day 21 part 1 complete

1 files changed, 43 insertions, 0 deletions

diff --git a/2021/day-21/day-21.rkt b/2021/day-21/day-21.rkt
new file mode 100644
index 0000000..30a94d2
--- /dev/null
+++ b/2021/day-21/day-21.rkt
@@ -0,0 +1,43 @@
+#lang racket
+(require "../../jj-aoc.rkt"
+         threading)
+
+;; 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-space (stream-tail track (sub1 player-1-start))]
+               [player-2-score 0]
+               [player-2-space (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 current-turn]
+               #:break (or (player-1-score . >= . 1000)
+                           (player-2-score . >= . 1000)))
+      (define rolls (apply + (stream->list (stream-take dice 3))))
+      (cond
+        [(equal? turn 'player-1)
+         (values (+ player-1-score
+                    (stream-first (stream-tail player-1-space rolls)))
+                 (stream-tail player-1-space rolls)
+                 player-2-score
+                 player-2-space
+                 (stream-tail dice 3)
+                 turn-count)]
+        [(equal? turn 'player-2)
+         (values player-1-score
+                 player-1-space
+                 (+ player-2-score
+                    (stream-first (stream-tail player-2-space rolls)))
+                 (stream-tail player-2-space rolls)
+                 (stream-tail dice 3)
+                 turn-count)]))
+  (apply * _))
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