From 77ef527041d04b32db91f1e77f104bd78647bc46 Mon Sep 17 00:00:00 2001
From: Shadowfacts <me@shadowfacts.net>
Date: Sun, 2 Dec 2018 11:04:05 -0500
Subject: [PATCH] Day 1: Use MapSet to speed up part 2

---
 lib/day1/day1.ex | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/lib/day1/day1.ex b/lib/day1/day1.ex
index 03a399f..3005aa8 100644
--- a/lib/day1/day1.ex
+++ b/lib/day1/day1.ex
@@ -28,6 +28,18 @@ defmodule Day1 do
       end)
   end
 
+  def first_repetition_mapset(shifts) do
+    shifts
+      |> Stream.cycle()
+      |> Enum.reduce_while({0, MapSet.new([0])}, fn shift, {sum, seen} ->
+        sum = sum + shift
+        cond do
+          sum in seen -> {:halt, sum}
+          true -> {:cont, {sum, MapSet.put(seen, sum)}}
+        end
+      end)
+  end
+
   def parse_input() do
     File.read!("lib/day1/input.txt")
       |> String.split("\n")
@@ -44,6 +56,6 @@ defmodule Day1 do
 
   def part2() do
     parse_input()
-      |> first_repetition_stream()
+      |> first_repetition_mapset()
   end
 end
\ No newline at end of file