child_i;
72            ++__child;
73        }
74
75        // check if we are in heap-order
76    } while (!__comp(*__child_i, __top));
77    *__start = _VSTD::move(__top);
78}
79
80_LIBCPP_END_NAMESPACE_STD
81
82_LIBCPP_POP_MACROS
83
84#endif // _LIBCPP___ALGORITHM_SIFT_DOWN_H
85