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Neo4j kernel is a lightweight, embedded Java database designed to store data structured as graphs rather than tables. For more information, see http://neo4j.org.

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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.kernel.impl.traversal;

import java.util.Iterator;
import org.neo4j.function.Factory;
import org.neo4j.graphdb.Node;
import org.neo4j.graphdb.Path;
import org.neo4j.graphdb.Relationship;
import org.neo4j.graphdb.traversal.TraversalMetadata;
import org.neo4j.graphdb.traversal.Traverser;
import org.neo4j.internal.helpers.collection.PrefetchingIterator;

public class DefaultTraverser implements Traverser {
    private final Factory traverserIteratorFactory;

    private TraversalMetadata lastIterator;

    DefaultTraverser(Factory traverserIteratorFactory) {
        this.traverserIteratorFactory = traverserIteratorFactory;
    }

    @Override
    public Iterable nodes() {
        return new PathIterableWrapper<>(this) {
            @Override
            protected Node convert(Path path) {
                return path.endNode();
            }
        };
    }

    @Override
    public Iterable relationships() {
        return new PathIterableWrapper<>(this) {
            @Override
            public Iterator iterator() {
                var pathIterator = pathIterator();
                return new PrefetchingIterator<>() {
                    @Override
                    protected Relationship fetchNextOrNull() {
                        while (pathIterator.hasNext()) {
                            Path path = pathIterator.next();
                            if (path.length() > 0) {
                                return path.lastRelationship();
                            }
                        }
                        return null;
                    }
                };
            }

            @Override
            protected Relationship convert(Path path) {
                return path.lastRelationship();
            }
        };
    }

    @Override
    public Iterator iterator() {
        TraverserIterator traverserIterator = traverserIteratorFactory.newInstance();
        lastIterator = traverserIterator;
        return traverserIterator;
    }

    @Override
    public TraversalMetadata metadata() {
        return lastIterator;
    }

    private abstract static class PathIterableWrapper implements Iterable {
        private final Iterable iterableToWrap;

        PathIterableWrapper(Iterable iterableToWrap) {
            this.iterableToWrap = iterableToWrap;
        }

        Iterator pathIterator() {
            return iterableToWrap.iterator();
        }

        @Override
        public Iterator iterator() {
            var iterator = pathIterator();
            return new PrefetchingIterator<>() {
                @Override
                protected T fetchNextOrNull() {
                    return iterator.hasNext() ? convert(iterator.next()) : null;
                }
            };
        }

        protected abstract T convert(Path path);
    }
}