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 <a href="michal.maczka@dimatics.com">Michal Maczka</a>
   *
   */
  public class CycleDetector {
  
      private static final Integer NOT_VISITED = 0;
  
      private static final Integer VISITING = 1;
  
      private static final Integer VISITED = 2;
  
      public static List<String> hasCycle(final DAG graph) {
          final List<Vertex> vertices = graph.getVertices();
  
          final Map<Vertex, Integer> vertexStateMap = new HashMap<>();
  
          List<String> 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 the vertex
       * @param vertexStateMap the vertex Map
       * @return the found cycle
       */
      public static List<String> introducesCycle(final Vertex vertex, final Map<Vertex, Integer> vertexStateMap) {
          final LinkedList<String> 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<String> cycle = cycleStack.subList(0, pos + 1);
  
              Collections.reverse(cycle);
  
              return cycle;
          }
  
          return null;
      }
  
      public static List<String> introducesCycle(final Vertex vertex) {
          final Map<Vertex, Integer> vertexStateMap = new HashMap<>();
  
          return introducesCycle(vertex, vertexStateMap);
      }
  
      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 boolean isVisiting(final Vertex vertex, final Map<Vertex, Integer> vertexStateMap) {
         final Integer state = vertexStateMap.get(vertex);
 
         return VISITING.equals(state);
     }
 
     private static boolean dfsVisit(
             final Vertex vertex, final LinkedList<String> cycle, final Map<Vertex, Integer> 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;
     }
 }