Series で L-99 P09-11

pack はもうちょっと何とかなりそうなんだけどね。

;; P09 (**) Pack consecutive duplicates of list elements into sublists.
;; If a list contains repeated elements they should be placed in separate sublists.
;;
;; Example:
;; * (pack '(a a a a b c c a a d e e e e))
;; ((A A A A) (B) (C C) (A A) (D) (E E E E))
(defun pack (x)
  (%pack (catenate x (scan (list (gensym)))))) ; #z(a b c) => #z(a b c #:G1)
(defun %pack (x &aux acc)
  (choose
   (mapping ((a x)
             (b (previous x (collect-first x))))
     (if (eql a b)
         (progn (push a acc) nil)
         (prog1 acc
           (setf acc (list a)))))))
(pack #z(a a a a b c c a a d e e e e))


;; P10 (*) Run-length encoding of a list.
;; Use the result of problem P09 to implement the so-called run-length encoding data compression method. Consecutive duplicates of elements are encoded as lists (N E) where N is the number of duplicates of the element E.
;;
;; Example:
;; * (encode '(a a a a b c c a a d e e e e))
;; ((4 A) (1 B) (2 C) (2 A) (1 D)(4 E))
(defun encode (x)
  (#M(lambda (x) (list (length x) (car x)))
     (pack x)))
(encode #z(a a a a b c c a a d e e e e))


;; P11 (*) Modified run-length encoding.
;; Modify the result of problem P10 in such a way that if an element has no duplicates it is simply copied into the result list. Only elements with duplicates are transferred as (N E) lists.
;;
;; Example:
;; * (encode-modified '(a a a a b c c a a d e e e e))
;; ((4 A) B (2 C) (2 A) D (4 E))
(defun encode-modified (x)
  (mapping ((e (pack x)))
    (let ((length (length e))
          (car (car e)))
      (if (= 1 length)
          car
          (list length car)))))
(encode-modified #z(a a a a b c c a a d e e e e))