/**
 * Copyright (c) 2014 Cisco Systems, Inc. and others.  All rights reserved.
 * 
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v1.0 which accompanies this distribution,
 * and is available at http://www.eclipse.org/legal/epl-v10.html
 */
package org.opendaylight.openflowplugin.api.openflow.md.util;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Queue;

import com.google.common.collect.Iterators;

/**
 * Zipper groups together a list of queues and exposes one poll method. Polling iterates through
 * all groups and returns first not-null result of poll method on each queue. If after polling each 
 * grouped queue for one time there is still null result, poll will return null. 
 * <br>
 * Iterating keeps last position so this polling is supposed to be fairly distributed.
 * 
 * @param <T> common item type of zipped queues
 */
public class PollableQueuesZipper<T> {
    
    private List<Queue<T>> sources;
    private Iterator<Queue<T>> cursor;
    
    /**
     * default ctor
     */
    public PollableQueuesZipper() {
        sources = new ArrayList<>();
    }
    
    /**
     * Add all member queues before first invocation of {@link PollableQueuesZipper#poll()}
     * @param queue to be added to group
     */
    public void addSource(Queue<T> queue) {
        sources.add(queue);
    }

    /**
     * @return next common product of polling member groups
     */
    public T poll() {
        T item = null;
        if (cursor == null) {
            cursor = Iterators.cycle(sources);
        }
        
        Queue<T> queue;
        for (int i = 0; i < sources.size(); i++) {
            queue = cursor.next();
            item = queue.poll();
            if (item != null) {
                break;
            }
        }
        
        return item;
    }
}