day 21 part 1

21-dirac-dice/code.scm

@@ -0,0 +1,64 @@
+(load "../common.scm")
+
+(define (read-starting-positions filename)
+  (call-with-input-file
+    filename
+    (lambda (file)
+      (let* [(player-1 (begin
+			 (get-string-n file (string-length "Player 1 starting position: "))
+			 (get-number file)))
+	     (player-2 (begin
+			 (get-string-n file (string-length "Player 2 starting position: "))
+			 (get-number file)))]
+	; subtracting one to play with a board numbered 0..9 instead of 1..10, just easier
+	(values (- player-1 1) (- player-2 1))))))
+
+(define (deterministic-dice)
+  (let [(value -1)]
+    (lambda ()
+      (set! value (mod (+ value 1) 100))
+      (+ value 1))))
+
+(define (roll! position-score dice)
+  (set-car! position-score (mod (+ (car position-score) (dice) (dice) (dice)) 10))
+  (set-cdr! position-score (+ (car position-score) (cdr position-score) 1)))
+
+(define (play-deterministic position-1 position-2)
+  (let [(player-1 (cons position-1 0))
+	(player-2 (cons position-2 0))
+	(dice (deterministic-dice))
+	(rolls 0)]
+    (let loop ()
+      (roll! player-1 dice)
+      (set! rolls (+ rolls 3))
+      (if (>= (cdr player-1) 1000)
+	(* (cdr player-2) rolls)
+	(begin
+	  (roll! player-2 dice)
+	  (set! rolls (+ rolls 3))
+	  (if (>= (cdr player-1) 1000)
+	    (* (cdr player-1) rolls)
+	    (loop)))))))
+
+(define (dirac-dice outcome)
+  (let [(next outcome)]
+    (lambda ()
+      (if (null? next)
+	#f
+	(let [(value (car next))]
+	  (set! next (cdr next))
+	  value)))))
+
+(define (play-dirac position-1 position-2)
+  (let [(player-1 (list (cons position-1 0)))
+	(player-2 (list (cons position-2 0)))
+	(wins-1 0)
+	(wins-2 0)]
+    (let loop ()
+      'no-idea-yet)))
+
+(let-values [((position-1 position-2) (read-starting-positions "input"))]
+  (printf "part 1:~% Rolls * Opponent Score = ~a~%"
+	  (play-deterministic position-1 position-2))
+  (printf "part 2:~% Max Winning Universes: ~a~%"
+	  (play-dirac position-1 position-2)))

21-dirac-dice/input

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+Player 1 starting position: 10
+Player 2 starting position: 6