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

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

    private final static Integer NOT_VISITED = 0;

    private final static Integer VISITING = 1;

    private final static Integer VISITED = 2;

    /**
     * @param graph
     * @return List of String (vertex labels)
     */

    public static List sort( final DAG graph )
    {
        return dfs( graph );
    }

    public static List sort( final Vertex vertex )
    {
        // we need to use addFirst method so we will use LinkedList explicitly
        final List retValue = new LinkedList();

        dfsVisit( vertex, new HashMap(), retValue );

        return retValue;
    }

    private static List dfs( final DAG graph )
    {
        // we need to use addFirst method so we will use LinkedList explicitly
        final List retValue = new LinkedList();
        final Map vertexStateMap = new HashMap();

        for ( Vertex vertex : graph.getVertices() )
        {
            if ( isNotVisited( vertex, vertexStateMap ) )
            {
                dfsVisit( vertex, vertexStateMap, retValue );
            }
        }

        return retValue;
    }

    /**
     * @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 );
    }

    private static void dfsVisit( final Vertex vertex, final Map vertexStateMap,
                                  final List list )
    {
        vertexStateMap.put( vertex, VISITING );

        for ( Vertex v : vertex.getChildren() )
        {
            if ( isNotVisited( v, vertexStateMap ) )
            {
                dfsVisit( v, vertexStateMap, list );
            }
        }

        vertexStateMap.put( vertex, VISITED );

        list.add( vertex.getLabel() );
    }

}