day 15

2020/15.rkt

@@ -0,0 +1,18 @@
+#lang br
+(require racket/file rackunit)
+
+(define (solve str max-turns)
+  (define nums (map string->number (string-split str ",")))
+  (define previously-seen (make-hasheq))
+  (for ([(num turn) (in-indexed (drop-right nums 1))])
+    (hash-set! previously-seen num (add1 turn)))
+  (for/fold ([last-seen (last nums)])
+            ([turn (in-range (add1 (length nums)) (add1 max-turns))])
+    (begin0
+      (match (hash-ref previously-seen last-seen #false)
+        [#false 0]
+        [where-previously-seen (- (sub1 turn) where-previously-seen)])
+      (hash-set! previously-seen last-seen (sub1 turn)))))
+
+(check-equal? (solve "11,0,1,10,5,19" 2020) 870)
+(check-equal? (solve "11,0,1,10,5,19" 30000000) 9136)