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#lang racket
(require advent-of-code
threading)
(struct exits (left right) #:transparent)
(match-define (list raw-directions raw-maze)
(~> (fetch-aoc-input (find-session) 2023 8 #:cache #true) (string-split "\n\n")))
(define directions (string->list raw-directions))
(define maze
(for/hash ([line (in-list (string-split raw-maze "\n"))])
(match (regexp-match #rx"(...) = \\((...), (...)\\)" line)
[(list _ name left right) (values name (exits left right))])))
(define (to-next-node start end dirs maze)
(for/fold ([current start]
[acc 0]
#:result acc)
([dir (in-cycle dirs)])
#:break (string-suffix? current end)
(define node (hash-ref maze current))
(case dir
[(#\L) (values (exits-left node) (add1 acc))]
[(#\R) (values (exits-right node) (add1 acc))])))
;; part 1
(to-next-node "AAA" "ZZZ" directions maze)
;; part 2
(for/lists (ns #:result (apply lcm ns))
([start (in-list (hash-keys maze))]
#:when (string-suffix? start "A"))
(to-next-node start "Z" directions maze))
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