diff --git a/src/15.rkt b/src/15.rkt
index d283602..7204a81 100644
--- a/src/15.rkt
+++ b/src/15.rkt
@@ -5,17 +5,19 @@
 (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)])
+  (define turns (make-vector end #f))
+  (for-each #{vector-set! turns %1 (add1 %2)}
+            input (range (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)
+      [(vector-ref turns curr)
+       (let ([next (- turn (vector-ref turns curr))])
+         (vector-set! turns curr turn)
          (loop (add1 turn) next))]
       [else
-       (hash-set! turns curr turn)
+       (vector-set! turns curr turn)
        (loop (add1 turn) 0)])))
 
 (show-solution (play 2020) (play 30000000))