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#lang racket
(require advent-of-code
threading)
(struct posn (r c) #:transparent)
(struct light (posn dir) #:transparent)
(define input (fetch-aoc-input (find-session) 2023 16 #:cache #true))
(define grid
(for*/hash ([(row r) (in-indexed (string-split input "\n"))]
[(cell c) (in-indexed (in-string row))])
(values (posn r c) cell)))
(define/match (move _l)
[((light (posn r c) 'up)) (light (posn (sub1 r) c) 'up)]
[((light (posn r c) 'right)) (light (posn r (add1 c)) 'right)]
[((light (posn r c) 'left)) (light (posn r (sub1 c)) 'left)]
[((light (posn r c) 'down)) (light (posn (add1 r) c) 'down)])
(define/match (transform l _cell)
[(_ #\.) l]
[(_ 'off) '()]
[((light _ (or 'up 'down)) #\|) l]
[((light _ (or 'left 'right)) #\-) l]
[((light p 'left) #\/) (light p 'down)]
[((light p 'down) #\/) (light p 'left)]
[((light p 'right) #\/) (light p 'up)]
[((light p 'up) #\/) (light p 'right)]
[((light p 'left) #\\) (light p 'up)]
[((light p 'up) #\\) (light p 'left)]
[((light p 'right) #\\) (light p 'down)]
[((light p 'down) #\\) (light p 'right)]
[((light p (or 'left 'right)) #\|) (list (light p 'up) (light p 'down))]
[((light p (or 'up 'down)) #\-) (list (light p 'left) (light p 'right))])
;; part 1
(define (get-energized start)
(for/fold ([energized (set)]
[previously-visited (set)]
[beam-tips (set start)]
#:result (set-count energized))
([_ (in-naturals)])
(define next-positions
(list->set
(flatten (for/list ([b (in-set beam-tips)])
(~> b move ((λ (b) (transform b (hash-ref grid (light-posn b) 'off)))))))))
(define all-visited (set-union previously-visited next-positions))
(define next-energized (set-union energized (list->set (set-map next-positions light-posn))))
#:break (equal? previously-visited all-visited)
(values next-energized all-visited (set-subtract next-positions previously-visited))))
(get-energized (light (posn 0 -1) 'right))
;; part 2
(define rows (~> input (string-split "\n") length))
(define cols (~> input (string-split #rx"\n.*") first string-length))
(define all-starting-positions
(append (map (λ (r) (light (posn r -1) 'right)) (range rows))
(map (λ (r) (light (posn r cols) 'left)) (range rows))
(map (λ (c) (light (posn -1 c) 'down)) (range cols))
(map (λ (c) (light (posn rows c) 'up)) (range cols))))
(get-energized (argmax get-energized all-starting-positions))
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