ConnectedSubgraphFinder.java

/*
* The JTS Topology Suite is a collection of Java classes that
* implement the fundamental operations required to validate a given
* geo-spatial data set to a known topological specification.
*
* Copyright (C) 2001 Vivid Solutions
*
* 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; either
* version 2.1 of the License, or (at your option) any later version.
*
* 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.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
*
* For more information, contact:
*
*     Vivid Solutions
*     Suite #1A
*     2328 Government Street
*     Victoria BC  V8T 5G5
*     Canada
*
*     (250)385-6040
*     www.vividsolutions.com
*/

package com.vividsolutions.jts.planargraph.algorithm;

import java.util.*;
import com.vividsolutions.jts.planargraph.*;

/**
 * Finds all connected {@link Subgraph}s of a {@link PlanarGraph}.
 * <p>
 * <b>Note:</b> uses the <code>isVisited</code> flag on the nodes.
 */
public class ConnectedSubgraphFinder
{

  private PlanarGraph graph;

  public ConnectedSubgraphFinder(PlanarGraph graph) {
    this.graph = graph;
  }

  public List getConnectedSubgraphs()
  {
    List subgraphs = new ArrayList();

    GraphComponent.setVisited(graph.nodeIterator(), false);
    for (Iterator i = graph.edgeIterator(); i.hasNext(); ) {
      Edge e = (Edge) i.next();
      Node node = e.getDirEdge(0).getFromNode();
      if (! node.isVisited()) {
        subgraphs.add(findSubgraph(node));
      }
    }
    return subgraphs;
  }

  private Subgraph findSubgraph(Node node)
  {
    Subgraph subgraph = new Subgraph(graph);
    addReachable(node, subgraph);
    return subgraph;
  }

  /**
   * Adds all nodes and edges reachable from this node to the subgraph.
   * Uses an explicit stack to avoid a large depth of recursion.
   *
   * @param node a node known to be in the subgraph
   */
  private void addReachable(Node startNode, Subgraph subgraph)
  {
    Stack nodeStack = new Stack();
    nodeStack.add(startNode);
    while (! nodeStack.empty()) {
      Node node = (Node) nodeStack.pop();
      addEdges(node, nodeStack, subgraph);
    }
  }

  /**
   * Adds the argument node and all its out edges to the subgraph.
   * @param node the node to add
   * @param nodeStack the current set of nodes being traversed
   */
  private void addEdges(Node node, Stack nodeStack, Subgraph subgraph)
  {
    node.setVisited(true);
    for (Iterator i = ((DirectedEdgeStar) node.getOutEdges()).iterator(); i.hasNext(); ) {
      DirectedEdge de = (DirectedEdge) i.next();
      subgraph.add(de.getEdge());
      Node toNode = de.getToNode();
      if (! toNode.isVisited()) nodeStack.push(toNode);
    }
  }

}