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(ql:quickload '(fiveam))
(defpackage :day21
(:use :cl :fiveam))
(in-package :day21)
;;22:55
;;23:32
(defun find-start (field)
(destructuring-bind (rows cols)
(array-dimensions field)
(loop for row below rows do
(loop for col below cols do
(when (eq (aref field row col) #\S)
(return-from find-start (list row col)))))))
(defun next-moves (field)
(destructuring-bind (rows cols)
(array-dimensions field)
(lambda (pos)
(destructuring-bind (row col) pos
(loop for (dr dc) in '((0 1) (0 -1) (1 0) (-1 0))
for nr = (+ row dr)
for nc = (+ col dc)
when (and (< -1 nr rows)
(< -1 nc cols)
(not (eq #\# (aref field nr nc))))
collect (list nr nc))))))
(let* ((lines
(uiop:read-file-lines "input"))
(field (make-array (list (length lines) (length (car lines)))
:initial-contents lines))
(walker (next-moves field)))
(labels ((all-places (positions)
(delete-duplicates (mapcan walker positions) :test #'equal)))
(loop
repeat 64
for positions = (all-places (list (find-start field)))
then (all-places positions)
;; do (print positions)
finally (return (length positions))))
)
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