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/*
* Copyright 2014-2015 Open Networking Laboratory
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.onlab.graph;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.Set;
/**
* Dijkstra shortest-path graph search algorithm capable of finding not just
* one, but all shortest paths between the source and destinations.
*/
public class DijkstraGraphSearch<V extends Vertex, E extends Edge<V>>
extends AbstractGraphPathSearch<V, E> {
@Override
public Result<V, E> search(Graph<V, E> graph, V src, V dst,
EdgeWeight<V, E> weight, int maxPaths) {
checkArguments(graph, src, dst);
// Use the default result to remember cumulative costs and parent
// edges to each each respective vertex.
DefaultResult result = new DefaultResult(src, dst, maxPaths);
// Cost to reach the source vertex is 0 of course.
result.updateVertex(src, null, 0.0, false);
if (graph.getEdges().isEmpty()) {
result.buildPaths();
return result;
}
// Use the min priority queue to progressively find each nearest
// vertex until we reach the desired destination, if one was given,
// or until we reach all possible destinations.
Heap<V> minQueue = createMinQueue(graph.getVertexes(),
new PathCostComparator(result));
while (!minQueue.isEmpty()) {
// Get the nearest vertex
V nearest = minQueue.extractExtreme();
if (nearest.equals(dst)) {
break;
}
// Find its cost and use it to determine if the vertex is reachable.
double cost = result.cost(nearest);
if (cost < Double.MAX_VALUE) {
// If the vertex is reachable, relax all its egress edges.
for (E e : graph.getEdgesFrom(nearest)) {
result.relaxEdge(e, cost, weight, true);
}
}
// Re-prioritize the min queue.
minQueue.heapify();
}
// Now construct a set of paths from the results.
result.buildPaths();
return result;
}
// Compares path weights using their accrued costs; used for sorting the
// min priority queue.
private final class PathCostComparator implements Comparator<V> {
private final DefaultResult result;
private PathCostComparator(DefaultResult result) {
this.result = result;
}
@Override
public int compare(V v1, V v2) {
double delta = result.cost(v2) - result.cost(v1);
return delta < 0 ? -1 : (delta > 0 ? 1 : 0);
}
}
// Creates a min priority queue from the specified vertexes and comparator.
private Heap<V> createMinQueue(Set<V> vertexes, Comparator<V> comparator) {
return new Heap<>(new ArrayList<>(vertexes), comparator);
}
}
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