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
24 template <class _AlgPolicy, class _Compare, class _RandomAccessIterator>
25 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void
__sift_down(_RandomAccessIterator __first,_Compare && __comp,typename iterator_traits<_RandomAccessIterator>::difference_type __len,_RandomAccessIterator __start)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
80 template <class _AlgPolicy, class _Compare, class _RandomAccessIterator>
81 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _RandomAccessIterator
__floyd_sift_down(_RandomAccessIterator __first,_Compare && __comp,typename iterator_traits<_RandomAccessIterator>::difference_type __len)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