(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))))
  )