#lang racket

(define (string->char-list text)
  (string->list text))

(define (char-list->string clist)
  (list->string clist))

(define (char-list->ascii-list char-list)
  (if (eq? char-list '())
      '()
      (cons (char->integer (car char-list))
            (char-list->ascii-list (cdr char-list)))))


(define (ascii-list->char-list ascii-list)
  (if (eq? ascii-list '())
      '()
      (cons (integer->char (car ascii-list))
            (ascii-list->char-list (cdr ascii-list)))))

(define (encode text i)
  (let ((ascii-list (char-list->ascii-list
                     (string->char-list
                      text))))
    (define (encode-iter ascii)
      (if (eq? ascii '())
          '()
          (cons (round (modulo (+ (car ascii) i) 26))
                (encode-iter (cdr ascii)))))
    (char-list->string
     (ascii-list->char-list (encode-iter ascii-list)))))

(define (decode text i)
  (let ((ascii-list (char-list->ascii-list
                     (string->char-list
                      text))))
    (define (decode-iter ascii)
      (if (eq? ascii '())
          '()
          (cons (- (car ascii) i)
                (decode-iter (cdr ascii)))))
    (char-list->string
     (ascii-list->char-list
      (decode-iter ascii-list)))))

(define hello "helloz")

(define encoded (encode hello 2))

(decode encoded 2)