Simple rot13 encoder written in CLISP.

rot13.lisp

#|
| A Simple rot13 - encoder written in CLISP
|
    rot13.lisp version 0.0.1
    Copyright (C) 2012  @d3f (http://identi.ca/d3f)
 
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU Affero 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 Affero General Public License for more details.

    You should have received a copy of the GNU Affero General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
|
| How-to:
| type in `(load "rot13.lisp") and run it with (rot13)
|#

(defparameter code-list (list))
(defparameter char-list (list))

(defun coder (input-string)
    (map 'list #'make-code-list input-string)
    (map 'list #'make-char-list code-list)
    (format t "~%Your string in rot13: ~a~%" (concatenate 'string char-list))
    (setf code-list (list))
    (setf char-list (list))
  )

(defun make-code-list (char)
  (setf code-list (cons (char-code char) code-list))
  )

(defun make-char-list (code)
  (when (and (>= code 97) (<= code 122)) ;; a == 97 and z == 122, that's why we did this.
    (incf code 13)
    (if (> code 122)
      (setf code (- code 26)) 
      )
    )
  (when (and (>= code 65) (<= code 90)) ;; Same for A & Z
    (incf code 13)
    (if (> code 90)
      (setf code (- code 26))
      )
    )
  (setf char-list (cons (code-char code) char-list))
  )

(defun rot13 ()
  (format t "Type in, what you want. I will give you rot13. But `quit' will exit.~%")
  (let ((str (read-line)))
    (cond
      ((equal str "quit") (format t "Good bye ~%"))
      (t (coder str) (rot13)))))