libcxx/include/__algorithm/sift_down.h

   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
  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
  82 template <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