#!/usr/bin/env mzscheme

; Special thanks to Vixey in #lispcafe

; within? x lower upper
; Is x is within lower to upper?
(define (within? x lower upper)
  (<= lower x upper))

; wrap-range x lower upper
; If x is above range, it will wrap from lower
; If x is below range, it will wrap from upper
; It will repeat if necessary
(define (wrap-range x lower upper)
  (cond
    ((> x upper) (wrap-range (+ lower (- x upper) -1) lower upper))
    ((< x lower) (wrap-range (- upper (- lower x) -1) lower upper))
    (else x))) 

; rotate-char char amount lower upper -> char
; If char is within lower and upper range, increase char by amount, and if
; it goes over the upper range, then wrap around to the lower range.
(define (rotate-char x amount lower upper)
  (let
      ((x (char->integer x))
       (lower (char->integer lower))
       (upper (char->integer upper)))
    (integer->char
     (if (within? x lower upper) 
         (wrap-range (+ x amount) lower upper)
         x))))

; rot13 x
; Converts char x to its rot13 equivalent
(define (rot13 x)
  (cond 
      ((char-lower-case? x) (rotate-char x 13 #\a #\z))
      ((char-upper-case? in) (rotate-char x 13 #\A #\Z))
      (else x)))

; rot13-string string
; Converts a string to its rot13 equivalent
(define (rot13-string string)
  (list->string (map rot13 (string->list string))))