RosettaCodeData/Task/Evolutionary-algorithm/Common-Lisp/evolutionary-algorithm-2.lisp

(defun unfit (s1 s2)
  (loop for a across s1
    for b across s2 count(char/= a b)))

(defun mutate (str alp n) ; n: number of chars to mutate
  (let ((out (copy-seq str)))
    (dotimes (i n) (setf (char out (random (length str)))
             (char alp (random (length alp)))))
    out))

(defun evolve (changes alpha target)
  (loop for gen from 1
    with f2 with s2
    with str = (mutate target alpha 100)
    with fit = (unfit target str)
    while (plusp fit) do
    (setf s2 (mutate str alpha changes)
          f2 (unfit target s2))
    (when (> fit f2)
      (setf str s2 fit f2)
      (format t "~5d: ~a (~d)~%" gen str fit))))

(evolve 1 " ABCDEFGHIJKLMNOPQRSTUVWXYZ" "METHINKS IT IS LIKE A WEASEL")