https://adventofcode.com/2017/day/9

day9.el

(defun calculate-score (str)
  (let ((list (split-string str "" t)) garbage (depth 0) (s 0) (g-count 0) ch)
    (loop
     for i from 0 to (1- (length list))
     do
     (progn
       (setq ch (nth i list))
       (cond
        ((string= ch "!")
         (setq i (+ i 1))
         )
        ((and (not (string= ch ">")) garbage)
         (setq g-count (1+ g-count))
         )
        ((string= ch "<")
         (setq garbage t)
         )
        ((string= ch ">")
         (setq garbage nil)
         )
        ((string= ch "{")
         (progn
           (setq depth (1+ depth))
           )
         )
        ((string= ch "}")
         (setq s (+ s depth))
         (setq depth (1- depth))
         )
        )
       )
     )
    (list s g-count)
    )
  )

(calculate-score "{}") ;; 1
(calculate-score "{{{}}}") ;; 6
(calculate-score "{{},{}}") ;; 5
(calculate-score "{{{},{},{{}}}}") ;; 16
(calculate-score "{<a>,<a>,<a>,<a>}") ;; 1
(calculate-score "{{<ab>},{<ab>},{<ab>},{<ab>}}") ;; 9
(calculate-score "{{<!!>},{<!!>},{<!!>},{<!!>}}") ;; 9
(calculate-score "{{<a!>},{<a!>},{<a!>},{<ab>}}") ;; 3

(defun read-file-as-string (file-path)
  (with-temp-buffer
    (insert-file-contents file-path)
    (buffer-string)
    )
)

(calculate-score (read-file-as-string "./input.txt"))