/**
 * Copyright (c) 2002-2010 "Neo Technology,"
 * Network Engine for Objects in Lund AB [http://neotechnology.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation, either version 3 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 */

package org.neo4j.kernel;

import java.util.LinkedList;
import java.util.Queue;

import org.neo4j.graphdb.traversal.TraversalBranch;
import org.neo4j.graphdb.traversal.BranchSelector;

/**
 * Selects {@link TraversalBranch}s according to breadth first
 * pattern, the most natural ordering in a breadth first search, see
 * http://en.wikipedia.org/wiki/Breadth-first_search
 */
class PreorderBreadthFirstSelector implements BranchSelector
{
    private final Queue<TraversalBranch> queue = new LinkedList<TraversalBranch>();
    private TraversalBranch current;
    
    PreorderBreadthFirstSelector( TraversalBranch startSource )
    {
        this.current = startSource;
    }

    public TraversalBranch next()
    {
        TraversalBranch result = null;
        while ( result == null )
        {
            TraversalBranch next = current.next();
            if ( next != null )
            {
                queue.add( next );
                result = next;
            }
            else
            {
                current = queue.poll();
                if ( current == null )
                {
                    return null;
                }
            }
        }
        return result;
    }
}