-(define (make-fifo)
- (define (cell val prev next)
- (list val prev next))
- (define cell-val car)
- (define cell-prev cadr)
- (define cell-next caddr)
- (define (set-cell-prev! cell prev-cell)
- (set-car! (cdr cell) prev-cell))
- (define (set-cell-next! cell next-cell)
- (set-car! (cddr cell) next-cell))
-
- (let ((head '())
- (tail '()))
- (lambda (cmd . args)
- (case cmd
- ((empty?) (null? head))
- ((push) (if (not (= (length args) 1))
- (error "Wrong number of arguments to push.")
- (if (not (null? head))
- (let ((old-head head))
- (set! head (cell (car args) '() old-head))
- (set-cell-prev! old-head head))
- (begin
- ;; Initialize list
- (set! head (cell (car args) '() '()))
- (set! tail head)))))
- ((pop) (if (not (= (length args) 0))
- (error "Wrong number of arguments to pop.")
- (if (null? head)
- (error "FIFO empty.")
- (let ((old-tail tail))
- (set! tail (cell-prev old-tail))
- (if (null? tail)
- (set! head '())
- (set-cell-next! tail '()))
- (cell-val old-tail)))))))))
-
-(define (fifo-push fifo x)
- (fifo 'push x))
-
-(define (fifo-pop fifo)
- (fifo 'pop))
-
-(define (fifo-empty? fifo)
- (fifo 'empty?))
+ (define (make-fifo)
+ (define (cell val prev next)
+ (list val prev next))
+ (define cell-val car)
+ (define cell-prev cadr)
+ (define cell-next caddr)
+ (define (set-cell-prev! cell prev-cell)
+ (set-car! (cdr cell) prev-cell))
+ (define (set-cell-next! cell next-cell)
+ (set-car! (cddr cell) next-cell))
+
+ (let ((head '())
+ (tail '()))
+ (lambda (cmd . args)
+ (case cmd
+ ((empty?) (null? head))
+ ((push) (if (not (= (length args) 1))
+ (error "Wrong number of arguments to push.")
+ (if (not (null? head))
+ (let ((old-head head))
+ (set! head (cell (car args) '() old-head))
+ (set-cell-prev! old-head head))
+ (begin
+ ;; Initialize list
+ (set! head (cell (car args) '() '()))
+ (set! tail head)))))
+ ((pop) (if (not (= (length args) 0))
+ (error "Wrong number of arguments to pop.")
+ (if (null? head)
+ (error "FIFO empty.")
+ (let ((old-tail tail))
+ (set! tail (cell-prev old-tail))
+ (if (null? tail)
+ (set! head '())
+ (set-cell-next! tail '()))
+ (cell-val old-tail)))))))))
+
+ (define (fifo-push fifo x)
+ (fifo 'push x))
+
+ (define (fifo-pop fifo)
+ (fifo 'pop))
+
+ (define (fifo-empty? fifo)
+ (fifo 'empty?))