1 files changed, 48 insertions, 0 deletions
diff --git a/racket/aoc2022/day-20/day-20.rkt b/racket/aoc2022/day-20/day-20.rkt
new file mode 100644
index 0000000..6dd1070
--- /dev/null
+++ b/racket/aoc2022/day-20/day-20.rkt
@@ -0,0 +1,48 @@
+#lang racket
+(require advent-of-code
+         threading)
+
+(define data (port->list read (open-aoc-input (find-session) 2022 20 #:cache #true)))
+
+(define gps-lst data)
+(define gps-len (length gps-lst))
+(define gps-indexed (map cons (inclusive-range 1 gps-len) gps-lst))
+
+(define (mix pt data)
+  (match-define (list left ... (== pt) right ...) data)
+  (define start (index-of data pt))
+  (define move-by (modulo (cdr pt) (sub1 gps-len)))
+  (cond
+    [(= 0 move-by) data]
+    [(<= move-by (length right))
+     (match-define-values (new-left new-right)
+       (split-at (append left right) (modulo (+ move-by start) (sub1 gps-len))))
+     (append new-left (list pt) new-right)]
+    [else
+     (match-define-values (new-left new-right)
+       (split-at (append left right) (modulo (+ move-by start) (sub1 gps-len))))
+     (append new-left (list pt) new-right)]))
+
+(define (mix-gps data original)
+  (for/fold ([pts data]) ([pt original])
+    (mix pt pts)))
+
+(define (cycle-mixed-gps mixed)
+  (define lst (map cdr mixed))
+  (in-sequences (drop lst (index-of lst 0)) (in-cycle lst)))
+
+(define (calculate-answer seq)
+  (for/sum ([id '(1000 2000 3000)]) (sequence-ref seq id)))
+
+;; part 1
+(~> gps-indexed (mix-gps _ gps-indexed) cycle-mixed-gps calculate-answer)
+
+;; part 2
+(define encrypted-gps-indexed
+  (for/list ([pt (in-list gps-indexed)] #:do [(match-define (cons i v) pt)])
+    (cons i (* 811589153 v))))
+
+(~>> encrypted-gps-indexed
+     ((λ (data) (foldr (λ (_ pts) (mix-gps pts data)) data (range 10))))
+     cycle-mixed-gps
+     calculate-answer)