diff options
Diffstat (limited to 'racket/aoc2023/day-16/day-16.rkt')
| -rw-r--r-- | racket/aoc2023/day-16/day-16.rkt | 70 |
1 files changed, 70 insertions, 0 deletions
diff --git a/racket/aoc2023/day-16/day-16.rkt b/racket/aoc2023/day-16/day-16.rkt new file mode 100644 index 0000000..4a70de8 --- /dev/null +++ b/racket/aoc2023/day-16/day-16.rkt @@ -0,0 +1,70 @@ +#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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