1 (define (front-ptr deque) (car deque))
2 (define (rear-ptr deque) (cdr deque))
3 (define (set-front-ptr! deque item) (set-car! deque item))
4 (define (set-rear-ptr! deque item) (set-cdr! deque item))
5 
6 (define (make-deque) (cons '() '()))
7 (define (empty-deque? deque)
8   (and (null? (front-ptr deque))
9        (null? (rear-ptr deque))))
10 (define (print-deque deque)
11   (define (make-printable-list q)
12     (if (null? q)
13 	'()
14 	(cons (car q)
15 	      (make-printable-list (cddr q)))))
16   (newline)
17   (display (make-printable-list (front-ptr deque))))
18 (define (rear-insert-deque! deque item)
19   (let ((new-pair (cons item (cons '() '()))))
20     (cond ((empty-deque? deque)
21 	   (set-front-ptr! deque new-pair)
22 	   (set-rear-ptr! deque new-pair))
23 	  (else
24 	   (set-car! (cdr new-pair) (rear-ptr deque))
25 	   (set-cdr! (cdr (rear-ptr deque)) new-pair)
26 	   (set-rear-ptr! deque new-pair)))))
27 (define (front-insert-deque! deque item)
28   (let ((new-pair (cons item (cons '() '()))))
29     (cond ((empty-deque? deque)
30 	   (set-front-ptr! deque new-pair)
31 	   (set-rear-ptr! deque new-pair))
32 	  (else
33 	   (set-cdr! (cdr new-pair) (front-ptr deque))
34 	   (set-car! (cdr (front-ptr deque)) new-pair)
35 	   (set-front-ptr! deque new-pair)))))
36 (define (front-deque deque)
37   (if (empty-deque? deque)
38       (error "FRONT called with an empty deque" deque)
39       (car (front-ptr deque))))
40 (define (rear-deque deque)
41   (if (empty-deque? deque)
42       (error "REAR called with an empty deque" deque)
43       (car (rear-ptr deque))))
44 (define (front-delete-deque! deque)
45   (cond ((empty-deque? deque)
46 	 (error "FRONT-DELETE! called with an empty deque" deque))
47 	((eq? (front-ptr deque) (rear-ptr deque))
48 	 (set-front-ptr! deque '())
49 	 (set-rear-ptr! deque '()))
50 	(else
51 	 (set-front-ptr! deque (cddr (front-ptr deque)))
52 	 (set-car! (cdr (front-ptr deque)) '()))))
53 (define (rear-delete-deque! deque)
54   (cond ((empty-deque? deque)
55 	 (error "REAR-DELETE! called with an empty deque" deque))
56 	((eq? (front-ptr deque) (rear-ptr deque))
57 	 (set-front-ptr! deque '())
58 	 (set-rear-ptr! deque '()))
59 	(else
60 	 (set-rear-ptr! deque (cadr (rear-ptr deque)))
61 	 (set-cdr! (cdr (rear-ptr deque)) '()))))
62 
63