From eb17b2431a8c591b64559962afbd88fd64b868ca Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <jdkaplan@metagram.net>
Date: Mon, 14 Dec 2020 21:24:58 -0800
Subject: [PATCH] Day 15 Part 1

---
 2020/day15/day15.ex | 45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)
 create mode 100644 2020/day15/day15.ex

diff --git a/2020/day15/day15.ex b/2020/day15/day15.ex
new file mode 100644
index 0000000..d0d7a8f
--- /dev/null
+++ b/2020/day15/day15.ex
@@ -0,0 +1,45 @@
+defmodule Day15 do
+  defp read_input do
+    Path.expand('input', Path.dirname(__ENV__.file))
+    |> File.read!()
+  end
+
+  defp test_input do
+    """
+    0,3,6
+    """
+  end
+
+  defp parse_numbers(text) do
+    text
+    |> String.trim()
+    |> String.split(",")
+    |> Enum.map(&String.to_integer/1)
+  end
+
+  defp next([previous | rest]) do
+    {_, idx} =
+      rest
+      |> Enum.with_index()
+      |> Enum.find({previous, -1}, fn {num, idx} -> num == previous end)
+
+    idx + 1
+  end
+
+  defp run(log, limit) do
+    if Enum.count(log) == limit do
+      Enum.at(log, 0)
+    else
+      run([next(log) | log], limit)
+    end
+  end
+
+  def part1 do
+    read_input()
+    |> parse_numbers()
+    |> Enum.reverse()
+    |> run(2020)
+  end
+end
+
+Day15.part1() |> IO.inspect()