;;; -*- Scheme -*-

(declare (usual-integrations))

;;;; Message queues

(define (make-empty-queue tag)
  (let ((data (list tag)))
    (cons data data)))

(define-integrable (queue/items q) (car q))
(define-integrable (queue/last q) (cdr q))
(define-integrable (set-queue/last! q item)
  (set-cdr! q item))

(define (queue/empty? q)
  (eq? (queue/items q) (queue/last q)))

(define (queue/add! q item)
  ;; make sure this executes atomically
  (let ((last-element (list item)))
    (set-cdr! (queue/last q) last-element)
    (set-queue/last! q last-element)
    q))

(define (queue/remove! q)
  ;; make sure this executes atomically
  (let ((data (queue/items q)))
    (let ((msg (cadr data)))
      (if (eq? (cdr data) (queue/last q))
	  (set-queue/last! q data))
      (set-cdr! data (cddr data))
      msg)))

(define (queue/length q)
  ;; subtract 1 for the tag
  (- (length (queue/items q)) 1))