Day 1: add alternate part 2 solution

lib/day1/day1.ex

@@ -16,6 +16,18 @@ defmodule Day1 do
     end
   end
 
+  def first_repetition_stream(shifts) do
+    shifts
+      |> Stream.cycle()
+      |> Enum.reduce_while({0, [0]}, fn shift, {sum, seen} ->
+        sum = sum + shift
+        cond do
+          Enum.member?(seen, sum) -> {:halt, sum}
+          true -> {:cont, {sum, [sum | seen]}}
+        end
+      end)
+  end
+
   def parse_input() do
     File.read!("lib/day1/input.txt")
       |> String.split("\n")
@@ -32,6 +44,6 @@ defmodule Day1 do
 
   def part2() do
     parse_input()
-      |> first_repetition()
+      |> first_repetition_stream()
   end
 end

test/day1_test.exs

@@ -16,4 +16,12 @@ defmodule Day1Test do
     assert Day1.first_repetition([-6, 3, 8, 5, -6]) == 5
     assert Day1.first_repetition([7, 7, -2, -7, -4]) == 14
   end
+
+  test "first repetition stream" do
+    assert Day1.first_repetition_stream([1, -2, 3, 1]) == 2
+    assert Day1.first_repetition_stream([1, -1]) == 0
+    assert Day1.first_repetition_stream([3, 3, 4, -2, -4]) == 10
+    assert Day1.first_repetition_stream([-6, 3, 8, 5, -6]) == 5
+    assert Day1.first_repetition_stream([7, 7, -2, -7, -4]) == 14
+  end
 end