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A collection of various utility classes to ease working with strings, files, command lines, XML and more.
There is a newer version: 4.0.1
package org.codehaus.plexus.util.dag;

/*
 * Copyright The Codehaus Foundation.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.util.Collections;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;

/**
 * @author Michal Maczka
 * @version $Id$
 */
public class CycleDetector
{

    private final static Integer NOT_VISITED = 0;

    private final static Integer VISITING = 1;

    private final static Integer VISITED = 2;

    public static List hasCycle( final DAG graph )
    {
        final List vertices = graph.getVertices();

        final Map vertexStateMap = new HashMap();

        List retValue = null;

        for ( Vertex vertex : vertices )
        {
            if ( isNotVisited( vertex, vertexStateMap ) )
            {
                retValue = introducesCycle( vertex, vertexStateMap );

                if ( retValue != null )
                {
                    break;
                }
            }
        }

        return retValue;
    }

    /**
     * This method will be called when an edge leading to given vertex was added and we want to check if introduction of
     * this edge has not resulted in apparition of cycle in the graph
     *
     * @param vertex
     * @param vertexStateMap
     * @return
     */
    public static List introducesCycle( final Vertex vertex, final Map vertexStateMap )
    {
        final LinkedList cycleStack = new LinkedList();

        final boolean hasCycle = dfsVisit( vertex, cycleStack, vertexStateMap );

        if ( hasCycle )
        {
            // we have a situation like: [b, a, c, d, b, f, g, h].
            // Label of Vertex which introduced the cycle is at the first position in the list
            // We have to find second occurrence of this label and use its position in the list
            // for getting the sublist of vertex labels of cycle participants
            //
            // So in our case we are searching for [b, a, c, d, b]
            final String label = cycleStack.getFirst();

            final int pos = cycleStack.lastIndexOf( label );

            final List cycle = cycleStack.subList( 0, pos + 1 );

            Collections.reverse( cycle );

            return cycle;
        }

        return null;
    }

    public static List introducesCycle( final Vertex vertex )
    {
        final Map vertexStateMap = new HashMap();

        return introducesCycle( vertex, vertexStateMap );
    }

    /**
     * @param vertex
     * @param vertexStateMap
     * @return
     */
    private static boolean isNotVisited( final Vertex vertex, final Map vertexStateMap )
    {
        final Integer state = vertexStateMap.get( vertex );

        return ( state == null ) || NOT_VISITED.equals( state );
    }

    /**
     * @param vertex
     * @param vertexStateMap
     * @return
     */
    private static boolean isVisiting( final Vertex vertex, final Map vertexStateMap )
    {
        final Integer state = vertexStateMap.get( vertex );

        return VISITING.equals( state );
    }

    private static boolean dfsVisit( final Vertex vertex, final LinkedList cycle,
                                     final Map vertexStateMap )
    {
        cycle.addFirst( vertex.getLabel() );

        vertexStateMap.put( vertex, VISITING );

        for ( Vertex v : vertex.getChildren() )
        {
            if ( isNotVisited( v, vertexStateMap ) )
            {
                final boolean hasCycle = dfsVisit( v, cycle, vertexStateMap );

                if ( hasCycle )
                {
                    return true;
                }
            }
            else if ( isVisiting( v, vertexStateMap ) )
            {
                cycle.addFirst( v.getLabel() );

                return true;
            }
        }
        vertexStateMap.put( vertex, VISITED );

        cycle.removeFirst();

        return false;
    }

}