;; Producer-Consumer-Problem mit asynchronem Message-Passing

(define-record-type :buffer
  (really-make-buffer empty-channel
		      insert-channel remove-channel
		      remove-ack-channel)
  buffer?
  (empty-channel buffer-empty-channel)
  (insert-channel buffer-insert-channel)
  (remove-channel buffer-remove-channel)
  (remove-ack-channel buffer-remove-ack-channel))

(define (make-buffer)
  (let ((empty-channel (make-async-channel))
	(insert-channel (make-async-channel))
	(remove-channel (make-async-channel))
	(remove-ack-channel (make-async-channel)))
    (spawn
     (lambda ()
       (letrec
	   ((empty
	     (lambda ()
	       (send-async empty-channel #f)
	       (full (receive-async insert-channel))))
	    (full
	     (lambda (value)
	       (send-async remove-channel value)
	       (receive-async remove-ack-channel)
	       (empty))))
	 (empty))))

    (really-make-buffer empty-channel
			insert-channel
			remove-channel
			remove-ack-channel)))

(define (insert buffer value)
  (receive-async (buffer-empty-channel buffer))
  (send-async (buffer-insert-channel buffer) value))

(define (remove buffer)
  (let ((value
	 (receive-async (buffer-remove-channel buffer))))
    (send-async (buffer-remove-ack-channel buffer) #f)
    value))