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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp
Subject: Re: Exercises please
Date: Sat, 21 Jun 2025 20:26:29 -0000 (UTC)
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Pascal J. Bourguignon wrote:

> and my solutions (still incomplete): 
> http://www.informatimago.com/develop/lisp/l99/index.html

Where we find:

(---------------------------------------------------------------
P13 (**) Run-length encoding of a list (direct solution).

   Example:
    * (encode-direct '(a a a a b c c a a d e e e e))
    ((4 A) B (2 C) (2 A) D (4 E))
"

;; Iterative solution, uses only O(r) space:

(defun encode-modified (list)
  (let ((result    '())
        (count     0)
        (last-item nil))
    (labels ((collect-result ()
               (push (if (= 1 count)
                         last-item
                         (list count last-item))
                     result))
             (new-item (item)
               (setf count 1
                     last-item item))
             (same-item ()
               (incf count))
             (return-result ()
               (when (plusp count)
                 (collect-result))
               (nreverse result)))
      (dolist (item list  (return-result))
        (cond
          ((zerop count)        (new-item item))
          ((eql item last-item) (same-item))
          (t                    (collect-result)
                                (new-item item)))))))
---------------------------------------------------------------)

Gauche Scheme

(define (encode List)
  (if (null? List)
    ()
    (let
      ((tmp (fold
              (lambda(x accum)
                (if (equal? x (caar accum))
                  (cons (cons x (car accum)) (cdr accum))
                  (cons (list x) accum)))
              `((,(car List)))
              (cdr List))))
      (reverse
        (map
          (lambda(xs) (let1 len (length xs)
            (if (= 1 len) (car xs) (list len (car xs)))))
          tmp)))))

gosh> (encode '(a a a a b c c a a d e e e e))
((4 a) b (2 c) (2 a) d (4 e))