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post.scala

private def getQueueForRunnable(command: Runnable) = {

  val runnableHash = command.hashCode()
  def sbhash(i: Int) = reverseBytes(i * 0x9e3775cd) * 0x9e3775cd
  def getNext = executionCounter.incrementAndGet() % parallelism

  def updateIfAbsentAndGetQueueIndex(
    workerQueueIndex: AtomicReference[ImmutableIntMap],
    runnableHash:     Int, queueIndex: ⇒ Int): Int = {
    @tailrec
    def updateIndex(): Unit = {
      val prev = workerQueueIndex.get()
      if (!runnableToWorkerQueueIndex.
      	compareAndSet(prev, prev.updateIfAbsent(runnableHash, queueIndex))) {
        updateIndex()
      }
    }
    updateIndex()
    workerQueueIndex.get().get(runnableHash) // can safely call get..
  }

  val workQueueIndex =
    if (fairDistributionThreshold == 0
     || runnableToWorkerQueueIndex.get().size > fairDistributionThreshold)
      Math.abs(sbhash(runnableHash)) % parallelism
    else
      updateIfAbsentAndGetQueueIndex(runnableToWorkerQueueIndex, runnableHash, getNext)

  workQueues(workQueueIndex)
}