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 <__algorithm/iterator_operations.h>
13#include <__assert>
14#include <__config>
15#include <__iterator/iterator_traits.h>
16#include <__utility/move.h>
17
18#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
19#  pragma GCC system_header
20#endif
21
22_LIBCPP_PUSH_MACROS
23#include <__undef_macros>
24
25_LIBCPP_BEGIN_NAMESPACE_STD
26
27template <class _AlgPolicy, class _Compare, class _RandomAccessIterator>
28_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void
29__sift_down(_RandomAccessIterator __first,
30            _Compare&& __comp,
31            typename iterator_traits<_RandomAccessIterator>::difference_type __len,
32            _RandomAccessIterator __start) {
33  using _Ops = _IterOps<_AlgPolicy>;
34
35  typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
36  typedef typename iterator_traits<_RandomAccessIterator>::value_type value_type;
37  // left-child of __start is at 2 * __start + 1
38  // right-child of __start is at 2 * __start + 2
39  difference_type __child = __start - __first;
40
41  if (__len < 2 || (__len - 2) / 2 < __child)
42    return;
43
44  __child                         = 2 * __child + 1;
45  _RandomAccessIterator __child_i = __first + __child;
46
47  if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + difference_type(1)))) {
48    // right-child exists and is greater than left-child
49    ++__child_i;
50    ++__child;
51  }
52
53  // check if we are in heap-order
54  if (__comp(*__child_i, *__start))
55    // we are, __start is larger than its largest child
56    return;
57
58  value_type __top(_Ops::__iter_move(__start));
59  do {
60    // we are not in heap-order, swap the parent with its largest child
61    *__start = _Ops::__iter_move(__child_i);
62    __start  = __child_i;
63
64    if ((__len - 2) / 2 < __child)
65      break;
66
67    // recompute the child based off of the updated parent
68    __child   = 2 * __child + 1;
69    __child_i = __first + __child;
70
71    if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + difference_type(1)))) {
72      // right-child exists and is greater than left-child
73      ++__child_i;
74      ++__child;
75    }
76
77    // check if we are in heap-order
78  } while (!__comp(*__child_i, __top));
79  *__start = std::move(__top);
80}
81
82template <class _AlgPolicy, class _Compare, class _RandomAccessIterator>
83_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _RandomAccessIterator __floyd_sift_down(
84    _RandomAccessIterator __first,
85    _Compare&& __comp,
86    typename iterator_traits<_RandomAccessIterator>::difference_type __len) {
87  using difference_type = typename iterator_traits<_RandomAccessIterator>::difference_type;
88  _LIBCPP_ASSERT_INTERNAL(__len >= 2, "shouldn't be called unless __len >= 2");
89
90  _RandomAccessIterator __hole    = __first;
91  _RandomAccessIterator __child_i = __first;
92  difference_type __child         = 0;
93
94  while (true) {
95    __child_i += difference_type(__child + 1);
96    __child = 2 * __child + 1;
97
98    if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + difference_type(1)))) {
99      // right-child exists and is greater than left-child
100      ++__child_i;
101      ++__child;
102    }
103
104    // swap __hole with its largest child
105    *__hole = _IterOps<_AlgPolicy>::__iter_move(__child_i);
106    __hole  = __child_i;
107
108    // if __hole is now a leaf, we're done
109    if (__child > (__len - 2) / 2)
110      return __hole;
111  }
112}
113
114_LIBCPP_END_NAMESPACE_STD
115
116_LIBCPP_POP_MACROS
117
118#endif // _LIBCPP___ALGORITHM_SIFT_DOWN_H
119