/*
* GeoTools - The Open Source Java GIS Toolkit
* http://geotools.org
*
* (C) 2002-2008, Open Source Geospatial Foundation (OSGeo)
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation;
* version 2.1 of the License.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*/
package org.geotools.graph.traverse.standard;
import java.util.Iterator;
import org.geotools.graph.structure.Graph;
import org.geotools.graph.structure.GraphVisitor;
import org.geotools.graph.structure.Graphable;
import org.geotools.graph.structure.Node;
import org.geotools.graph.traverse.GraphTraversal;
import org.geotools.graph.traverse.basic.AbstractGraphIterator;
import org.geotools.graph.util.FIFOQueue;
import org.geotools.graph.util.Queue;
/**
* Iterates over the nodes of a graph in <B>Breadth First Topological Sort</B>
* pattern. The following is an illustration of the iteration.<BR>
* <IMG src="doc-files/breadth_topo.gif"><BR>
* <BR>
* Initially all nodes of degree less than two are <B>active</B>
* (ready to be visited). As nodes are visited, a node can become active
* when all but one of its related nodes have been visited (
* degree = counter + 1). When a node becomes active it is placed into the
* <B>active node queue</B> (queue of nodes to be visited).
* The Breadth First Topological iterator places
* nodes into the queue in <B>First In First Out</B> order.<BR>
* <BR>
* To determine when a node is to become active the iterator uses the counter
* associated with each node. If these counters are modified by an entity
* other then the iterator, the iteration may be affected in undefined ways.
*
* @author Justin Deoliveira, Refractions Research Inc, jdeolive@refractions.net
*
*
*
* @source $URL$
*/
public class BreadthFirstTopologicalIterator extends AbstractGraphIterator {
/** Queue of active nodes **/
private Queue m_queue;
/**
* Creates the active queue, and populates it with all nodes of degree less
* than 2. Counters of all nodes are also reset to 0.
*
* @see org.geotools.graph.traverse.GraphIterator#init(Graph)
*/
public void init(Graph graph, GraphTraversal traversal) {
//create queue
m_queue = buildQueue(graph);
//initialize nodes
graph.visitNodes(
new GraphVisitor() {
public int visit(Graphable component) {
Node node = (Node)component;
//reset counter to zero
node.setCount(0);
if (node.getDegree() < 2) m_queue.enq(node);
return(0);
}
}
);
}
/**
* Returns the next node in the active node queue.
*
* @see org.geotools.graph.traverse.GraphIterator#next()
*/
public Graphable next(GraphTraversal traversal) {
return(!m_queue.isEmpty() ? (Graphable)m_queue.deq() : null);
}
/**
* Continues the iteration by incrementing the counters of any unvisited
* nodes related to the current node. If any related nodes have counters
* equal to degree of that node - 1, it is placed into the active queue.
*
* @see org.geotools.graph.traverse.GraphIterator#cont(Graphable)
*/
public void cont(Graphable current, GraphTraversal traversal) {
//increment the count of all adjacent nodes by one
// if the result count is 1 less than the degree, place it into the queue
for (Iterator itr = current.getRelated(); itr.hasNext();) {
Node related = (Node)itr.next();
if (!traversal.isVisited(related)) {
related.setCount(related.getCount()+1);
if (related.getDegree()-1 == related.getCount()) m_queue.enq(related);
}
}
}
/**
* Kills the current branch of the traversal by not incremening the counters
* of any related nodes.
*
* @see org.geotools.graph.traverse.GraphIterator#killBranch(Graphable)
*/
public void killBranch(Graphable current, GraphTraversal traversal) {
//do nothing
}
/**
* Builds the active node queue.
*
* @param graph The Graph whose components are being iterated over.
*
* @return A first in first out queue
*/
protected Queue buildQueue(Graph graph) {
return(new FIFOQueue(graph.getNodes().size()));
}
}