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