;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;
;                               LQUEUE
;
;              Queue implemented using a list (scm list)
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define queue (make-record-type "queue" '(lq)))
(define create-queue (record-constructor queue '(lq)))
(define (make-queue) (create-queue '()))
;;;
;;; make a queue, and it's empty
;;;

(define queue? (record-predicate queue))

(define lq (record-accessor queue 'lq))

(define lq! (record-modifier queue 'lq))

(define (empty? q)
  (null? (lq q)))

(define q (make-queue))

(define (enqueue e q)
  (lq! q (append (lq q) (list e))))

(define (dequeue q)
  (cond ((empty? q) 'underflow)
	(t (let ((e (car (lq q))))
	     (lq! q (cdr (lq q)))
	     e))))

(define (front q)
  (cond ((empty? q) 'empty)
	(t (car (lq q)))))

;;;
;;; NOTE that how append operates is outwith our control
;;; consequently this "could" be a very inefficient implementation 
;;; of a queue, but it is simple, it works, it allows the
;;; queue to grow to a size limited only by virtual memeory (vm)
;;; and it is simple (you guessed, I like simplicity)
;;;
;;;