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#!/usr/bin/sbcl --script
(defparameter *numbers*
'())
(defun main (filename)
(setq *numbers* (read-stones filename))
(loop repeat 25
do (blink)
finally (return (length *numbers*))))
(defun read-stones (filename)
(with-open-file (stream filename)
(let ((contents (make-string (file-length stream))))
(read-sequence contents stream)
(read-from-string (concatenate 'string "(" contents ")")))))
(defun blink ()
(setq *numbers*
(loop for number in *numbers*
for after-blink = (stone-change-or-split number)
if (listp after-blink)
append after-blink
else
collect after-blink
finally (setq *numbers* after-blink))))
(defun stone-change-or-split (number)
(cond ((= number 0)
1)
((evenp (digit-count number))
(number-split number))
(:else
(* 2024 number))))
(defun digit-count (number)
(loop with x = 0
while (/= number 0)
do (progn
(incf x)
(setq number (floor number 10)))
finally (return x)))
(defun number-split (number)
(let* ((length (digit-count number))
(string (write-to-string number))
(half (/ length 2)))
(list (parse-integer (subseq string 0 half))
(parse-integer (subseq string half (length string))))))
(format t "~d~%" (main "input"))
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