utils/misc/src/main/java/org/onlab/graph/BellmanFordGraphSearch.java

/*
 * Copyright 2014-present 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;

/**
 * Bellman-Ford graph search algorithm for locating shortest-paths in
 * directed graphs that may contain negative cycles.
 */
public class BellmanFordGraphSearch<V extends Vertex, E extends Edge<V>>
        extends AbstractGraphPathSearch<V, E> {

    @Override
    protected Result<V, E> internalSearch(Graph<V, E> graph, V src, V dst,
                               EdgeWeigher<V, E> weigher, int maxPaths) {

        // Prepare the graph search result.
        DefaultResult result = new DefaultResult(src, dst, maxPaths);

        // The source vertex has cost 0, of course.
        result.updateVertex(src, null, weigher.getInitialWeight(), true);

        int max = graph.getVertexes().size() - 1;
        for (int i = 0; i < max; i++) {
            // Relax, if possible, all egress edges of the current vertex.
            for (E edge : graph.getEdges()) {
                if (result.hasCost(edge.src())) {
                    result.relaxEdge(edge, result.cost(edge.src()), weigher);
                }
            }
        }

        // Remove any vertexes reached by traversing edges with negative weights.
        for (E edge : graph.getEdges()) {
            if (result.hasCost(edge.src())) {
                if (result.relaxEdge(edge, result.cost(edge.src()), weigher)) {
                    result.removeVertex(edge.dst());
                }
            }
        }

        // Finally, but the paths on the search result and return.
        result.buildPaths();
        return result;
    }
}