mirror of https://gitee.com/bigwinds/arangodb
397 lines
14 KiB
C++
397 lines
14 KiB
C++
////////////////////////////////////////////////////////////////////////////////
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/// DISCLAIMER
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///
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/// Copyright 2014-2016 ArangoDB GmbH, Cologne, Germany
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/// Copyright 2004-2014 triAGENS GmbH, Cologne, Germany
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///
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/// Licensed under the Apache License, Version 2.0 (the "License");
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/// you may not use this file except in compliance with the License.
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/// You may obtain a copy of the License at
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///
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/// http://www.apache.org/licenses/LICENSE-2.0
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///
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/// Unless required by applicable law or agreed to in writing, software
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/// distributed under the License is distributed on an "AS IS" BASIS,
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/// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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/// See the License for the specific language governing permissions and
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/// limitations under the License.
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///
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/// Copyright holder is ArangoDB GmbH, Cologne, Germany
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///
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/// @author Michael Hackstein
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////////////////////////////////////////////////////////////////////////////////
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#ifndef ARANGODB_GRAPH_SHORTEST_PATH_PRIORITY_QUEUE_H
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#define ARANGODB_GRAPH_SHORTEST_PATH_PRIORITY_QUEUE_H 1
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#include "Basics/Common.h"
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namespace arangodb {
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namespace graph {
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template <typename Key, typename Value, typename Weight>
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class ShortestPathPriorityQueue {
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// This class implements a data structure that is a key/value
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// store with the additional property that every Value has a
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// positive Weight (provided by the weight() and setWeight(w)
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// methods), which is a numerical type, and for which operator<
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// is defined. With respect to this weight the data structure
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// is at the same time a priority queue in that it is possible
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// to ask for (one of) the value(s) with the smallest weight and
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// remove this efficiently.
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// All methods only work with pointers to Values for efficiency
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// reasons. This class deletes all Value* that are stored on
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// destruction.
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// The Value type must have a method getKey that returns a Key
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// const&.
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// This data structure makes the following complexity promises
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// (amortized), where n is the number of key/value pairs stored
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// in the queue:
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// insert: O(log(n)) (but see below)
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// lookup value by key: O(1)
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// get smallest: O(1)
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// get and erase smallest: O(log(n)) (but see below)
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// lower weight by key O(log(n)) (but see below)
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// Additionally, if we only ever insert pairs whose value is not
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// smaller than any other value that is already in the structure,
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// and if we do not use lower weight by key, then we even get:
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// insert: O(1)
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// get and erase smallest: O(1)
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// With the "get and erase smallest" operation one has the option
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// of retaining the erased value in the key/value store. It can then
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// still be looked up but will no longer be considered for the
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// priority queue.
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public:
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ShortestPathPriorityQueue() : _popped(0), _isHeap(false), _maxWeight(0) {}
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~ShortestPathPriorityQueue() {
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for (Value* v : _heap) {
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delete v;
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}
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for (Value* v : _history) {
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delete v;
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}
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}
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//////////////////////////////////////////////////////////////////////////////
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/// @brief empty
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//////////////////////////////////////////////////////////////////////////////
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bool empty() { return _heap.empty(); }
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//////////////////////////////////////////////////////////////////////////////
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/// @brief size
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//////////////////////////////////////////////////////////////////////////////
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size_t size() { return _heap.size(); }
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//////////////////////////////////////////////////////////////////////////////
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/// @brief insert, data will be copied, returns true, if the key did not
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/// yet exist, and false, in which case nothing else is changed.
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//////////////////////////////////////////////////////////////////////////////
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bool insert(Key const& k, Value* v) {
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auto it = _lookup.find(k);
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if (it != _lookup.end()) {
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return false;
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}
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// Are we still in the simple case of a deque?
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if (!_isHeap) {
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Weight w = v->weight();
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if (w < _maxWeight) {
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// Oh dear, we have to upgrade to heap:
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_isHeap = true;
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// fall through intentionally
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} else {
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if (w > _maxWeight) {
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_maxWeight = w;
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}
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_heap.push_back(v);
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try {
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_lookup.insert(std::make_pair(k, static_cast<ssize_t>(_heap.size() - 1 + _popped)));
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} catch (...) {
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_heap.pop_back();
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throw;
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}
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return true;
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}
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}
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// If we get here, we have to insert into a proper binary heap:
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_heap.push_back(v);
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try {
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size_t newpos = _heap.size() - 1;
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_lookup.insert(std::make_pair(k, static_cast<ssize_t>(newpos + _popped)));
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repairUp(newpos);
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} catch (...) {
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_heap.pop_back();
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throw;
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}
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return true;
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}
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//////////////////////////////////////////////////////////////////////////////
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/// @brief find, note that the resulting pointer is only valid until the
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/// the next modification of the data structure happens (insert or lowerWeight
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/// or popMinimal). The weight in the Value type must not be modified other
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/// than via lowerWeight, otherwise the queue order could be violated.
