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;; 9:39 start
;; 10:17 part 1
;;
(ql:quickload '(fiveam))
(defun roll-rocks (field)
(loop for row from 1 below (array-dimension field 0)
sum
(loop for col below (array-dimension field 1)
when (and (eq #\O (aref field row col))
(eq #\. (aref field (1- row) col)))
do (setf (aref field (1- row) col) #\O
(aref field row col) #\.)
and count t)))
(defun array-slice (arr row)
(make-array (array-dimension arr 1)
:displaced-to arr
:displaced-index-offset (* row (array-dimension arr 1))))
(defun solve (input)
(let* ((rows (length input))
(field
(make-array (list rows (length (car input))) :initial-contents input)))
(loop
for result = (roll-rocks field)
until (zerop result))
(loop for row below rows
sum (* (- rows row) (count #\O (array-slice field row))))))
(fiveam:test solutions
(fiveam:is (= 136 (solve (uiop:read-file-lines "eg-in"))))
(fiveam:is (= 108935 (solve (uiop:read-file-lines "input")))))
(fiveam:run!)
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