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(defsubst char-to-number (char)
(declare (pure t) (side-effect-free t))
(- char ?0))
(defun first-nil (vector start)
(cl-loop for i from (1+ start) below (length vector)
when (null (aref vector i))
return i
finally return (length vector)))
(defun last-number (vector start)
(cl-loop for i from (1- start) downto 0
when (aref vector i)
return i
finally return 0))
(defun solve (input-file)
(let* ((input (with-temp-buffer
(insert-file-contents-literally input-file)
(buffer-string)))
(nums (mapcar #'char-to-number input))
(vector (make-vector (apply #'+ nums) nil))
(i 0) (j 0))
;; Populate ‘vector’
(dolist (num nums)
(when (cl-evenp i)
(dotimes (k num)
(aset vector (+ j k) (/ i 2))))
(cl-incf i)
(cl-incf j num))
;; Swap elements
(setq i (first-nil vector 0)
j (last-number vector (length vector)))
(while (< i j)
(let ((x (aref vector i))
(y (aref vector j)))
(aset vector i y)
(aset vector j x))
(setq i (first-nil vector i))
(setq j (last-number vector j)))
;; Compute checksum
(cl-loop for i from 0 below (length vector)
for x = (aref vector i)
while x sum (* i x))))
(message "%d" (solve "input"))
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