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 static final Integer NOT_VISITED = 0;
  
      private static final Integer VISITING = 1;
  
      private static final 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());
      }
  }