d6

2017/d6/main.rkt

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+#lang reader "../aoc-lang.rkt"
+
+(provide (rename-out [#%mb #%module-begin]))
+(define-macro (#%mb (STARS) (BANK ...))
+  #`(#%module-begin
+     (count-redists (list->vector '(BANK ...)) 'STARS)))
+
+(define (redist starting-vec)
+  (define vec (vector-copy starting-vec))
+  (define max-blocks (vector-argmax values vec))
+  (define start-at (vector-member max-blocks vec))
+  (vector-set! vec start-at 0)
+  (for ([block (in-range max-blocks)]
+        [idx (in-cycle (shift-left-cycle (range (vector-length vec)) (add1 start-at)))])
+    (vector-set! vec idx (+ (vector-ref vec idx) 1)))
+  vec)
+
+(define (count-redists bankvec stars)
+  (let/ec exit
+    (for/fold ([bankvecs-seen (list bankvec)])
+              ([i (in-naturals)])
+      (cond
+        [(member (car bankvecs-seen) (cdr bankvecs-seen))
+         => (λ (tail) (exit (if (eq? stars '★)
+                                i
+                                (- (length bankvecs-seen) (length tail)))))])
+      (cons (redist (car bankvecs-seen)) bankvecs-seen))))
+

2017/d6/star1.rkt

@@ -0,0 +1,2 @@
+#lang reader "main.rkt" ★ ; 14029
+10	3	15	10	5	15	5	15	9	2	5	8	5	2	3	6

2017/d6/star2.rkt

@@ -0,0 +1,2 @@
+#lang reader "main.rkt" ★★ ; 2765
+10	3	15	10	5	15	5	15	9	2	5	8	5	2	3	6

2017/d6/test1.rkt

@@ -0,0 +1,2 @@
+#lang reader "main.rkt" ★
+0 2 7 0

2017/d6/test2.rkt

@@ -0,0 +1,2 @@
+#lang reader "main.rkt" ★★
+0 2 7 0