Solution to lisp-koan' scoring-project, Greed dice game.

scoring-project.lsp

(defun score (dice)
  (let ((dice (sort dice #'>)))
    (flet ((is-1-or-5 (x)
             (or (= x 1) (= x 5)))
           (triple-p (dice)
             (and (= (length dice) 3) (apply #'= dice)))
           (triple-score (dice)
             (if (= (first dice) 1)
                 1000
                 (* 100 (first dice)))))
      (cond ((null dice) 0)
            ;; when length = 1
            ((equal dice '(5)) 50)
            ((equal dice '(1)) 100)
            ((= (length dice) 1) 0)
            ;; when length = 2
            ((= (length dice) 2)
             (+ (score (list (first dice)))
                (score (rest dice))))
            ;; when length = 3
            ((= (length dice) 3)
             (if (triple-p dice)
                 (triple-score dice)
                 (+ (score (subseq dice 0 1)) (score (subseq dice 1)))))
            ;; when length >= 4
            (t
             (+ (score (subseq dice 0 1)) (score (subseq dice 1))))))))