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99 LISP problems: Problem #11

Solution to the 99 LISP Problems #11

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(defun pack-first (alist)
  (if (null alist)
    nil
    (if (equalp
          (car alist)
          (cadr alist))
      (append (list (car alist)) (pack-first (cdr alist)))
      (list (car alist)))))

(defun trim-first (alist)
  (if (null alist)
    nil
    (if (not (equalp
               (car alist)
               (cadr alist)))
      (cdr alist)
      (trim-first (cdr alist)))))

(defun encode-modified (alist)
  (if (null alist)
    nil
    (append
      (if (= (length (pack-first alist)) 1)
        (list (car alist))
        (list (list (length (pack-first alist)) (car alist))))
      (encode-modified (trim-first alist)))))

Lisp dialect: Steel Bank Common Lisp