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1 files changed, 22 insertions, 0 deletions

diff --git a/racket/aoc2020/day-15/day-15.rkt b/racket/aoc2020/day-15/day-15.rkt
new file mode 100644
index 0000000..4dd9e88
--- /dev/null
+++ b/racket/aoc2020/day-15/day-15.rkt
@@ -0,0 +1,22 @@
+#lang rackjure
+
+(define first-numbers '(2 20 0 4 1 17))
+
+(define number-hash
+  (for/hash ([(xs i) (in-indexed (drop-right first-numbers 1))])
+    (values xs (add1 i))))
+
+(define starting-round (~> number-hash hash-values (apply max _) (+ 2)))
+
+(define (find-spoken-number-at end)
+  (for/fold ([ns number-hash] [previous-number (last first-numbers)] #:result previous-number)
+            ([rnd (inclusive-range starting-round end)])
+    (define next-spoken-number
+      (match (ns previous-number)
+        [#f 0]
+        [n (- (sub1 rnd) n)]))
+    (values (ns previous-number (sub1 rnd)) next-spoken-number)))
+
+(find-spoken-number-at 2020)
+
+(find-spoken-number-at 30000000)
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