diff --git a/15.rkt b/15.rkt
index 34e602e..ab7bc24 100644
--- a/15.rkt
+++ b/15.rkt
@@ -5,20 +5,20 @@
 ;; solution for day 15
 
 (define (naive-solution input limit)
-  (define seen-numbers (make-hash))
+  (define seen-numbers (make-vector limit #f))
   (define prev
     (for/last ([num (in-list input)] [i (in-naturals)])
-      (begin0 num (hash-set! seen-numbers num i))))
+      (begin0 num (vector-set! seen-numbers num i))))
   ;; delay "seeing" numbers by one iteration
-  (hash-remove! seen-numbers prev)
+  (vector-set! seen-numbers prev #f)
 
   (for/fold ([prev prev]) ([i (in-range (length input) limit)])
     (begin0
-      (match (hash-ref seen-numbers prev #f)
+      (match (vector-ref seen-numbers prev)
         [#f 0]
         [idx (- i 1 idx)])
       ;; _now_ we've "seen" it, but it happened on the last turn
-      (hash-set! seen-numbers prev (sub1 i)))))
+      (vector-set! seen-numbers prev (sub1 i)))))
 
 (define part1 #{naive-solution % 2020})
 (define part2 #{naive-solution % 30000000})