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//////////////////////////////////////////////////////////////////////////////
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Value* find(Key const& k) const {
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auto it = _lookup.find(k);
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if (it == _lookup.end()) {
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return nullptr;
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}
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if (it->second >= 0) { // still in the queue
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return _heap.at(static_cast<size_t>(it->second) - _popped);
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} else { // already in the history
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return _history.at(static_cast<size_t>(-it->second) - 1);
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}
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}
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//////////////////////////////////////////////////////////////////////////////
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/// @brief erase, returns whether the key was found
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//////////////////////////////////////////////////////////////////////////////
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bool lowerWeight(Key const& k, Weight newWeight) {
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if (!_isHeap) {
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_isHeap = true;
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}
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auto it = _lookup.find(k);
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if (it == _lookup.end()) {
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return false;
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}
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if (it->second >= 0) { // still in the queue
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size_t pos = static_cast<size_t>(it->second) - _popped;
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_heap[pos]->setWeight(newWeight);
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repairUp(pos);
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} else { // already in the history
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size_t pos = static_cast<size_t>(-it->second) - 1;
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_history[pos]->setWeight(newWeight);
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}
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return true;
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}
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//////////////////////////////////////////////////////////////////////////////
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/// @brief getMinimal, note that the resulting pointer is only valid until the
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/// the next modification of the data structure happens (insert or lowerWeight
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/// or popMinimal). The weight in the Value type must not be modified other
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/// than via lowerWeight, otherwise the queue order could be violated.
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//////////////////////////////////////////////////////////////////////////////
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Value* getMinimal() {
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if (_heap.empty()) {
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return nullptr;
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}
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return _heap[0];
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}
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//////////////////////////////////////////////////////////////////////////////
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/// @brief popMinimal, returns true if something was returned and false
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/// if the structure is empty. Key and Value are stored in k and v.
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/// If keepForLookup is true then the Value is kept for lookup in the
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/// hash table but removed from the priority queue.
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//////////////////////////////////////////////////////////////////////////////
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bool popMinimal(Key& k, Value*& v, bool keepForLookup = false) {
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if (_heap.empty()) {
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return false;
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}
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k = _heap[0]->getKey();
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v = _heap[0];
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if (!_isHeap) {
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auto it = _lookup.find(k);
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TRI_ASSERT(it != _lookup.end());
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if (keepForLookup) {
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_history.push_back(_heap[0]);
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it->second = -static_cast<ssize_t>(_history.size());
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// Note: This is intentionally one too large to shift by 1
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} else {
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_lookup.erase(it);
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}
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_heap.pop_front();
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_popped++;
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} else {
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removeFromHeap(keepForLookup);
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}
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return true;
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}
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private:
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//////////////////////////////////////////////////////////////////////////////
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/// @brief swap, two positions in the heap, adjusts the _lookup table
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//////////////////////////////////////////////////////////////////////////////
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void swap(size_t p, size_t q) {
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Value* v = _heap[p];
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_heap[p] = _heap[q];
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_heap[q] = v;
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// Now fix the lookup:
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Key const& keyp(_heap[p]->getKey());
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auto it = _lookup.find(keyp);
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TRI_ASSERT(it != _lookup.end());
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TRI_ASSERT(it->second - _popped == q);
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it->second = static_cast<ssize_t>(p) + static_cast<ssize_t>(_popped);
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Key const& keyq(_heap[q]->getKey());
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it = _lookup.find(keyq);
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TRI_ASSERT(it != _lookup.end());
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TRI_ASSERT(it->second - _popped == p);
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it->second = static_cast<ssize_t>(q) + static_cast<ssize_t>(_popped);
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}
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//////////////////////////////////////////////////////////////////////////////
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/// @brief parent, find the parent node in heap
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//////////////////////////////////////////////////////////////////////////////
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size_t parent(size_t pos) { return ((pos + 1) >> 1) - 1; }
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//////////////////////////////////////////////////////////////////////////////
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/// @brief lchild, find the node of the left child in the heap
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//////////////////////////////////////////////////////////////////////////////
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size_t lchild(size_t pos) { return 2 * (pos + 1) - 1; }
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//////////////////////////////////////////////////////////////////////////////
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/// @brief rchild, find the node of the right child in the heap
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//////////////////////////////////////////////////////////////////////////////
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size_t rchild(size_t pos) { return 2 * (pos + 1); }
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//////////////////////////////////////////////////////////////////////////////
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/// @brief repairUp, fix the heap property between position pos and its parent
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//////////////////////////////////////////////////////////////////////////////
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void repairUp(size_t pos) {
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while (pos > 0) {
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size_t par = parent(pos);
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Weight wpos = _heap[pos]->weight();
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Weight wpar = _heap[par]->weight();
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if (wpos < wpar) {
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swap(pos, par);
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pos = par;
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} else {
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return;
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}
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}
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}
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//////////////////////////////////////////////////////////////////////////////
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/// @brief repairDown, fix the heap property between position pos and its
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/// children.
