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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.Iterator;
import java.util.LinkedList;
import java.util.List;
import org.neo4j.graphdb.PathExpander;

/**
 * Selects {@link TraversalBranch}s according to postorder breadth first
 * pattern which basically is a reverse to preorder breadth first in that
 * deepest levels are returned first, see
 * http://en.wikipedia.org/wiki/Breadth-first_search
 */
class PostorderBreadthFirstSelector implements BranchSelector {
    private Iterator sourceIterator;
    private final TraversalBranch current;
    private final PathExpander expander;

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

    @Override
    public TraversalBranch next(TraversalContext metadata) {
        if (sourceIterator == null) {
            sourceIterator = gatherSourceIterator(metadata);
        }
        return sourceIterator.hasNext() ? sourceIterator.next() : null;
    }

    private Iterator gatherSourceIterator(TraversalContext metadata) {
        List queue = new LinkedList<>();
        queue.add(current.next(expander, metadata));
        while (true) {
            List level = gatherOneLevel(queue, metadata);
            if (level.isEmpty()) {
                break;
            }
            queue.addAll(0, level);
        }
        return queue.iterator();
    }

    private List gatherOneLevel(List queue, TraversalContext metadata) {
        List level = new LinkedList<>();
        Integer depth = null;
        for (TraversalBranch source : queue) {
            if (depth == null) {
                depth = source.length();
            } else if (source.length() != depth) {
                break;
            }

            while (true) {
                TraversalBranch next = source.next(expander, metadata);
                if (next == null) {
                    break;
                }
                level.add(next);
            }
        }
        return level;
    }
}