Day 15, but in Typed Racket.

src/15-typed.rkt

@@ -0,0 +1,26 @@
+#lang typed/racket
+
+(require/typed "../lib.rkt"
+               [show-solution (-> Any Any Void)])
+
+(: input (Listof Natural))
+(define input '(5 1 9 18 13 8 0))
+
+(: play (-> Exact-Positive-Integer Integer))
+(define (play end)
+  (let ([turns : (Vectorof Natural) (make-vector end 0)])
+    (for-each (λ ([i : Natural] [j : Natural]) (vector-set! turns i (add1 j)))
+              input (range (length input)))
+    (let loop ([turn : Natural (length input)]
+               [curr : Integer (last input)])
+      (cond
+        [(>= turn end) curr]
+        [(positive? (vector-ref turns curr))
+         (let ([next : Integer (- turn (vector-ref turns curr))])
+           (vector-set! turns curr turn)
+           (loop (add1 turn) next))]
+        [else
+         (vector-set! turns curr turn)
+         (loop (add1 turn) 0)]))))
+
+(show-solution (play 2020) (play 30000000))