From c07c38f17a56c26a5fa529c7a1777876c45da340 Mon Sep 17 00:00:00 2001
From: Glenn Griffin <ggriffiniii@gmail.com>
Date: Thu, 24 Dec 2020 21:29:17 -0800
Subject: [PATCH] day25

---
 input/2020/day25.txt |  2 ++
 src/day25.rs         | 33 +++++++++++++++++++++++++++++++++
 src/lib.rs           |  1 +
 3 files changed, 36 insertions(+)
 create mode 100644 input/2020/day25.txt
 create mode 100644 src/day25.rs

diff --git a/input/2020/day25.txt b/input/2020/day25.txt
new file mode 100644
index 0000000..3e56505
--- /dev/null
+++ b/input/2020/day25.txt
@@ -0,0 +1,2 @@
+17773298
+15530095
diff --git a/src/day25.rs b/src/day25.rs
new file mode 100644
index 0000000..d05abfe
--- /dev/null
+++ b/src/day25.rs
@@ -0,0 +1,33 @@
+use crate::split_once;
+use aoc_runner_derive::aoc;
+
+const MOD: usize = 20201227;
+
+#[aoc(day25, part1)]
+fn solve_d25_p1(input: &str) -> usize {
+    let (pubkey1, pubkey2) = split_once(input, "\n").unwrap();
+    let pubkey1: usize = pubkey1.parse().unwrap();
+    let pubkey2: usize = pubkey2.parse().unwrap();
+    let loop_size = loop_size_from_pubkey(pubkey1);
+    encryption_key_from_pubkey(pubkey2, loop_size)
+}
+
+fn encryption_key_from_pubkey(pubkey: usize, loop_size: usize) -> usize {
+    let mut value = 1;
+    for _ in 0..loop_size {
+        value *= pubkey;
+        value %= MOD;
+    }
+    value
+}
+
+fn loop_size_from_pubkey(pubkey: usize) -> usize {
+    let mut loop_size = 0;
+    let mut value = 1;
+    while value != pubkey {
+        loop_size += 1;
+        value *= 7;
+        value %= MOD;
+    }
+    loop_size
+}
diff --git a/src/lib.rs b/src/lib.rs
index 78f4c3b..ffe62b0 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -15,6 +15,7 @@ pub mod day21;
 pub mod day22;
 pub mod day23;
 pub mod day24;
+pub mod day25;
 pub mod day3;
 pub mod day4;
 pub mod day5;