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#lang racket
(require "../lib/utils.rkt")
(define lines
(call-with-input-file "input"
(λ (fp)
(get-lines fp))))
(define diagram (list->vector lines))
(define height (vector-length diagram))
(define width (string-length (vector-ref diagram 0)))
(define (char-at x y)
(string-ref (vector-ref diagram y) x))
(define (find-start)
(let loop ((x 0) (y 0))
(if (>= y height)
#f
(if (>= x width)
(loop 0 (+ y 1))
(if (char=? #\S (char-at x y))
(cons x y)
(loop (+ x 1) y))))))
(define start (find-start))
(define (valid-pos? pos)
(define x (car pos))
(define y (cdr pos))
(and (and (>= x 0) (< x width))
(and (>= y 0) (< y height))))
(define first-pos
(let ()
(define up (cons (car start) (- (cdr start) 1)))
(define down (cons (car start) (+ (cdr start) 1)))
(define left (cons (- (car start) 1) (cdr start)))
(define right (cons (+ (car start) 1) (cdr start)))
(cond ((and (valid-pos? up)
(member (char-at (car up) (cdr up)) (list #\| #\F #\7)))
up)
((and (valid-pos? down)
(member (char-at (car down) (cdr down)) (list #\| #\L #\J )))
down)
((and (valid-pos? left)
(member (char-at (car left) (cdr left)) (list #\- #\F #\L )))
left)
((and (valid-pos? right)
(member (char-at (car right) (cdr right)) (list #\- #\7 #\J)))
right))))
(define (next cur recent)
(define up (cons (car cur) (- (cdr cur) 1)))
(define down (cons (car cur) (+ (cdr cur) 1)))
(define left (cons (- (car cur) 1) (cdr cur)))
(define right (cons (+ (car cur) 1) (cdr cur)))
(define c (char-at (car cur) (cdr cur)))
(define possible-pos
(cond ((char=? c #\|) (list up down))
((char=? c #\-) (list left right))
((char=? c #\F) (list right down))
((char=? c #\L) (list up right))
((char=? c #\7) (list left down))
((char=? c #\J) (list up left))))
(let loop ((cur possible-pos))
(if (not (equal? (car cur) recent))
(car cur)
(loop (cdr cur)))))
(define (simulate)
(let loop ((i 0) (cur first-pos) (recent start))
(if (equal? cur start)
i
(let ()
(define next-loc (next cur recent))
(loop (+ i 1) next-loc cur)))))
(define loop-length (simulate))
(if (= 0 (modulo loop-length 2))
(/ loop-length 2)
(+ 1 (quotient loop-length 2)))
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