;; Message-Queues für je einen Producer/Consumer

(define-record-type :mqueue
  (really-make-mqueue front rear semaphore)
  mqueue?
  (front mqueue-front set-mqueue-front!)
  (rear mqueue-rear set-mqueue-rear!)
  (semaphore mqueue-semaphore))

(define *nothing* (list '*nothing*))

(define (make-mqueue)
  (let ((pair (cons *nothing* '())))
    (really-make-mqueue pair
		       pair
		       (make-semaphore 0))))

(define (add-to-mqueue! mqueue value)
  (let ((pair (cons value '())))
    (set-cdr! (mqueue-rear mqueue) pair)
    (set-mqueue-rear! mqueue pair))
  (semaphore-post (mqueue-semaphore mqueue)))

(define (remove-from-mqueue mqueue)
  (semaphore-wait (mqueue-semaphore mqueue))
  (let* ((front (mqueue-front mqueue))
	 (next (cdr front))
	 (value (car next)))
    (set-mqueue-front! mqueue next)
    (set-car! next (car front))
    value))