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//////////////////////////////////////////////////////////////////////////////
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void repairDown() {
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size_t pos = 0;
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while (pos < _heap.size()) {
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size_t lchi = lchild(pos);
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if (lchi >= _heap.size()) {
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return;
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}
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Weight wpos = _heap[pos]->weight();
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Weight wlchi = _heap[lchi]->weight();
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size_t rchi = rchild(pos);
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if (rchi >= _heap.size()) {
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if (wpos > wlchi) {
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swap(pos, lchi);
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}
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return;
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}
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Weight wrchi = _heap[rchi]->weight();
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if (wlchi <= wrchi) {
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if (wpos <= wlchi) {
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return;
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}
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swap(pos, lchi);
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pos = lchi;
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} else {
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if (wpos <= wrchi) {
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return;
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}
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swap(pos, rchi);
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pos = rchi;
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}
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}
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}
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//////////////////////////////////////////////////////////////////////////////
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/// @brief removeFromHeap, remove first position in the heap
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//////////////////////////////////////////////////////////////////////////////
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void removeFromHeap(bool keepForLookup) {
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auto it = _lookup.find(_heap[0]->getKey());
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TRI_ASSERT(it != _lookup.end());
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if (keepForLookup) {
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_history.push_back(_heap[0]);
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it->second = -static_cast<ssize_t>(_history.size());
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// Note: This is intentionally one too large to shift by 1
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} else {
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_lookup.erase(it);
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}
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if (_heap.size() == 1) {
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_heap.clear();
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_popped = 0;
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_isHeap = false;
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_maxWeight = 0;
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return;
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}
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// Move one in front:
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_heap[0] = _heap.back();
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_heap.pop_back();
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it = _lookup.find(_heap[0]->getKey());
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TRI_ASSERT(it != _lookup.end());
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it->second = static_cast<ssize_t>(_popped);
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repairDown();
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}
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//////////////////////////////////////////////////////////////////////////////
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/// @brief _popped, number of elements that have been popped from the
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/// beginning
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/// of the deque, this is necessary to interpret positions stored in the
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/// unordered_map _lookup
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//////////////////////////////////////////////////////////////////////////////
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private:
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size_t _popped;
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//////////////////////////////////////////////////////////////////////////////
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/// @brief _lookup, this provides O(1) lookup by Key
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//////////////////////////////////////////////////////////////////////////////
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std::unordered_map<Key, ssize_t> _lookup;
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//////////////////////////////////////////////////////////////////////////////
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/// @brief _isHeap, starts as false, in which case we only use a deque,
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/// if true, then _heap is an actual binary heap and we do no longer modify
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/// _popped.
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//////////////////////////////////////////////////////////////////////////////
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bool _isHeap;
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//////////////////////////////////////////////////////////////////////////////
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/// @brief _heap, the actual data
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//////////////////////////////////////////////////////////////////////////////
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std::deque<Value*> _heap;
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//////////////////////////////////////////////////////////////////////////////
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/// @brief _maxWeight, the current maximal weight ever seen
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//////////////////////////////////////////////////////////////////////////////
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Weight _maxWeight;
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//////////////////////////////////////////////////////////////////////////////
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/// @brief _history, the actual data that is only in the key/value store
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//////////////////////////////////////////////////////////////////////////////
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std::vector<Value*> _history;
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};
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} // namespace graph
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} // namespace arangodb
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#endif
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