content: package backtype.storm.utils;
  
  import java.util.ArrayList;
  import java.util.HashMap;
  import java.util.LinkedList;
  import java.util.List;
  import java.util.Map;
  import java.util.Queue;
  import java.util.concurrent.Semaphore;
  
  public class KeyedRoundRobinQueue<V> {
      private final Object _lock = new Object();
      private Semaphore _size = new Semaphore(0);
      private Map<Object, Queue<V>> _queues = new HashMap<Object, Queue<V>>();
      private List<Object> _keyOrder = new ArrayList<Object>();
      private int _currIndex = 0;
  
      public void add(Object key, V val) {
          synchronized(_lock) {
              Queue<V> queue = _queues.get(key);
              if(queue==null) {
                  queue = new LinkedList<V>();
                  _queues.put(key, queue);
                  _keyOrder.add(key);
              }
              queue.add(val);
          }
          _size.release();
      }
  
      public V take() throws InterruptedException {
          _size.acquire();
          synchronized(_lock) {
              Object key = _keyOrder.get(_currIndex);
              Queue<V> queue = _queues.get(key);
              V ret = queue.remove();
              if(queue.isEmpty()) {
                  _keyOrder.remove(_currIndex);
                  _queues.remove(key);
                  if(_keyOrder.size()==0) {
                      _currIndex = 0;
                  } else {
                      _currIndex = _currIndex % _keyOrder.size();
                  }
              } else {
                  _currIndex = (_currIndex + 1) % _keyOrder.size();
              }
              return ret;
          }
      }
  }
  