rle.lisp

(defun encode(input)
  (defun enc(out cur cnt inp)
    (defun flush() (cons cnt (cons cur out)))
    (cond
      ((null inp) (if (null cur) out (flush)))
      ((null cur) (enc out (car inp) 1 (cdr inp)))
      ((eq cur (car inp)) (enc out cur (+ cnt 1) (cdr inp)))
      (t (enc (flush) (car inp) 1 (cdr inp)))))
  (nreverse (enc '() '() '() input)))

(defun test(input)
  (format t "~a => ~a~%" input (encode input)))

(test '(a b b b c))
(test '(a b))
(test '())