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.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
/**
* @author <a href="michal.maczka@dimatics.com">Michal Maczka</a>
* @version $Id$
*/
public class CycleDetector
{
private final static Integer NOT_VISTITED = new Integer( 0 );
private final static Integer VISITING = new Integer( 1 );
private final static Integer VISITED = new Integer( 2 );
public static List hasCycle( final DAG graph )
{
final List verticies = graph.getVerticies();
final Map vertexStateMap = new HashMap();
List retValue = null;
for ( final Iterator iter = verticies.iterator(); iter.hasNext(); )
{
final Vertex vertex = ( Vertex ) iter.next();
if ( isNotVisited( vertex, vertexStateMap ) )
{
retValue = introducesCycle( vertex, vertexStateMap );
if ( retValue != null )
{
break;
}
}
}
return retValue;
}
/**
* This method will be called when an egde 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
* @param vertexStateMap
* @return
*/
public static List introducesCycle( final Vertex vertex, final Map vertexStateMap )
{
final LinkedList 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 occurence of this label and use its position in the list
// for getting the sublist of vertex labels of cycle paricipants
//
// So in our case we are seraching for [b, a, c, d, b]
final String label = ( String ) cycleStack.getFirst();
final int pos = cycleStack.lastIndexOf( label );
final List cycle = cycleStack.subList( 0, pos + 1 );
Collections.reverse( cycle );
return cycle;
}
return null;
}
public static List introducesCycle( final Vertex vertex )
{
final Map vertexStateMap = new HashMap();
return introducesCycle( vertex, vertexStateMap );
}
/**
* @param vertex
* @param vertexStateMap
* @return
*/
private static boolean isNotVisited( final Vertex vertex, final Map vertexStateMap )
{
if ( !vertexStateMap.containsKey( vertex ) )
{
return true;
}
final Integer state = ( Integer ) vertexStateMap.get( vertex );
return NOT_VISTITED.equals( state );
}
/**
* @param vertex
* @param vertexStateMap
* @return
*/
private static boolean isVisiting( final Vertex vertex, final Map vertexStateMap )
{
final Integer state = ( Integer ) vertexStateMap.get( vertex );
return VISITING.equals( state );
}
private static boolean dfsVisit( final Vertex vertex, final LinkedList cycle, final Map vertexStateMap )
{
cycle.addFirst( vertex.getLabel() );
vertexStateMap.put( vertex, VISITING );
final List verticies = vertex.getChildren();
for ( final Iterator iter = verticies.iterator(); iter.hasNext(); )
{
final Vertex v = ( Vertex ) iter.next();
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;
}
}