KShortestPaths.java

/* ==========================================
 * JGraphT : a free Java graph-theory library
 * ==========================================
 *
 * Project Info:  http://jgrapht.sourceforge.net/
 * Project Creator:  Barak Naveh (http://sourceforge.net/users/barak_naveh)
 *
 * (C) Copyright 2003-2010, by Barak Naveh and Contributors.
 *
 * This program and the accompanying materials are dual-licensed under
 * either
 *
 * (a) the terms of the GNU Lesser General Public License version 2.1
 * as published by the Free Software Foundation, or (at your option) any
 * later version.
 *
 * or (per the licensee's choosing)
 *
 * (b) the terms of the Eclipse Public License v1.0 as published by
 * the Eclipse Foundation.
 */
/* -------------------------
 * KShortestPaths.java
 * -------------------------
 * (C) Copyright 2007-2010, by France Telecom
 *
 * Original Author:  Guillaume Boulmier and Contributors.
 * Contributor(s):   John V. Sichi
 *
 * $Id$
 *
 * Changes
 * -------
 * 05-Jun-2007 : Initial revision (GB);
 * 05-Jul-2007 : Added support for generics (JVS);
 * 06-Dec-2010 : Bugfixes (GB);
 *
 */
package org.jgrapht.alg;

import java.util.*;

import org.jgrapht.*;


/**
 * The algorithm determines the k shortest simple paths in increasing order of
 * weight. Weights can be negative (but no negative cycle is allowed), and paths
 * can be constrained by a maximum number of edges. Multigraphs are allowed.
 *
 * <p>The algorithm is a variant of the Bellman-Ford algorithm but instead of
 * only storing the best path it stores the "k" best paths at each pass,
 * yielding a complexity of O(k*n*(m^2)) where m is the number of edges and n is
 * the number of vertices.
 *
 * @author Guillaume Boulmier
 * @since July 5, 2007
 */
public class KShortestPaths<V, E>
{
    

    /**
     * Graph on which shortest paths are searched.
     */
    private Graph<V, E> graph;

    private int nMaxHops;

    private int nPaths;

    private V startVertex;

    

    /**
     * Creates an object to compute ranking shortest paths between the start
     * vertex and others vertices.
     *
     * @param graph
     * @param startVertex
     * @param k number of paths to be computed.
     */
    public KShortestPaths(Graph<V, E> graph, V startVertex, int k)
    {
        this(graph, startVertex, k, graph.vertexSet().size() - 1);
    }

    /**
     * Creates an object to calculate ranking shortest paths between the start
     * vertex and others vertices.
     *
     * @param graph graph on which shortest paths are searched.
     * @param startVertex start vertex of the calculated paths.
     * @param nPaths number of ranking paths between the start vertex and an end
     * vertex.
     * @param nMaxHops maximum number of edges of the calculated paths.
     *
     * @throws NullPointerException if the specified graph or startVertex is
     * <code>null</code>.
     * @throws IllegalArgumentException if nPaths is negative or 0.
     * @throws IllegalArgumentException if nMaxHops is negative or 0.
     */
    public KShortestPaths(
        Graph<V, E> graph,
        V startVertex,
        int nPaths,
        int nMaxHops)
    {
        assertKShortestPathsFinder(graph, startVertex, nPaths, nMaxHops);

        this.graph = graph;
        this.startVertex = startVertex;
        this.nPaths = nPaths;
        this.nMaxHops = nMaxHops;
    }

    

    /**
     * Returns the k shortest simple paths in increasing order of weight.
     *
     * @param endVertex target vertex of the calculated paths.
     *
     * @return list of paths, or <code>null</code> if no path exists between the
     * start vertex and the end vertex.
     */
    public List<GraphPath<V, E>> getPaths(V endVertex)
    {
        assertGetPaths(endVertex);

        KShortestPathsIterator<V, E> iter =
            new KShortestPathsIterator<V, E>(
                this.graph,
                this.startVertex,
                endVertex,
                this.nPaths);

        // at the i-th pass the shortest paths with less (or equal) than i edges
        // are calculated.
        for (
            int passNumber = 1;
            (passNumber <= this.nMaxHops)
            && iter.hasNext();
            passNumber++)
        {
            iter.next();
        }

        List<RankingPathElement<V, E>> list = iter.getPathElements(endVertex);

        if (list == null) {
            return null;
        }

        List<GraphPath<V, E>> pathList = new ArrayList<GraphPath<V, E>>();

        for (RankingPathElement<V, E> element : list) {
            pathList.add(new PathWrapper(element));
        }

        return pathList;
    }

    private void assertGetPaths(V endVertex)
    {
        if (endVertex == null) {
            throw new NullPointerException("endVertex is null");
        }
        if (endVertex.equals(this.startVertex)) {
            throw new IllegalArgumentException(
                "The end vertex is the same as the start vertex!");
        }
        if (!this.graph.vertexSet().contains(endVertex)) {
            throw new IllegalArgumentException(
                "Graph must contain the end vertex!");
        }
    }

    private void assertKShortestPathsFinder(
        Graph<V, E> graph,
        V startVertex,
        int nPaths,
        int nMaxHops)
    {
        if (graph == null) {
            throw new NullPointerException("graph is null");
        }
        if (startVertex == null) {
            throw new NullPointerException("startVertex is null");
        }
        if (nPaths <= 0) {
            throw new NullPointerException("nPaths is negative or 0");
        }
        if (nMaxHops <= 0) {
            throw new NullPointerException("nMaxHops is negative or 0");
        }
    }

    

    private class PathWrapper
        implements GraphPath<V, E>
    {
        private RankingPathElement<V, E> rankingPathElement;

        private List<E> edgeList;

        PathWrapper(RankingPathElement<V, E> rankingPathElement)
        {
            this.rankingPathElement = rankingPathElement;
        }

        // implement GraphPath
        public Graph<V, E> getGraph()
        {
            return graph;
        }

        // implement GraphPath
        public V getStartVertex()
        {
            return startVertex;
        }

        // implement GraphPath
        public V getEndVertex()
        {
            return rankingPathElement.getVertex();
        }

        // implement GraphPath
        public List<E> getEdgeList()
        {
            if (edgeList == null) {
                edgeList = rankingPathElement.createEdgeListPath();
            }
            return edgeList;
        }

        // implement GraphPath
        public double getWeight()
        {
            return rankingPathElement.getWeight();
        }

        // override Object
        public String toString()
        {
            return getEdgeList().toString();
        }
    }
}

// End KShortestPaths.java