package org.eclipse.aether.util.graph.visitor;

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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 org.eclipse.aether.graph.DependencyNode;

/**
 * Generates a sequence of dependency nodes from a dependeny graph by traversing the graph in postorder. This visitor
 * visits each node exactly once regardless how many paths within the dependency graph lead to the node such that the
 * resulting node sequence is free of duplicates.
 */
public final class PostorderNodeListGenerator
    extends AbstractDepthFirstNodeListGenerator
{

    private final Stack<Boolean> visits;

    /**
     * Creates a new postorder list generator.
     */
    public PostorderNodeListGenerator()
    {
        visits = new Stack<Boolean>();
    }

    @Override
    public boolean visitEnter( DependencyNode node )
    {
        boolean visited = !setVisited( node );

        visits.push( visited );

        if ( visited )
        {
            return false;
        }

        return true;
    }

    @Override
    public boolean visitLeave( DependencyNode node )
    {
        Boolean visited = visits.pop();

        if ( visited )
        {
            return true;
        }

        if ( node.getDependency() != null )
        {
            nodes.add( node );
        }

        return true;
    }

}