Day 23: Part 1.

src/23.rkt

@@ -0,0 +1,39 @@
+#lang curly-fn racket
+
+(require "../lib.rkt")
+
+(define input '(3 6 2 9 8 1 7 5 4))
+
+(define (succ idx)
+  (modulo (add1 idx) 9))
+
+(define (add1* c)
+  (if (= c 9) 1 (add1 c)))
+
+(define (sub1* c)
+  (if (= c 1) 9 (sub1 c)))
+
+(define (play current cups moves stop)
+  (if (= moves stop)
+      cups
+      (let* ([curr-idx (index-of cups current)]
+             [picked-up (list (list-ref cups (succ curr-idx))
+                              (list-ref cups (succ (succ curr-idx)))
+                              (list-ref cups (succ (succ (succ curr-idx)))))]
+             [dest-idx (let loop ([c (sub1* current)])
+                         (if (member c picked-up)
+                             (loop (sub1* c))
+                             (index-of cups c)))])
+        (define-values (front back) (split-at cups (+ dest-idx 1)))
+        (let ([cups (append (remove* picked-up front) picked-up (remove* picked-up back))])
+          (play (list-ref cups (succ (index-of cups current))) cups (add1 moves) stop)))))
+
+(define part1
+  (let* ([cups (play (first input) input 0 100)]
+         [i (index-of cups 1)]
+         [labels (append (drop cups (succ i)) (take cups i))])
+    (apply string-append (map number->string labels))))
+
+(define part2 #f)
+
+(show-solution part1 part2)