From 818b64f9c71c6485ea3f4ad987b0d698fff4fd8e Mon Sep 17 00:00:00 2001
From: Jonathan Chan <jcxz@cs.ubc.ca>
Date: Thu, 24 Dec 2020 22:19:41 -0800
Subject: [PATCH] Day 25.

---
 src/25.rkt | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)
 create mode 100644 src/25.rkt

diff --git a/src/25.rkt b/src/25.rkt
new file mode 100644
index 0000000..d22702e
--- /dev/null
+++ b/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!")