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_BEGIN_NAMESPACE_STD
23
24template <class _AlgPolicy, class _Compare, class _RandomAccessIterator>
25_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void
26__sift_down(_RandomAccessIterator __first, _Compare&& __comp,
27            typename iterator_traits<_RandomAccessIterator>::difference_type __len,
28            _RandomAccessIterator __start)
29{
30    using _Ops = _IterOps<_AlgPolicy>;
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 + difference_type(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 its largest child
53        return;
54
55    value_type __top(_Ops::__iter_move(__start));
56    do
57    {
58        // we are not in heap-order, swap the parent with its largest child
59        *__start = _Ops::__iter_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 + difference_type(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
80template <class _AlgPolicy, class _Compare, class _RandomAccessIterator>
81_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _RandomAccessIterator
82__floyd_sift_down(_RandomAccessIterator __first, _Compare&& __comp,
83                  typename iterator_traits<_RandomAccessIterator>::difference_type __len)
84{
85    using difference_type = typename iterator_traits<_RandomAccessIterator>::difference_type;
86    _LIBCPP_ASSERT(__len >= 2, "shouldn't be called unless __len >= 2");
87
88    _RandomAccessIterator __hole = __first;
89    _RandomAccessIterator __child_i = __first;
90    difference_type __child = 0;
91
92    while (true) {
93        __child_i += difference_type(__child + 1);
94        __child = 2 * __child + 1;
95
96        if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + difference_type(1)))) {
97            // right-child exists and is greater than left-child
98            ++__child_i;
99            ++__child;
100        }
101
102        // swap __hole with its largest child
103        *__hole = _IterOps<_AlgPolicy>::__iter_move(__child_i);
104        __hole = __child_i;
105
106        // if __hole is now a leaf, we're done
107        if (__child > (__len - 2) / 2)
108            return __hole;
109    }
110}
111
112_LIBCPP_END_NAMESPACE_STD
113
114#endif // _LIBCPP___ALGORITHM_SIFT_DOWN_H
115