From 2786c716278286e2246138318d46e88fd4457492 Mon Sep 17 00:00:00 2001
From: Adam Millerchip <adam@millerchip.net>
Date: Fri, 25 Dec 2020 16:45:16 +0900
Subject: [PATCH] Day 25 Part 1

---
 README.md            |  2 +-
 day25/README         | 26 ++++++++++++++++++++++++++
 day25/day25part1.exs | 29 +++++++++++++++++++++++++++++
 day25/input          |  2 ++
 4 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100644 day25/README
 create mode 100644 day25/day25part1.exs
 create mode 100644 day25/input

diff --git a/README.md b/README.md
index 4b8e232..3cf6de6 100644
--- a/README.md
+++ b/README.md
@@ -2,7 +2,7 @@
 
 My (attempted) solutions to [Advent of Code 2020](https://adventofcode.com/2020) in Elixir.
 
-<img width="978" alt="image" src="https://user-images.githubusercontent.com/498229/103093696-6321ac80-463e-11eb-933c-8c6b23d15ee0.png">
+<img width="982" alt="image" src="https://user-images.githubusercontent.com/498229/103125884-e8a16d00-46cf-11eb-854d-67e600374e82.png">
 
 ## Strategy
 
diff --git a/day25/README b/day25/README
new file mode 100644
index 0000000..3705f45
--- /dev/null
+++ b/day25/README
@@ -0,0 +1,26 @@
+Day 25 Notes
+
++--------+
+| Part 1 |
++--------+
+
+$ elixir day25part1.exs
+4968512
+
+Thoughts:
+
+Was a bit difficult to decipher the requirements, but after that the implementation was
+simple enough. Wondering if there's a significance of the numbers 7 and 2020_12_27?
+
++--------+
+| Part 2 |
++--------+
+
+Arg. Need all the previous stars. So far I'm 10 under.
+
+
++------------------+
+| Overall Thoughts |
++------------------+
+
+
diff --git a/day25/day25part1.exs b/day25/day25part1.exs
new file mode 100644
index 0000000..366784c
--- /dev/null
+++ b/day25/day25part1.exs
@@ -0,0 +1,29 @@
+defmodule Day25Part1 do
+  def run do
+    [card_key, door_key] =
+      File.read!("input")
+      |> String.split("\n", trim: true)
+      |> Enum.map(&String.to_integer/1)
+
+    door_loop_size = find_loop_size(1, door_key, 0)
+
+    transform(1, card_key, door_loop_size)
+    |> IO.puts()
+  end
+
+  def find_loop_size(public_key, public_key, size), do: size
+
+  def find_loop_size(value, key, size) do
+    operation(value, 7) |> find_loop_size(key, size + 1)
+  end
+
+  def transform(encryption_key, _subject_number, 0), do: encryption_key
+
+  def transform(value, subject_number, size) do
+    operation(value, subject_number) |> transform(subject_number, size - 1)
+  end
+
+  def operation(value, subject_number), do: rem(value * subject_number, 2020_12_27)
+end
+
+Day25Part1.run()
diff --git a/day25/input b/day25/input
new file mode 100644
index 0000000..b1473b7
--- /dev/null
+++ b/day25/input
@@ -0,0 +1,2 @@
+10604480
+4126658