001package org.eclipse.aether.util.graph.visitor;
002
003/*
004 * Licensed to the Apache Software Foundation (ASF) under one
005 * or more contributor license agreements.  See the NOTICE file
006 * distributed with this work for additional information
007 * regarding copyright ownership.  The ASF licenses this file
008 * to you under the Apache License, Version 2.0 (the
009 * "License"); you may not use this file except in compliance
010 * with the License.  You may obtain a copy of the License at
011 * 
012 *  http://www.apache.org/licenses/LICENSE-2.0
013 * 
014 * Unless required by applicable law or agreed to in writing,
015 * software distributed under the License is distributed on an
016 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
017 * KIND, either express or implied.  See the License for the
018 * specific language governing permissions and limitations
019 * under the License.
020 */
021
022import java.util.IdentityHashMap;
023import java.util.Map;
024
025import org.eclipse.aether.graph.DependencyNode;
026import org.eclipse.aether.graph.DependencyVisitor;
027
028/**
029 * A dependency visitor that delegates to another visitor if a node hasn't been visited before. In other words, this
030 * visitor provides a tree-view of a dependency graph which generally can have multiple paths to the same node or even
031 * cycles.
032 */
033public final class TreeDependencyVisitor
034    implements DependencyVisitor
035{
036
037    private final Map<DependencyNode, Object> visitedNodes;
038
039    private final DependencyVisitor visitor;
040
041    private final Stack<Boolean> visits;
042
043    /**
044     * Creates a new visitor that delegates to the specified visitor.
045     * 
046     * @param visitor The visitor to delegate to, must not be {@code null}.
047     */
048    public TreeDependencyVisitor( DependencyVisitor visitor )
049    {
050        if ( visitor == null )
051        {
052            throw new IllegalArgumentException( "no visitor delegate specified" );
053        }
054        this.visitor = visitor;
055        visitedNodes = new IdentityHashMap<DependencyNode, Object>( 512 );
056        visits = new Stack<Boolean>();
057    }
058
059    public boolean visitEnter( DependencyNode node )
060    {
061        boolean visited = visitedNodes.put( node, Boolean.TRUE ) != null;
062
063        visits.push( visited );
064
065        if ( visited )
066        {
067            return false;
068        }
069
070        return visitor.visitEnter( node );
071    }
072
073    public boolean visitLeave( DependencyNode node )
074    {
075        Boolean visited = visits.pop();
076
077        if ( visited )
078        {
079            return true;
080        }
081
082        return visitor.visitLeave( node );
083    }
084
085}