1 files changed, 59 insertions, 0 deletions
diff --git a/racket/aoc2021/day-21/day-21.rkt b/racket/aoc2021/day-21/day-21.rkt
new file mode 100644
index 0000000..9ca9b1b
--- /dev/null
+++ b/racket/aoc2021/day-21/day-21.rkt
@@ -0,0 +1,59 @@
+#lang racket
+(require threading
+         memoize)
+
+;; not going to bother importing the data since it's just two lines of text
+(define player-1-start 4)
+(define player-2-start 6)
+
+(define track (sequence->stream (in-cycle (inclusive-range 1 10))))
+(define current-turn (in-cycle (list 'player-1 'player-2)))
+(define die-rolls (sequence->stream (in-cycle (inclusive-range 1 100))))
+
+;; part 1
+(~> (for/fold ([player-1-score 0]
+               [player-1-track (stream-tail track (sub1 player-1-start))]
+               [player-2-score 0]
+               [player-2-track (stream-tail track (sub1 player-2-start))]
+               [dice die-rolls]
+               [last-turn 0]
+               #:result (list (min player-1-score player-2-score) (* 3 last-turn)))
+              ([turn-count (in-naturals 1)]
+               [turn-for current-turn]
+               #:break (or (player-1-score . >= . 1000) (player-2-score . >= . 1000)))
+      (define rolls (apply + (stream->list (stream-take dice 3))))
+      (match turn-for
+        ['player-1
+         (define next-track (stream-tail player-1-track rolls))
+         (values (+ player-1-score (stream-first next-track))
+                 next-track
+                 player-2-score
+                 player-2-track
+                 (stream-tail dice 3)
+                 turn-count)]
+        ['player-2
+         (define next-track (stream-tail player-2-track rolls))
+         (values player-1-score
+                 player-1-track
+                 (+ player-2-score (stream-first next-track))
+                 next-track
+                 (stream-tail dice 3)
+                 turn-count)]))
+    (apply * _))
+
+;; part 2
+(define d3 (list 1 2 3))
+(define roll-space (~>> (cartesian-product d3 d3 d3) (map (λ~>> (apply +)))))
+
+(define/memo
+ (next-turns p1-score p2-score p1-start p2-start)
+ (cond
+   [(p1-score . >= . 21) '(1 0)]
+   [(p2-score . >= . 21) '(0 1)]
+   [else
+    (for/fold ([wins '(0 0)]) ([roll (in-list roll-space)])
+      (define move-to (add1 (modulo (sub1 (+ roll p1-start)) 10)))
+      (define possible-wins (next-turns p2-score (+ p1-score move-to) p2-start move-to))
+      (list (+ (first wins) (second possible-wins)) (+ (second wins) (first possible-wins))))]))
+
+(~>> (next-turns 0 0 player-1-start player-2-start) (apply max))