(define-module consqueue
  (export make-queue queue? queue-empty?
          queue-push!
          queue-pop! queue-front list->queue))

(select-module consqueue)

(define-syntax apply-car!
  (syntax-rules ()
    ((_ pair fn) (set-car! pair (fn (car pair))))))
(define-syntax apply-cdr!
  (syntax-rules ()
    ((_ pair fn) (set-cdr! pair (fn (cdr pair))))))

(define (make-queue)
  '(() . ()))

(define (queue? obj)
  (and (list? (car obj)) (list? (cdr obj))))

(define (queue-empty? queue)
  (and (null? (car obj)) (null? (cdr obj))))

(define (queue-push! queue obj)
  (apply-car queue (cut cons obj <>)))

(define (queue-pop! que fallback)
  (cond
    ((queue-empty? que) fallback)
    ((null? (cdr que))
     (let1 reversed (reverse (car que))
       (set-car! que '())
       (set-cdr! que (cdr reversed))
       (values (car reversed) que)))
    (else
      (let1 obj (cadr que)
        (apply-cdr! que cdr)
        (values obj que)))))

(define (queue-front queue fallback)
  (cond
    ((queue-empty? queue) fallback)
    ((null? (cdr queue))
     (let1 reversed (reverse (car queue))
       (set-car! queue '())
       (set-cdr! queue reversed)
       (cadr queue)))
    (else
      (cadr queue))))

(define (list->queue lst)
  (cons lst '()))