From 3d2d6e3a39e1dae20343702e22a30fc0e1a48eac Mon Sep 17 00:00:00 2001
From: Adam Millerchip <adam@millerchip.net>
Date: Tue, 1 Dec 2020 14:36:44 +0900
Subject: [PATCH] Day 1 Part 2

---
 README.md          |  2 +-
 day1/README        | 19 +++++++++++++++++++
 day1/day1part2.exs | 27 +++++++++++++++++++++++++++
 3 files changed, 47 insertions(+), 1 deletion(-)
 create mode 100644 day1/day1part2.exs

diff --git a/README.md b/README.md
index 42dfb60..666324f 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="967" alt="image" src="https://user-images.githubusercontent.com/498229/100191430-6fc5be80-2f33-11eb-8a96-21d87c503c74.png">
+<img width="976" alt="image" src="https://user-images.githubusercontent.com/498229/100701084-45b44680-33e2-11eb-9fb9-11c6161a49ef.png">
 
 ## Strategy
 
diff --git a/day1/README b/day1/README
index 1f97bca..dd58507 100644
--- a/day1/README
+++ b/day1/README
@@ -1,6 +1,25 @@
 Day 1 Notes
 
+------
 Part1:
+------
 
 $ elixir day1part1.exs
 618 x 1402 = 866436
+
+Thoughts:
+
+Avoid iterating seen elements by consuming the list, and comparing each list element only to the remaining
+elements.
+
+------
+Part2:
+------
+
+$ elixir day1part2.exs
+547 x 545 x 928 = 276650720
+
+Thoughts:
+
+Use a comprehension over two versions of the remaining list to efficiently compare the three items.
+
diff --git a/day1/day1part2.exs b/day1/day1part2.exs
new file mode 100644
index 0000000..ad0a73d
--- /dev/null
+++ b/day1/day1part2.exs
@@ -0,0 +1,27 @@
+defmodule Day1Part2 do
+  def run do
+    {a, b, c} =
+      File.read!("input")
+      |> String.trim()
+      |> String.split("\n")
+      |> Enum.map(&String.to_integer/1)
+      |> find_2020()
+
+    IO.puts("#{a} x #{b} x #{c} = #{a * b * c}")
+  end
+
+  def find_2020([current | rest]) do
+    pair =
+      for i <- rest, j <- rest do
+        if current + i + j == 2020, do: {i, j}, else: nil
+      end
+      |> Enum.find(&match?({_, _}, &1))
+
+    case pair do
+      {a, b} -> {a, b, current}
+      nil -> find_2020(rest)
+    end
+  end
+end
+
+Day1Part2.run()