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CycleDetector.java

/* ==========================================
 * JGraphT : a free Java graph-theory library
 * ==========================================
 *
 * Project Info:  http://jgrapht.sourceforge.net/
 * Project Lead:  Barak Naveh (http://sourceforge.net/users/barak_naveh)
 *
 * (C) Copyright 2003-2004, by Barak Naveh and Contributors.
 *
 * 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.
 */
/* ------------------
 * CycleDetector.java
 * ------------------
 * (C) Copyright 2004, by John V. Sichi and Contributors.
 *
 * Original Author:  John V. Sichi
 * Contributor(s):   -
 *
 * $Id: CycleDetector.java,v 1.4 2005/04/23 08:09:28 perfecthash Exp $
 *
 * Changes
 * -------
 * 16-Sept-2004 : Initial revision (JVS);
 *
 */
package org._3pq.jgrapht.alg;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

import org._3pq.jgrapht.DirectedGraph;
import org._3pq.jgrapht.Edge;
import org._3pq.jgrapht.Graph;
import org._3pq.jgrapht.traverse.DepthFirstIterator;

/**
 * Performs cycle detection on a graph. The <i>inspected graph</i> is specified
 * at construction time and cannot be modified. Currently, the detector
 * supports only directed graphs.
 *
 * @author John V. Sichi
 *
 * @since Sept 16, 2004
 */
00060 public class CycleDetector {
    /** Graph on which cycle detection is being performed. */
00062     Graph m_graph;

    /**
     * Creates a cycle detector for the specified graph.  Currently only
     * directed graphs are supported.
     *
     * @param graph the DirectedGraph in which to detect cycles
     */
00070     public CycleDetector( DirectedGraph graph ) {
        m_graph = graph;
    }

    /**
     * Performs yes/no cycle detection on the entire graph.
     *
     * @return true iff the graph contains at least one cycle
     */
00079     public boolean detectCycles(  ) {
        try {
            execute( null, null );
        }
         catch( CycleDetectedException ex ) {
            return true;
        }

        return false;
    }


    /**
     * Performs yes/no cycle detection on an individual vertex.
     *
     * @param v the vertex to test
     *
     * @return true if v is on at least one cycle
     */
00098     public boolean detectCyclesContainingVertex( Object v ) {
        try {
            execute( null, v );
        }
         catch( CycleDetectedException ex ) {
            return true;
        }

        return false;
    }


    /**
     * Finds the vertex set for the subgraph of all cycles.
     *
     * @return set of all vertices which participate in at least one cycle in
     *         this graph
     */
00116     public Set findCycles(  ) {
        Set set = new HashSet(  );
        execute( set, null );

        return set;
    }


    /**
     * Finds the vertex set for the subgraph of all cycles which contain a
     * particular vertex.
     *
     * @param v the vertex to test
     *
     * @return set of all vertices reachable from v via at least one cycle
     */
00132     public Set findCyclesContainingVertex( Object v ) {
        Set set = new HashSet(  );
        execute( set, v );

        return set;
    }


    private void execute( Set s, Object v ) {
        ProbeIterator iter = new ProbeIterator( s, v );

        while( iter.hasNext(  ) ) {
            iter.next(  );
        }
    }

    /**
     * Exception thrown internally when a cycle is detected during a yes/no
     * cycle test.  Must be caught by top-level detection method.
     */
00152     private static class CycleDetectedException extends RuntimeException {
        private static final long serialVersionUID = 3834305137802950712L;
    }


    /**
     * Version of DFS which maintains a backtracking path used to probe for
     * cycles.
     */
00161     private class ProbeIterator extends DepthFirstIterator {
        private List m_path;
        private Set  m_cycleSet;

        ProbeIterator( Set cycleSet, Object startVertex ) {
            super( m_graph, startVertex );
            m_cycleSet     = cycleSet;
            m_path         = new ArrayList(  );
        }

        /**
         * {@inheritDoc}
         */
00174         protected void encounterVertexAgain( Object vertex, Edge edge ) {
            super.encounterVertexAgain( vertex, edge );

            int i = m_path.indexOf( vertex );

            if( i > -1 ) {
                if( m_cycleSet == null ) {
                    // we're doing yes/no cycle detection
                    throw new CycleDetectedException(  );
                }

                for( ; i < m_path.size(  ); ++i ) {
                    m_cycleSet.add( m_path.get( i ) );
                }
            }
        }


        /**
         * {@inheritDoc}
         */
00195         protected Object provideNextVertex(  ) {
            Object v = super.provideNextVertex(  );

            // backtrack
            for( int i = m_path.size(  ) - 1; i >= 0; --i ) {
                if( m_graph.containsEdge( m_path.get( i ), v ) ) {
                    break;
                }

                m_path.remove( i );
            }

            m_path.add( v );

            return v;
        }
    }
}

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