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 <a href="michal.maczka@dimatics.com">Michal Maczka</a>
   *
   */
  public class TopologicalSorter
  {
  
      private final static Integer NOT_VISITED = 0;
  
      private final static Integer VISITING = 1;
  
      private final static Integer VISITED = 2;
  
      /**
       * @param graph the graph
       * @return List of String (vertex labels)
       */
      public static List<String> sort( final DAG graph )
      {
          return dfs( graph );
      }
  
      public static List<String> sort( final Vertex vertex )
      {
          // we need to use addFirst method so we will use LinkedList explicitly
          final List<String> retValue = new LinkedList<>();
  
          dfsVisit( vertex, new HashMap<Vertex, Integer>(), retValue );
  
          return retValue;
      }
  
      private static List<String> dfs( final DAG graph )
      {
          // we need to use addFirst method so we will use LinkedList explicitly
          final List<String> retValue = new LinkedList<>();
          final Map<Vertex, Integer> vertexStateMap = new HashMap<>();
  
          for ( Vertex vertex : graph.getVertices() )
          {
              if ( isNotVisited( vertex, vertexStateMap ) )
              {
                  dfsVisit( vertex, vertexStateMap, retValue );
              }
          }
  
          return retValue;
      }
  
      private static boolean isNotVisited( final Vertex vertex, final Map<Vertex, Integer> vertexStateMap )
      {
          final Integer state = vertexStateMap.get( vertex );
  
          return ( state == null ) || NOT_VISITED.equals( state );
      }
  
      private static void dfsVisit( final Vertex vertex, final Map<Vertex, Integer> vertexStateMap,
                                    final List<String> 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() );
      }
  
  }