Rot-13: Difference between revisions

=={{header|ACL2}}==

<lang lisp>(include-book "arithmetic-3/top" :dir :system)

(defun char-btn (c low high)

(and (char>= c low)

(char<= c high)))

(defun rot-13-cs (cs)

(cond ((endp cs) nil)

((or (char-btn (first cs) #\a #\m)

(char-btn (first cs) #\A #\M))

(cons (code-char (+ (char-code (first cs)) 13))

(rot-13-cs (rest cs))))

((or (char-btn (first cs) #\n #\z)

(char-btn (first cs) #\N #\Z))

(cons (code-char (- (char-code (first cs)) 13))

(rot-13-cs (rest cs))))

(t (cons (first cs) (rot-13-cs (rest cs))))))

(defun rot-13 (s)

(coerce (rot-13-cs (coerce s 'list)) 'string))</lang>