Add day15

Author: opsound

2020/day15.rkt

@@ -0,0 +1,22 @@
+#lang racket
+
+(define INPUT (map string->number (string-split "2,20,0,4,1,17" ",")))
+
+(define (initialize nums)
+  (for/hash ([n (in-list nums)]
+             [i (in-range (length nums))])
+    (values n i)))
+
+(define (nth-spoken input n)
+  (for/fold ([spoken-nums (initialize (drop-right input 1))]
+             [just-said (last input)]
+             #:result just-said)
+            ([turn (in-range (length input) n)])
+    (define to-speak
+      (match (hash-ref spoken-nums just-said #f)
+        [#f 0]
+        [when-spoke (- (sub1 turn) when-spoke)]))
+    (values (hash-set spoken-nums just-said (sub1 turn)) to-speak)))
+
+(nth-spoken INPUT 2020)
+(nth-spoken INPUT 30000000)