(define-class
  'stack
  object-class
  '(num-items
   item-vector)
  '(push
    pop
    top
    height
    empty?
    display))

(class/set-method!
 stack-class 'init
 (lambda (this)
   (stack/set-num-items! this 0)
   (stack/set-item-vector! this (make-vector 10))))
 
(class/set-method!
 stack-class 'top
 (lambda (this)
   (vector-ref (stack/get-item-vector this)
               (- (stack/get-num-items this) 1))))

(class/set-method!
 stack-class 'push
 (lambda (this item)
    (vector-set! (stack/get-item-vector this)
                 (stack/get-num-items this) item)
    (stack/set-num-items! this
                          (+ (stack/get-num-items this) 1))))
   
(class/set-method!
 stack-class 'pop
 (lambda (this)
   (if (stack/empty? this)
       (error "can't pop an empty stack")
       (let ((item (stack/top this)))
         (stack/set-num-items! this
                               (- (stack/get-num-items this) 1))
         item))))

(class/set-method!
 stack-class 'height
 (lambda (this)
   (stack/get-num-items this)))

(class/set-method!
 stack-class 'empty?
 (lambda (this)
   (zero? (stack/height this))))

(class/set-method!
 stack-class 'display
 (lambda (this)
   (do ((n (- (stack/get-num-items this) 1) (- n 1)))
     ((negative? n))
     (display " ")
     (display (vector-ref (stack/get-item-vector this) n)))))