libcxx/include/__algorithm/sift_down.h

//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef _LIBCPP___ALGORITHM_SIFT_DOWN_H
#define _LIBCPP___ALGORITHM_SIFT_DOWN_H

#include <__config>
#include <__iterator/iterator_traits.h>
#include <__utility/move.h>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif

_LIBCPP_BEGIN_NAMESPACE_STD

template <class _Compare, class _RandomAccessIterator>
_LIBCPP_CONSTEXPR_AFTER_CXX11 void
__sift_down(_RandomAccessIterator __first, _RandomAccessIterator /*__last*/,
            _Compare __comp,
            typename iterator_traits<_RandomAccessIterator>::difference_type __len,
            _RandomAccessIterator __start)
{
    typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
    typedef typename iterator_traits<_RandomAccessIterator>::value_type value_type;
    // left-child of __start is at 2 * __start + 1
    // right-child of __start is at 2 * __start + 2
    difference_type __child = __start - __first;

    if (__len < 2 || (__len - 2) / 2 < __child)
        return;

    __child = 2 * __child + 1;
    _RandomAccessIterator __child_i = __first + __child;

    if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + difference_type(1)))) {
        // right-child exists and is greater than left-child
        ++__child_i;
        ++__child;
    }

    // check if we are in heap-order
    if (__comp(*__child_i, *__start))
        // we are, __start is larger than it's largest child
        return;

    value_type __top(_VSTD::move(*__start));
    do
    {
        // we are not in heap-order, swap the parent with its largest child
        *__start = _VSTD::move(*__child_i);
        __start = __child_i;

        if ((__len - 2) / 2 < __child)
            break;

        // recompute the child based off of the updated parent
        __child = 2 * __child + 1;
        __child_i = __first + __child;

        if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + difference_type(1)))) {
            // right-child exists and is greater than left-child
            ++__child_i;
            ++__child;
        }

        // check if we are in heap-order
    } while (!__comp(*__child_i, __top));
    *__start = _VSTD::move(__top);
}

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP___ALGORITHM_SIFT_DOWN_H