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
27 template <class _AlgPolicy, class _Compare, class _RandomAccessIterator>
28 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void
__sift_down(_RandomAccessIterator __first,_Compare && __comp,typename iterator_traits<_RandomAccessIterator>::difference_type __len,_RandomAccessIterator __start)29 __sift_down(_RandomAccessIterator __first, _Compare&& __comp,
30 typename iterator_traits<_RandomAccessIterator>::difference_type __len,
31 _RandomAccessIterator __start)
32 {
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 {
61 // we are not in heap-order, swap the parent with its largest child
62 *__start = _Ops::__iter_move(__child_i);
63 __start = __child_i;
64
65 if ((__len - 2) / 2 < __child)
66 break;
67
68 // recompute the child based off of the updated parent
69 __child = 2 * __child + 1;
70 __child_i = __first + __child;
71
72 if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + difference_type(1)))) {
73 // right-child exists and is greater than left-child
74 ++__child_i;
75 ++__child;
76 }
77
78 // check if we are in heap-order
79 } while (!__comp(*__child_i, __top));
80 *__start = _VSTD::move(__top);
81 }
82
83 template <class _AlgPolicy, class _Compare, class _RandomAccessIterator>
84 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _RandomAccessIterator
__floyd_sift_down(_RandomAccessIterator __first,_Compare && __comp,typename iterator_traits<_RandomAccessIterator>::difference_type __len)85 __floyd_sift_down(_RandomAccessIterator __first, _Compare&& __comp,
86 typename iterator_traits<_RandomAccessIterator>::difference_type __len)
87 {
88 using difference_type = typename iterator_traits<_RandomAccessIterator>::difference_type;
89 _LIBCPP_ASSERT_UNCATEGORIZED(__len >= 2, "shouldn't be called unless __len >= 2");
90
91 _RandomAccessIterator __hole = __first;
92 _RandomAccessIterator __child_i = __first;
93 difference_type __child = 0;
94
95 while (true) {
96 __child_i += difference_type(__child + 1);
97 __child = 2 * __child + 1;
98
99 if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + difference_type(1)))) {
100 // right-child exists and is greater than left-child
101 ++__child_i;
102 ++__child;
103 }
104
105 // swap __hole with its largest child
106 *__hole = _IterOps<_AlgPolicy>::__iter_move(__child_i);
107 __hole = __child_i;
108
109 // if __hole is now a leaf, we're done
110 if (__child > (__len - 2) / 2)
111 return __hole;
112 }
113 }
114
115 _LIBCPP_END_NAMESPACE_STD
116
117 _LIBCPP_POP_MACROS
118
119 #endif // _LIBCPP___ALGORITHM_SIFT_DOWN_H
120