Day 15.

.gitignore

@@ -1,4 +1,5 @@
 **/*.bak
 **/*.rkt~
 **/*.txt~
+**/*.exe
 **/compiled/

src/15.rkt

@@ -0,0 +1,21 @@
+#lang curly-fn racket
+
+(require "../lib.rkt")
+
+(define input '(5 1 9 18 13 8 0))
+
+(define (play end)
+  (define turns
+    (make-hash (map cons input (range 1 (add1 (length input))))))
+  (let loop ([turn (length input)] [curr (last input)])
+    (cond
+      [(>= turn end) curr]
+      [(hash-has-key? turns curr)
+       (let ([next (- turn (hash-ref turns curr))])
+         (hash-set! turns curr turn)
+         (loop (add1 turn) next))]
+      [else
+       (hash-set! turns curr turn)
+       (loop (add1 turn) 0)])))
+
+(show-solution (play 2020) (play 30000000))