day 15: improve solution spaghettiness somewhat

15.rkt

@@ -6,20 +6,21 @@
 
 (define (naive-solution input limit)
   (define seen-numbers (make-hash))
-  (define prev #f)
-  (for ([num (in-list input)] [i (in-naturals)])
+  (define prev
+    (for/last ([num (in-list input)] [i (in-naturals)])
       (hash-set! seen-numbers num i)
-    (set! prev num))
+      num))
+  ;; delay "seeing" numbers by one iteration
   (hash-remove! seen-numbers prev)
 
-  (for ([i (in-range (length input) limit)])
+  (for/fold ([prev prev]) ([i (in-range (length input) limit)])
     (define new-val
-      (cond
-        [(hash-has-key? seen-numbers prev) (sub1 (- i (hash-ref seen-numbers prev)))]
-        [else 0]))
+      (match (hash-ref seen-numbers prev #f)
+        [#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))
-    (set! prev new-val))
-  prev)
+    new-val))
 
 (define part1 #{naive-solution % 2020})
 (define part2 #{naive-solution % 30000000})