Initial commit

2021/day21/day21.scm

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+(import (chicken io)
+	(chicken format)
+	srfi-1
+	srfi-42
+	srfi-69
+	srfi-152)
+
+(define (read-input #!optional (port (current-input-port)))
+  (apply values (map (lambda (s) (string->number (string-drop s 28))) (read-lines port))))
+
+(define (part1 start1 start2)
+  (let lp ((k 0)
+	   (pos1 (- start1 1))
+	   (pos2 (- start2 1))
+	   (score1 0)
+	   (score2 0))
+    (cond
+     ((>= score1 1000) (* score2 k 3))
+     ((>= score2 1000) (* score1 k 3))
+     ((even? k) (let* ((dice (+ (* 9 k) 6))
+		       (new-pos1 (remainder (+ pos1 dice) 10)))
+		  (lp (+ k 1) new-pos1 pos2 (+ score1 new-pos1 1) score2)))
+     ((odd? k) (let* ((dice (+ (* 9 k) 6))
+		      (new-pos2 (remainder (+ pos2 dice) 10)))
+		 (lp (+ k 1) pos1 new-pos2 score1 (+ score2 new-pos2 1)))))))
+
+(define (part2 start1 start2)
+  (define cache (make-hash-table))
+  (define (play pos1 pos2 score1 score2)
+    (if (>= score2 21)
+	(list 0 1)
+	(fold-ec (list 0 0)
+		 (: d1 '(1 2 3)) (: d2 '(1 2 3)) (: d3 '(1 2 3))
+		 (:let new-pos1 (remainder (+ pos1 d1 d2 d3) 10))
+		 (:let key (list pos2 new-pos1 score2 (+ score1 new-pos1 1)))
+		 (reverse (if (hash-table-exists? cache key)
+			      (hash-table-ref cache key)
+			      (let ((val (apply play key)))
+				(hash-table-set! cache key val)
+				val)))
+		 (lambda (x acc) (list (+ (car x) (car acc)) (+ (cadr x) (cadr acc)))))))
+  (apply max (play (- start1 1) (- start2 1) 0 0)))
+
+(let-values (((start1 start2) (read-input)))
+  (print (part1 start1 start2))
+  (print (part2 start1 start2)))

2021/day21/input

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+Player 1 starting position: 8
+Player 2 starting position: 3

2021/day21/test

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+Player 1 starting position: 4
+Player 2 starting position: 8