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1 files changed, 36 insertions, 0 deletions

diff --git a/racket/aoc2023/day-08/day-08.rkt b/racket/aoc2023/day-08/day-08.rkt
new file mode 100644
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--- /dev/null
+++ b/racket/aoc2023/day-08/day-08.rkt
@@ -0,0 +1,36 @@
+#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))