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#lang racket
(require "../lib/utils.rkt")
(define lines
(call-with-input-file "input"
(λ (fp) (get-lines fp))))
(define (mat-copy mat)
(list->vector (map string-copy (vector->list mat))))
(define (flip-mat mat)
(list->vector (reverse (map string-copy (vector->list mat)))))
(define mat (list->vector lines))
(define (char-at mat x y)
(string-ref (vector-ref mat y) x))
(define (set-mat! mat x y c)
(string-set! (vector-ref mat y) x c))
(define (move-stone mat x1 y1 x2 y2)
(define t (char-at mat x1 y1))
(set-mat! mat x1 y1 (char-at mat x2 y2))
(set-mat! mat x2 y2 t))
(define (find-new-pos mat x y)
(let loop ((new-y y))
(if (or (= new-y 0)
(not (char=? #\. (char-at mat x (- new-y 1)))))
new-y
(loop (- new-y 1)))))
(define height (vector-length mat))
(define width (string-length (vector-ref mat 0)))
(define (tilt old-mat)
(define mat (mat-copy old-mat))
(do ((y 0 (+ y 1)))
((>= y height) (void))
(do ((x 0 (+ x 1)))
((>= x width) (void))
(when (char=? #\O (char-at mat x y))
(move-stone mat x y x (find-new-pos mat x y)))))
mat)
(define (count mat)
(define sum 0)
(do ((y 0 (+ y 1)))
((>= y height) (void))
(do ((x 0 (+ x 1)))
((>= x width) (void))
(when (char=? #\O (char-at mat x y))
(set! sum (+ sum (- height y))))))
sum)
(define (tilt-north mat)
(tilt mat))
(define (tilt-south mat)
(flip-mat (tilt (flip-mat mat))))
(define (transpose linesvec)
(define lines (vector->list linesvec))
(define width (string-length (vector-ref linesvec 0)))
(define new-lines (make-vector width))
(let loop ((i 0))
(if (>= i width)
new-lines
(let ()
(define char-list (map (λ (l) (string-ref l i)) lines))
(vector-set! new-lines i (list->string char-list))
(loop (+ i 1))))))
(define (tilt-west mat)
(transpose (tilt (transpose mat))))
(define (tilt-east mat)
(transpose (flip-mat (tilt (flip-mat (transpose mat))))))
(define mat-dict (make-hash))
(define r-mat-dict (make-hash))
(define (tilt-cycle mat)
(tilt-east (tilt-south (tilt-west (tilt-north mat)))))
;; return (start . end)
(define (find-loop mat)
(let loop ((i 1) (cur-mat mat))
(define m (tilt-cycle cur-mat))
(if (hash-has-key? mat-dict (vector->list m))
(cons (hash-ref mat-dict (vector->list m)) i)
(let ()
(hash-set! mat-dict (vector->list m) i)
(hash-set! r-mat-dict i (vector->list m))
(loop (+ 1 i) m)))))
(define tmp (find-loop mat))
(define loop-start (car tmp))
(define loop-len (- (cdr tmp) (car tmp)))
(define reduced-num
(+ loop-start (modulo (- 1000000000 loop-start) loop-len)))
(count (list->vector (hash-ref r-mat-dict reduced-num)))
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