MergeVertices.java

/*
 * Generic graph library
 * Copyright (C) 2000,2003,2004 University of Maryland
 *
 * 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
 */

// $Revision: 1.13 $

package edu.umd.cs.findbugs.graph;

import java.util.Iterator;
import java.util.Set;
import java.util.TreeSet;

/**
 * Algorithm to merge a set of vertices into a single vertex. Note that the
 * graph is modified as part of this process.
 */
public class MergeVertices<GraphType extends Graph<EdgeType, VertexType>, EdgeType extends GraphEdge<EdgeType, VertexType>, VertexType extends GraphVertex<VertexType>> {

    /**
     * Constructor.
     */
    public MergeVertices() {
    }

    /**
     * Merge the specified set of vertices into a single vertex.
     * 
     * @param vertexSet
     *            the set of vertices to be merged
     * @param g
     *            the graph to be modified
     * @param combinator
     *            object used to combine vertices
     * @param toolkit
     *            GraphToolkit used to copy auxiliary information for edges
     */
    public void mergeVertices(Set<VertexType> vertexSet, GraphType g, VertexCombinator<VertexType> combinator,
            GraphToolkit<GraphType, EdgeType, VertexType> toolkit) {

        // Special case: if the vertex set contains a single vertex
        // or is empty, there is nothing to do
        if (vertexSet.size() <= 1)
            return;

        // Get all vertices to which we have outgoing edges
        // or from which we have incoming edges, since they'll need
        // to be fixed
        TreeSet<EdgeType> edgeSet = new TreeSet<EdgeType>();
        for (Iterator<EdgeType> i = g.edgeIterator(); i.hasNext();) {
            EdgeType e = i.next();
            if (vertexSet.contains(e.getSource()) || vertexSet.contains(e.getTarget()))
                edgeSet.add(e);
        }

        // Combine all of the vertices into a single composite vertex
        VertexType compositeVertex = combinator.combineVertices(vertexSet);

        // For each original edge into or out of the vertex set,
        // create an equivalent edge referencing the composite
        // vertex
        for (EdgeType e : edgeSet) {
            VertexType source = vertexSet.contains(e.getSource()) ? compositeVertex : e.getSource();
            VertexType target = vertexSet.contains(e.getTarget()) ? compositeVertex : e.getTarget();

            // if (source != compositeVertex && target != compositeVertex)
            // System.out.println("BIG OOPS!");

            // Don't create a self edge for the composite vertex
            // unless one of the vertices in the vertex set
            // had a self edge
            if (source == compositeVertex && target == compositeVertex && e.getSource() != e.getTarget())
                continue;

            // Don't create duplicate edges.
            if (g.lookupEdge(source, target) != null)
                continue;

            EdgeType compositeEdge = g.createEdge(source, target);
            // FIXME: we really should have an EdgeCombinator here
            toolkit.copyEdge(e, compositeEdge);
        }

        // Remove all of the vertices in the vertex set; this will
        // automatically remove the edges into and out of those
        // vertices
        for (VertexType aVertexSet : vertexSet) {
            g.removeVertex(aVertexSet);
        }

    }

}

// vim:ts=4