; Copyright (C) 2014, 2015 by Alexandru Cojocaru

; This program is free software: you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation, either version 3 of the License, or
; (at your option) any later version.

; This program is distributed in the hope that it will be useful,
; but WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
; GNU General Public License for more details.

; You should have received a copy of the GNU General Public License
; along with this program.  If not, see <http://www.gnu.org/licenses/>.

(define (rot c a)
  (integer->char (+ (modulo (+ (- (char->integer c) (char->integer a)) 13) 26) (char->integer a))))

(define (rot13 str)
  (string-map (lambda (c)
                  (if (and (char>=? c #\a) (char<=? c #\z))
                      (rot c #\a)
                      (if (and (char>=? c #\A) (char<=? c #\Z))
                          (rot c #\A)
                          c))) str))

(display (rot13 "The butler did it!")) (newline)
(display (rot13 "Cebtenzzvat Cenkvf vf sha!")) (newline)