/*
 * 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.internal.helpers.collection;

import java.util.Iterator;

/**
 * Concatenates sub-iterators of an iterator.
 *
 * Iterates through each item in an iterator. For each item, the
 * {@link #createNestedIterator(Object)} is invoked to create a sub-iterator.
 * The resulting iterator iterates over each item in each sub-iterator. In
 * effect flattening the iteration.
 *
 * @param <T> the type of items to return
 * @param <U> the type of items in the surface item iterator
 */
public abstract class NestingIterator<T, U> extends PrefetchingIterator<T> {
    private final Iterator<U> source;
    private Iterator<T> currentNestedIterator;

    public NestingIterator(Iterator<U> source) {
        this.source = source;
    }

    protected abstract Iterator<T> createNestedIterator(U item);

    @Override
    protected T fetchNextOrNull() {
        if (currentNestedIterator == null || !currentNestedIterator.hasNext()) {
            while (source.hasNext()) {
                U currentSurfaceItem = source.next();
                currentNestedIterator = createNestedIterator(currentSurfaceItem);
                if (currentNestedIterator.hasNext()) {
                    break;
                }
            }
        }
        return currentNestedIterator != null && currentNestedIterator.hasNext() ? currentNestedIterator.next() : null;
    }
}
