;; -- reverse.scm -- (in class)
;;
;;   (1 2 3)  is  (cons 1 (cons 2 (cons 3 '())))
;;   (3 2 1)  is  (cons 3 (cons 2 (cons 1 '())))
;;
;;   -- steps to turn one to the other --
;;  
;;   original                            reverse
;;   ------------------------------      -----------------------------
;;   (cons 1 (cons 2 (cons 3 '())))      '()
;;           (cons 2 (cons 3 '()))       (cons 1 '())
;;                   (cons 3 '())        (cons 2 (cons 1 '()))
;;                           '()         (cons 3 (cons 2 (cons 1 '()))
;;
;;

(define (reverse items)
  (define (reverse-iter original backwards)
    (printf " debug: (reverse-iter ~a ~a)\n" original backwards)
    (if (null? original)
        backwards
        (let ((new-original   (cdr original))
              (new-backwards  (cons (car original) backwards)))
          (reverse-iter new-original new-backwards))))
  (reverse-iter items '()))

(define testcase (list 1 2 3 4))
(printf "testcase is ~a\n" testcase)
(printf "(reverse testcase) is ~a\n" (reverse testcase))

; $ scheme reverse.scm
; testcase is (1 2 3 4)
;  debug: (reverse-iter (1 2 3 4) ())
;  debug: (reverse-iter (2 3 4) (1))
;  debug: (reverse-iter (3 4) (2 1))
;  debug: (reverse-iter (4) (3 2 1))
;  debug: (reverse-iter () (4 3 2 1))
; (reverse testcase) is (4 3 2 1)