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Graph Database API for Neo4j.

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/*
 * Copyright (c) "Neo4j"
 * Neo4j Sweden AB [https://neo4j.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 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see .
 */
package org.neo4j.graphdb.traversal;

import java.util.LinkedList;
import java.util.Queue;
import org.neo4j.graphdb.PathExpander;

/**
 * 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 queue = new LinkedList<>();
    private TraversalBranch current;
    private final PathExpander expander;

    PreorderBreadthFirstSelector(TraversalBranch startSource, PathExpander expander) {
        this.current = startSource;
        this.expander = expander;
    }

    @Override
    public TraversalBranch next(TraversalContext metadata) {
        TraversalBranch result = null;
        while (result == null) {
            TraversalBranch next = current.next(expander, metadata);
            if (next != null) {
                queue.add(next);
                result = next;
            } else {
                current = queue.poll();
                if (current == null) {
                    return null;
                }
            }
        }
        return result;
    }
}