Day 06.

input/06.txt

@@ -0,0 +1 @@
+3,5,2,5,4,3,2,2,3,5,2,3,2,2,2,2,3,5,3,5,5,2,2,3,4,2,3,5,5,3,3,5,2,4,5,4,3,5,3,2,5,4,1,1,1,5,1,4,1,4,3,5,2,3,2,2,2,5,2,1,2,2,2,2,3,4,5,2,5,4,1,3,1,5,5,5,3,5,3,1,5,4,2,5,3,3,5,5,5,3,2,2,1,1,3,2,1,2,2,4,3,4,1,3,4,1,2,2,4,1,3,1,4,3,3,1,2,3,1,3,4,1,1,2,5,1,2,1,2,4,1,3,2,1,1,2,4,3,5,1,3,2,1,3,2,3,4,5,5,4,1,3,4,1,2,3,5,2,3,5,2,1,1,5,5,4,4,4,5,3,3,2,5,4,4,1,5,1,5,5,5,2,2,1,2,4,5,1,2,1,4,5,4,2,4,3,2,5,2,2,1,4,3,5,4,2,1,1,5,1,4,5,1,2,5,5,1,4,1,1,4,5,2,5,3,1,4,5,2,1,3,1,3,3,5,5,1,4,1,3,2,2,3,5,4,3,2,5,1,1,1,2,2,5,3,4,2,1,3,2,5,3,2,2,3,5,2,1,4,5,4,4,5,5,3,3,5,4,5,5,4,3,5,3,5,3,1,3,2,2,1,4,4,5,2,2,4,2,1,4

src/06.rkt

@@ -0,0 +1,28 @@
+#lang curly-fn racket
+
+(require "../lib.rkt")
+
+(define input
+  (~> (problem-input 6)
+      first
+      string-csv
+      (map string->number _)))
+
+(define assocf (∘ cdr assoc))
+
+(define fish
+  (for/list ([age (range 0 9)])
+    (cons age (count #{= % age} input))))
+
+(define (tick days)
+  (for/fold ([fish fish]
+             #:result (sum (map cdr fish)))
+            ([_ (range days)])
+    (for/list ([age (range 0 9)])
+      (match age
+        [6 (cons age (+ (assocf 0 fish)
+                        (assocf 7 fish)))]
+        [8 (cons age (assocf 0 fish))]
+        [_ (cons age (assocf (add1 age) fish))]))))
+
+(show-solution (tick 80) (tick 256))