Day 20

src/20.rkt

@@ -0,0 +1,63 @@
+#lang curly-fn racket
+
+(require "../lib.rkt")
+
+(define input
+  (map string->number (problem-input 20)))
+
+(struct element (value [prev #:mutable] [next #:mutable]))
+
+(define (parse)
+  (let ([hd (element (first input) #f #f)])
+    (for/fold ([curr hd]
+               [elements `(,hd)]
+               #:result (values (begin (set-element-next! curr hd)
+                                       (set-element-prev! hd curr)
+                                       (reverse elements))))
+              ([i (rest input)])
+      (define next (element i curr #f))
+      (set-element-next! curr next)
+      (values next (cons next elements)))))
+
+(define (mix! elements key)
+  (for ([e elements])
+    (set-element-next! (element-prev e) (element-next e))
+    (set-element-prev! (element-next e) (element-prev e))
+    (let loop ([i (% (* (element-value e) key) (sub1 (length elements)))]
+               [next (element-next e)])
+      (cond
+        [(zero? i)
+         (set-element-next! (element-prev next) e)
+         (set-element-prev! e (element-prev next))
+         (set-element-next! e next)
+         (set-element-prev! next e)]
+        [(positive? i) (loop (sub1 i) (element-next next))]
+        [(negative? i) (loop (add1 i) (element-prev next))]))))
+
+(define (grove elements key)
+  (define 0th
+    (let loop ([curr (first elements)])
+      (if (= (element-value curr) 0)
+          curr
+          (loop (element-next curr)))))
+  (define (get e i)
+    (for/fold ([curr e])
+              ([_ (in-range i)])
+      (element-next curr)))
+  (* (+ (element-value (get 0th (% 1000 (sub1 (length elements)))))
+        (element-value (get 0th (% 2000 (sub1 (length elements)))))
+        (element-value (get 0th (% 3000 (sub1 (length elements))))))
+     key))
+
+(define part1
+  (let ([elements (parse)])
+    (mix! elements 1)
+    (grove elements 1)))
+
+(define part2
+  (let ([elements (parse)])
+    (for ([_ (in-range 10)])
+      (mix! elements 811589153))
+    (grove elements 811589153)))
+
+(show-solution part1 part2)