/*
 *    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())); 
  }
}