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/* ==========================================
* 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 2003-2008, 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.
*/
/* -------------------------
* BellmanFordIterator.java
* -------------------------
* (C) Copyright 2006-2008, by France Telecom and Contributors.
*
* Original Author: Guillaume Boulmier and Contributors.
* Contributor(s): John V. Sichi
*
* $Id$
*
* Changes
* -------
* 05-Jan-2006 : Initial revision (GB);
* 14-Jan-2006 : Added support for generics (JVS);
*
*/
package org.jgrapht.alg;
import java.util.*;
import org.jgrapht.*;
/**
* Helper class for {@link BellmanFordShortestPath}; not intended for general
* use.
*/
class BellmanFordIterator
implements Iterator>
{
/**
* Error message.
*/
protected final static String NEGATIVE_UNDIRECTED_EDGE =
"Negative"
+ "edge-weights are not allowed in an unidrected graph!";
/**
* Graph on which shortest paths are searched.
*/
protected Graph graph;
/**
* Start vertex.
*/
protected V startVertex;
/**
* Vertices whose shortest path cost have been improved during the previous
* pass.
*/
private List prevImprovedVertices = new ArrayList();
private Map> prevVertexData;
private boolean startVertexEncountered = false;
/**
* Stores the vertices that have been seen during iteration and (optionally)
* some additional traversal info regarding each vertex.
*/
private Map> vertexData;
private double epsilon;
/**
* @param graph
* @param startVertex start vertex.
* @param epsilon tolerance factor.
*/
protected BellmanFordIterator(
Graph graph,
V startVertex,
double epsilon)
{
assertBellmanFordIterator(graph, startVertex);
this.graph = graph;
this.startVertex = startVertex;
this.epsilon = epsilon;
}
/**
* Returns the path element of the shortest path with less than
* nMaxHops edges between the start vertex and the end vertex.
*
* @param endVertex end vertex.
*
* @return .
*/
public BellmanFordPathElement getPathElement(V endVertex)
{
return getSeenData(endVertex);
}
/**
* @return true if at least one path has been improved during
* the previous pass, false otherwise.
*/
@Override public boolean hasNext()
{
if (!this.startVertexEncountered) {
encounterStartVertex();
}
return !(this.prevImprovedVertices.isEmpty());
}
/**
* Returns the list Collection of vertices whose path has been
* improved during the current pass.
*
* @see java.util.Iterator#next()
*/
@Override public List next()
{
if (!this.startVertexEncountered) {
encounterStartVertex();
}
if (hasNext()) {
List improvedVertices = new ArrayList();
for (int i = this.prevImprovedVertices.size() - 1; i >= 0; i--) {
V vertex = this.prevImprovedVertices.get(i);
for (
Iterator iter = edgesOfIterator(vertex);
iter.hasNext();)
{
E edge = iter.next();
V oppositeVertex =
Graphs.getOppositeVertex(
graph,
edge,
vertex);
if (getPathElement(oppositeVertex) != null) {
boolean relaxed =
relaxVertexAgain(oppositeVertex, edge);
if (relaxed) {
improvedVertices.add(oppositeVertex);
}
} else {
relaxVertex(oppositeVertex, edge);
improvedVertices.add(oppositeVertex);
}
}
}
savePassData(improvedVertices);
return improvedVertices;
}
throw new NoSuchElementException();
}
/**
* Unsupported
*
* @see java.util.Iterator#remove()
*/
@Override public void remove()
{
throw new UnsupportedOperationException();
}
/**
* @param edge
*
* @throws IllegalArgumentException if the graph is undirected and the
* edge-weight is negative.
*/
protected void assertValidEdge(E edge)
{
if (this.graph instanceof UndirectedGraph) {
if (graph.getEdgeWeight(edge) < 0) {
throw new IllegalArgumentException(NEGATIVE_UNDIRECTED_EDGE);
}
}
}
/**
* Costs taken into account are the weights stored in Edge
* objects.
*
* @param vertex a vertex which has just been encountered.
* @param edge the edge via which the vertex was encountered.
*
* @return the cost obtained by concatenation.
*
* @see Graph#getEdgeWeight(E)
*/
protected double calculatePathCost(V vertex, E edge)
{
V oppositeVertex = Graphs.getOppositeVertex(graph, edge, vertex);
// we get the data of the previous pass.
BellmanFordPathElement oppositePrevData =
getPrevSeenData(oppositeVertex);
double pathCost = graph.getEdgeWeight(edge);
if (!oppositePrevData.getVertex().equals(this.startVertex)) {
// if it's not the start vertex, we add the cost of the previous
// pass.
pathCost += oppositePrevData.getCost();
}
return pathCost;
}
/**
* Returns an iterator to loop over outgoing edges Edge of the
* vertex.
