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/*
* Copyright (C) 2017 Apple Inc. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#pragma once
#include <wtf/MathExtras.h>
#include <wtf/StdLibExtras.h>
#include <wtf/Vector.h>
namespace WTF {
// This class implements a basic priority queue. The class is backed as a binary heap, like std::priority_queue.
// PriorityQueue has a couple of advantages over std::priority_queue:
//
// 1) The backing vector is fastMalloced.
// 2) You can iterate the elements.
// 3) It has in-place decrease/increaseKey methods, although they are still O(n) rather than O(log(n)).
template<typename T, bool (*isHigherPriority)(const T&, const T&) = &isLessThan<T>, size_t inlineCapacity = 0>
class PriorityQueue final {
WTF_MAKE_FAST_ALLOCATED;
using BufferType = Vector<T, inlineCapacity>;
using const_iterator = typename BufferType::const_iterator;
public:
size_t size() const { return m_buffer.size(); }
bool isEmpty() const { return m_buffer.isEmpty(); }
void enqueue(T element)
{
size_t location = m_buffer.size();
m_buffer.append(std::forward<T>(element));
siftUp(location);
}
const T& peek() const { return m_buffer[0]; }
T dequeue()
{
std::swap(m_buffer[0], m_buffer.last());
T result = m_buffer.takeLast();
siftDown(0);
return result;
}
void decreaseKey(NOESCAPE const Invocable<bool(T&)> auto& desiredElement)
{
for (size_t i = 0; i < m_buffer.size(); ++i) {
if (desiredElement(m_buffer[i])) {
siftDown(i);
return;
}
}
ASSERT(isValidHeap());
}
void increaseKey(NOESCAPE const Invocable<bool(T&)> auto& desiredElement)
{
for (size_t i = 0; i < m_buffer.size(); ++i) {
if (desiredElement(m_buffer[i])) {
siftUp(i);
return;
}
}
ASSERT(isValidHeap());
}
const_iterator begin() const { return m_buffer.begin(); };
const_iterator end() const { return m_buffer.end(); };
bool isValidHeap() const
{
for (size_t i = 0; i < m_buffer.size(); ++i) {
if (leftChildOf(i) < m_buffer.size() && !isHigherPriority(m_buffer[i], m_buffer[leftChildOf(i)]))
return false;
if (rightChildOf(i) < m_buffer.size() && !isHigherPriority(m_buffer[i], m_buffer[rightChildOf(i)]))
return false;
}
return true;
}
protected:
static inline size_t parentOf(size_t location) { ASSERT(location); return (location - 1) / 2; }
static constexpr size_t leftChildOf(size_t location) { return location * 2 + 1; }
static constexpr size_t rightChildOf(size_t location) { return leftChildOf(location) + 1; }
void siftUp(size_t location)
{
while (location) {
auto parent = parentOf(location);
if (isHigherPriority(m_buffer[parent], m_buffer[location]))
return;
std::swap(m_buffer[parent], m_buffer[location]);
location = parent;
}
}
void siftDown(size_t location)
{
while (leftChildOf(location) < m_buffer.size()) {
size_t higherPriorityChild;
if (LIKELY(rightChildOf(location) < m_buffer.size()))
higherPriorityChild = isHigherPriority(m_buffer[leftChildOf(location)], m_buffer[rightChildOf(location)]) ? leftChildOf(location) : rightChildOf(location);
else
higherPriorityChild = leftChildOf(location);
if (isHigherPriority(m_buffer[location], m_buffer[higherPriorityChild]))
return;
std::swap(m_buffer[location], m_buffer[higherPriorityChild]);
location = higherPriorityChild;
}
}
Vector<T, inlineCapacity> m_buffer;
};
} // namespace WTF
using WTF::PriorityQueue;
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