summingbuffer.scala

// Adds an item to the buffer and returns None, or fills the buffer and returns the sum
  @tailrec
  final def offer(item: V, acc: Option[V] = None): Option[V] =
    if(!queue.offer(item)) {
      // Queue is full
      val toSum = ListBuffer[V]()
      queue.drainTo(toSum.asJava)
      // Add everything up and get the new acc:
      val newAcc = Monoid.plus(acc, Some(Monoid.sum(toSum)))
      // We never actually got the item in:
      offer(item, newAcc)
    }
    else {
      // We added, so just return the accumulator
      acc
    }