*
* @param vertex
*
* @return .
*/
protected Iterator edgesOfIterator(V vertex)
{
if (this.graph instanceof DirectedGraph) {
return ((DirectedGraph) this.graph).outgoingEdgesOf(vertex)
.iterator();
} else {
return this.graph.edgesOf(vertex).iterator();
}
}
/**
* Access the data stored for a seen vertex in the previous pass.
*
* @param vertex a vertex which has already been seen.
*
* @return data associated with the seen vertex or null if no
* data was associated with the vertex.
*/
protected BellmanFordPathElement getPrevSeenData(V vertex)
{
return this.prevVertexData.get(vertex);
}
/**
* Access the data stored for a seen vertex in the current pass.
*
* @param vertex a vertex which has already been seen.
*
* @return data associated with the seen vertex or null if no
* data was associated with the vertex.
*/
protected BellmanFordPathElement getSeenData(V vertex)
{
return this.vertexData.get(vertex);
}
/**
* Determines whether a vertex has been seen yet by this traversal.
*
* @param vertex vertex in question.
*
* @return true if vertex has already been seen.
*/
protected boolean isSeenVertex(V vertex)
{
return this.vertexData.containsKey(vertex);
}
/**
* @param vertex
* @param data
*
* @return .
*/
protected BellmanFordPathElement putPrevSeenData(
V vertex,
BellmanFordPathElement data)
{
if (this.prevVertexData == null) {
this.prevVertexData =
new HashMap>();
}
return this.prevVertexData.put(vertex, data);
}
/**
* Stores iterator-dependent data for a vertex that has been seen during the
* current pass.
*
* @param vertex a vertex which has been seen.
* @param data data to be associated with the seen vertex.
*
* @return previous value associated with specified vertex or
* null if no data was associated with the vertex.
*/
protected BellmanFordPathElement putSeenData(
V vertex,
BellmanFordPathElement data)
{
if (this.vertexData == null) {
this.vertexData = new HashMap>();
}
return this.vertexData.put(vertex, data);
}
private void assertBellmanFordIterator(Graph graph, V startVertex)
{
if (!(graph.containsVertex(startVertex))) {
throw new IllegalArgumentException(
"Graph must contain the start vertex!");
}
}
/**
* The first time we see a vertex, make up a new entry for it.
*
* @param vertex a vertex which has just been encountered.
* @param edge the edge via which the vertex was encountered.
* @param cost cost of the created path element.
*
* @return the new entry.
*/
private BellmanFordPathElement createSeenData(
V vertex,
E edge,
double cost)
{
BellmanFordPathElement prevPathElement =
getPrevSeenData(
Graphs.getOppositeVertex(graph, edge, vertex));
BellmanFordPathElement data =
new BellmanFordPathElement(
graph,
prevPathElement,
edge,
cost,
epsilon);
return data;
}
private void encounterStartVertex()
{
BellmanFordPathElement data =
new BellmanFordPathElement(
this.startVertex,
epsilon);
// first the only vertex considered as improved is the start vertex.
this.prevImprovedVertices.add(this.startVertex);
putSeenData(this.startVertex, data);
putPrevSeenData(this.startVertex, data);
this.startVertexEncountered = true;
}
/**
* Upates data first time a vertex is reached by a path.
*
* @param vertex a vertex which has just been encountered.
* @param edge the edge via which the vertex was encountered.
*/
private void relaxVertex(V vertex, E edge)
{
assertValidEdge(edge);
double shortestPathCost = calculatePathCost(vertex, edge);
BellmanFordPathElement data =
createSeenData(vertex, edge,
shortestPathCost);
putSeenData(vertex, data);
}
/**
* Check if the cost of the best path so far reaching the specified vertex
* could be improved if the vertex is reached through the specified edge.
*
* @param vertex a vertex which has just been encountered.
* @param edge the edge via which the vertex was encountered.
*
* @return true if the cost has been improved,
* false otherwise.
*/
private boolean relaxVertexAgain(V vertex, E edge)
{
assertValidEdge(edge);
double candidateCost = calculatePathCost(vertex, edge);
// we get the data of the previous pass.
BellmanFordPathElement oppositePrevData =
getPrevSeenData(
Graphs.getOppositeVertex(graph, edge, vertex));
BellmanFordPathElement pathElement = getSeenData(vertex);
return pathElement.improve(oppositePrevData, edge, candidateCost);
}
private void savePassData(List improvedVertices)
{
for (V vertex : improvedVertices) {
BellmanFordPathElement orig = getSeenData(vertex);
BellmanFordPathElement clonedData =
new BellmanFordPathElement(orig);
putPrevSeenData(vertex, clonedData);
}
this.prevImprovedVertices = improvedVertices;
}
}
// End BellmanFordIterator.java
