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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)))))))
(let* ((lines
(uiop:read-file-lines "input"))
(field (make-array (list (length lines) (length (car lines)))
:initial-contents lines))
end-positions
pending
(start (find-start field))
(visited (make-hash-table :test #'equal))
)
(push (cons 64 start) pending)
(setf (gethash start visited) t)
(destructuring-bind (rows cols)
(array-dimensions field)
(loop while pending
for (steps-left row col) = (pop pending) do
(when (zerop (mod steps-left 2))
(push (list row col) end-positions))
(unless (zerop steps-left)
(loop
for (dr dc) in '((0 1) (0 -1) (1 0) (-1 0))
for nr = (+ row dr)
for nc = (+ col dc)
for new-pos = (list nr nc)
when (and (< -1 nr rows)
(< -1 nc cols)
(not (eq #\# (aref field nr nc)))
(not (gethash new-pos visited)))
do (setf (gethash new-pos visited) t)
(push (cons (1- steps-left) new-pos) pending)))))
;; (list
;; end-positions)
;; (length (remove-duplicates end-positions :test #'equal))
(length
end-positions)
)
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