/**
 * Copyright (c) 2002-2010 "Neo Technology,"
 * Network Engine for Objects in Lund AB [http://neotechnology.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 Affero 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 Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 */

package org.neo4j.helpers.collection;

import java.util.Iterator;

/**
 * Combining one or more {@link Iterator}s, making them look like they were
 * one big iterator. All iteration/combining is done lazily.
 * 
 * @param <T> the type of items in the iteration.
 */
public class CombiningIterator<T> extends PrefetchingIterator<T>
{
    private Iterator<Iterator<T>> iterators;
    private Iterator<T> currentIterator;
    
    public CombiningIterator( Iterable<Iterator<T>> iterators )
    {
        this.iterators = iterators.iterator();
    }

    @Override
    protected T fetchNextOrNull()
    {
        if ( currentIterator == null || !currentIterator.hasNext() )
        {
            while ( iterators.hasNext() )
            {
                currentIterator = iterators.next();
                if ( currentIterator.hasNext() )
                {
                    break;
                }
            }
        }
        return currentIterator != null && currentIterator.hasNext() ?
            currentIterator.next() : null;
    }
}