day 21 part 2 WIP

21-dirac-dice/code.scm

@@ -49,13 +49,76 @@
 	  (set! next (cdr next))
 	  value)))))
 
+(define (vector-sum vec start end)
+  (vector-fold
+    (lambda (i a x)
+      (if (and (>= i start)
+	       (< i end))
+	(+ a x)
+	a))
+    0 vec))
+
+;;; TODO: handle not playing if opponent already won
+;;;       not loose winning games in the process x_x
+(define (dirac-roll! ongoings)
+  (let [(new (make-vector 220 0))]
+    ; keep the ones that already won
+    (vector-copy! new 210 ongoings 210) ; this should keep winning games, why do I loose some?
+    ; for each possible combination of 3 rolls of 3-sided dice
+    (for-each
+      (lambda (dice-1)
+	(for-each
+	  (lambda (dice-2)
+	    (for-each
+	      (lambda (dice-3)
+		; increase scores of ongoing games
+		(vector-for-each
+		  (lambda (index ongoing)
+		    (when (and (> ongoing 0) (< index 210))
+		      (let* [(position (mod index 10))
+			     (score (div index 10))
+			     (new-position (mod (+ position dice-1 dice-2 dice-3) 10))
+			     (new-score (min 21 (+ score new-position 1)))
+			     (new-index (+ new-position (* new-score 10)))]
+			(vector-set! new new-index
+				     (+ ongoing (vector-ref new new-index))))))
+		  ongoings))
+	      '(1 2 3)))
+	  '(1 2 3)))
+      '(1 2 3))
+    (vector-copy! ongoings 0 new)))
+
+(define (print-ongoing vec)
+  (vector-for-each
+    (lambda (index ongoing)
+      (when (> ongoing 0)
+	(printf "  ~12a games with score ~a at position ~a~%"
+		ongoing (div index 10) (+ (mod index 10) 1))))
+    vec))
+
 (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)]
+  ; the vectors track the number of games going on at given position and score,
+  ; where position = index % 10 and score = index / 10
+  ; 10 because there are 10 positions on board
+  ; max score is 21 so 22 entries total accounting for starting zero
+  ; => 10 * 22 = 220 entries
+  (let [(player-1 (make-vector 220 0))
+	(player-2 (make-vector 220 0))]
+    (vector-set! player-1 position-1 1)
+    (vector-set! player-2 position-2 1)
+    (printf "Player 1 starting at ~a, player 2 at ~a~%" position-1 position-2)
     (let loop ()
-      'no-idea-yet)))
+      (set! wins-1 (dirac-roll! player-1))
+      (set! wins-2 (dirac-roll! player-2))
+      (printf "P1:~%") (print-ongoing player-1)
+      (printf "P2:~%") (print-ongoing player-2)
+      ; compute how many games are still going (not reached 21)
+      (if (= (+ (vector-sum player-1 0 210) (vector-sum player-2 0 210)) 0)
+	; sum how many winning games (the ones that reached 21)
+	; and return biggest
+	(max (vector-sum player-1 210 220)
+	     (vector-sum player-2 210 220))
+	(loop)))))
 
 (let-values [((position-1 position-2) (read-starting-positions "input"))]
   (printf "part 1:~% Rolls * Opponent Score = ~a~%"