;;  -----------------------------------------------------------------------
;; |                                                                       |
;; |   FILE           :   factorial.rkt                                    |
;; |   AUTHOR         :   Eugene Wallingford                               |
;; |   CREATION DATE  :   2024/02/20                                       |
;; |                                                                       |
;; |   DESCRIPTION    :   A tail-recursive implementation of factorial.    |
;; |                      The second parameter on factorial-aps is an      |
;; |                      *accumulator variable*.  We then define          |
;; |                      factorial as an *interface procedure* that       |
;; |                      calls factorial-aps to do the work.              |
;; |                                                                       |
;; |   REQUIRES       :   Uses Rackunit for testing.                       |
;; |                                                                       |
;;  -----------------------------------------------------------------------
;; |                                                                       |
;; |   MODIFIED       :                                                    |
;; |                                                                       |
;;  -----------------------------------------------------------------------

#lang racket
(require rackunit)

; -------------------------------------------------------------------------
; factorial                                                (tail recursion)
; -------------------------------------------------------------------------

(define factorial-aps
  (lambda (n answer)
    (if (zero? n)
        answer
        (factorial-aps (sub1 n) (* n answer)))))

(check-equal? (factorial-aps 10 1)
              3628800)
(check-equal? (factorial-aps 40 1)
              815915283247897734345611269596115894272000000000)

; -------------------------------------------------------------------------
; factorial                                           (interface procedure)
; -------------------------------------------------------------------------

(define factorial
  (lambda (n)
    (factorial-aps n 1)))

(check-equal? (factorial 10)
              3628800)
(check-equal? (factorial 40)
              815915283247897734345611269596115894272000000000)

; -------------------------------------------------------------------------

;; (require racket/trace)
;; (trace factorial-aps)
;; (factorial-aps 6 1)

; -------------------------------------------------------------------------