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1 files changed, 35 insertions, 0 deletions

diff --git a/racket/aoc2021/day-25/day-25.rkt b/racket/aoc2021/day-25/day-25.rkt
new file mode 100644
index 0000000..7a3a5ca
--- /dev/null
+++ b/racket/aoc2021/day-25/day-25.rkt
@@ -0,0 +1,35 @@
+#lang racket
+(require "../../jj-aoc.rkt"
+         threading)
+
+(define sea-floor
+  (for*/hash ([(row i) (in-indexed (in-lines (open-day 25 2021)))] [(c j) (in-indexed row)])
+    (values (list i j) c)))
+
+(define-values (max-i max-j)
+  (~> sea-floor hash-keys (argmax (λ (coord) (apply + coord)) _) (apply values _)))
+
+(define (cucumber-movement h c delta-i delta-j)
+  (define new-hash (hash-copy h))
+  (for* ([(coord x) (in-hash h)] #:when (eq? x c))
+    (match-define (list i j) coord)
+    (define neighbor (list (+ delta-i i) (+ delta-j j)))
+    (define neighbor-or-wrap
+      (if (hash-has-key? h neighbor) neighbor (list (if (= delta-i 0) i 0) (if (= delta-j 0) j 0))))
+    (when (eq? #\. (hash-ref h neighbor-or-wrap))
+      (hash-set*! new-hash coord #\. neighbor-or-wrap c)))
+  new-hash)
+
+(define (eastwards-movement h)
+  (cucumber-movement h #\> 0 1))
+
+(define (southwards-movement h)
+  (cucumber-movement h #\v 1 0))
+
+;; part 1
+(for/fold ([f sea-floor] [step 0] #:result (add1 step)) ([i (in-naturals 1)])
+  (define f* (~> f eastwards-movement southwards-movement))
+  #:break (equal? f* f)
+  (values f* i))
+
+;; no part 2 -- merry Christmas