/*
 * Copyright (c) "Neo4j"
 * Neo4j Sweden AB [http://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 <http://www.gnu.org/licenses/>.
 */
package org.neo4j.graphalgo.impl.util;

import java.util.LinkedList;
import java.util.Queue;
import org.neo4j.graphdb.PathExpander;
import org.neo4j.graphdb.traversal.BranchSelector;
import org.neo4j.graphdb.traversal.TraversalBranch;
import org.neo4j.graphdb.traversal.TraversalContext;

/**
 * A preorder depth first selector which detects "super nodes", i.e. nodes
 * which has many relationships. It delays traversing those super nodes until
 * after all non-super nodes have been traversed.
 */
public class LiteDepthFirstSelector implements BranchSelector {
    private final Queue<TraversalBranch> superNodes = new LinkedList<>();
    private TraversalBranch current;
    private final int threshold;
    private final PathExpander expander;

    public LiteDepthFirstSelector(TraversalBranch startSource, int startThreshold, PathExpander expander) {
        this.current = startSource;
        this.threshold = startThreshold;
        this.expander = expander;
    }

    @Override
    public TraversalBranch next(TraversalContext metadata) {
        TraversalBranch result = null;
        while (result == null) {
            if (current == null) {
                current = superNodes.poll();
                if (current == null) {
                    return null;
                }
            } else if (current.expanded() > 0 && current.expanded() % threshold == 0) {
                superNodes.add(current);
                current = current.parent();
                continue;
            }

            TraversalBranch next = current.next(expander, metadata);
            if (next == null) {
                current = current.parent();
                continue;
            }
            current = next;
            result = current;
        }
        return result;
    }
}
