Day 17.

input/17.txt

@@ -0,0 +1,8 @@
+#.#.#.##
+.####..#
+#####.#.
+#####..#
+#....###
+###...##
+...#.#.#
+#.##..##

lib.rkt

@@ -24,6 +24,11 @@
   (curry apply))
 (define $ uncurry)
 
+;; iterate : (a -> a) -> number -> (a -> a)
+(define ((iterate f n) a)
+  (let loop ([a a] [n n])
+    (if (zero? n) a (loop (f a) (sub1 n)))))
+
 
 ;; IO helpers ;;
 

src/17.rkt

@@ -0,0 +1,49 @@
+#lang curly-fn racket
+
+(require "../lib.rkt")
+
+(define input
+  (map string->list (problem-input 17)))
+
+(define grid
+  (for*/fold ([grid (hash)])
+             ([x 8]
+              [y 8])
+    (hash-set grid (list x y 0 0) (list-ref (list-ref input y) x))))
+
+(define (actives chars)
+  (count #{char=? % #\#} chars))
+
+(define (cycle part1? grid)
+  (let* ([coords (hash-keys grid)]
+         [xs (map first  coords)] [min-x (apply min xs)] [max-x (apply max xs)]
+         [ys (map second coords)] [min-y (apply min ys)] [max-y (apply max ys)]
+         [zs (map third  coords)] [min-z (apply min zs)] [max-z (apply max zs)]
+         [ws (map fourth coords)] [min-w (apply min ws)] [max-w (apply max ws)])
+    (for*/fold ([grid* grid])
+               ([x (in-range (- min-x 1) (+ max-x 2))]
+                [y (in-range (- min-y 1) (+ max-y 2))]
+                [z (in-range (- min-z 1) (+ max-z 2))]
+                [w (if part1? '(0) (in-range (- min-w 1) (+ max-w 2)))])
+      (let* ([cube (hash-ref grid (list x y z w) #\.)]
+             [neighbours (for*/list ([x* (in-range (- x 1) (+ x 2))]
+                                     [y* (in-range (- y 1) (+ y 2))]
+                                     [z* (in-range (- z 1) (+ z 2))]
+                                     [w* (if part1? '(0) (in-range (- w 1) (+ w 2)))]
+                                     #:unless (and (= x x*) (= y y*) (= z z*) (= w w*)))
+                           (hash-ref grid (list x* y* z* w*) #\.))]
+             [active-neighbours (actives neighbours)])
+        (cond
+          [(and (char=? cube #\#) (not (<= 2 active-neighbours 3)))
+           (hash-set grid* (list x y z w) #\.)]
+          [(and (char=? cube #\.) (= active-neighbours 3))
+           (hash-set grid* (list x y z w) #\#)]
+          [else grid*])))))
+
+(define part1
+  (actives (hash-values ((iterate #{cycle #t} 6) grid))))
+
+(define part2
+  (actives (hash-values ((iterate #{cycle #f} 6) grid))))
+
+(show-solution part1 part2)