Day 25.

src/25.rkt

@@ -0,0 +1,17 @@
+#lang racket
+
+(require math/number-theory
+         "../lib.rkt")
+
+(define card 9033205)
+(define door 9281649)
+
+(define door-loop
+  (for/first ([loop (in-naturals)]
+              #:when (= (modular-expt 7 loop 20201227) door))
+    loop))
+
+(define part1
+  (modular-expt card door-loop 20201227))
+
+(show-solution part1 "Done!")