;;
;; length, copy, and append ... in class
;;

(define things1 (list 1 2 3 4 5))
(define things2 (list 10 20 30 40 50))

(define (length stuff)
  ;; O(n) time
  ;; O(n) space
  (if (null? stuff)
      0
      (+ 1 (length (cdr stuff)))))

(printf "(length things1 is ~a.\n" (length things1))

(define (length2 stuff)
  ;; O(n) time
  ;; O(1) space
  (define (length2-iter stuff total)
    (if (null? stuff)
        total
        (length2-iter (cdr stuff) (+ 1 total))))
  (length2-iter stuff 0))

(printf "(length2 things2 is ~a.\n" (length2 things2))

(define (copy items)
  ;; return a copy of a list
  (if (null? items)
      '()
      (cons (car items) (copy (cdr items)))))

(printf "(copy things1) is ~a.\n" (copy things1))

(define (append stuff1 stuff2)
  ;; make a new list with one list after the other list
  (if (null? stuff1)
      (copy stuff2)
      (cons (car stuff1)
            (append (cdr stuff1) stuff2))))

(printf "(append things1 things2) is ~a.\n"
        (append things1 things2))