day 21 complete

1 files changed, 39 insertions, 11 deletions

diff --git a/2021/day-21/day-21.rkt b/2021/day-21/day-21.rkt
index 30a94d2..d2e17b6 100644
--- a/2021/day-21/day-21.rkt
+++ b/2021/day-21/day-21.rkt
@@ -1,6 +1,6 @@
 #lang racket
-(require "../../jj-aoc.rkt"
-         threading)
+(require threading
+         memoize)
 
 ;; not going to bother importing the data since it's just two lines of text
 (define player-1-start 4)
@@ -12,9 +12,9 @@
 
 ;; part 1
 (~> (for/fold ([player-1-score 0]
-               [player-1-space (stream-tail track (sub1 player-1-start))]
+               [player-1-track (stream-tail track (sub1 player-1-start))]
                [player-2-score 0]
-               [player-2-space (stream-tail track (sub1 player-2-start))]
+               [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)))
@@ -26,18 +26,46 @@
       (cond
         [(equal? turn 'player-1)
          (values (+ player-1-score
-                    (stream-first (stream-tail player-1-space rolls)))
-                 (stream-tail player-1-space rolls)
+                    (stream-first (stream-tail player-1-track rolls)))
+                 (stream-tail player-1-track rolls)
                  player-2-score
-                 player-2-space
+                 player-2-track
                  (stream-tail dice 3)
                  turn-count)]
         [(equal? turn 'player-2)
          (values player-1-score
-                 player-1-space
+                 player-1-track
                  (+ player-2-score
-                    (stream-first (stream-tail player-2-space rolls)))
-                 (stream-tail player-2-space rolls)
+                    (stream-first (stream-tail player-2-track rolls)))
+                 (stream-tail player-2-track rolls)
                  (stream-tail dice 3)
                  turn-count)]))
-  (apply * _))
\ No newline at end of file
+  (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 ([p-wins '(0 0)])
+               ([roll (in-list roll-space)])
+       (define move-to (~> p1-start
+                         (+ roll)
+                         sub1
+                         (modulo 10)
+                         add1))
+       (match-define (list p1-wins p2-wins) p-wins)
+       (match-define (list p2-possible-win p1-possible-win)
+         (next-turns p2-score
+                     (+ p1-score move-to)
+                     p2-start
+                     move-to))
+       (list (+ p1-wins p1-possible-win)
+             (+ p2-wins p2-possible-win)))]))
+
+(~>> (next-turns 0 0 player-1-start player-2-start) (apply max))
\ No newline at end of file