Add Day 06 part 1

.formatter.exs

@@ -1,5 +1,5 @@
 # Used by "mix format"
 [
   inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"],
-  line_length: 130
+  line_length: 150
 ]

lib/advent_2021/day_06.ex

@@ -0,0 +1,29 @@
+defmodule Advent2021.Day06 do
+  def fish_count(input, days) do
+    input
+    |> retrieve_school
+    |> go_for_days(days)
+    |> Enum.count()
+  end
+
+  def go_for_days(school, 0), do: school
+
+  def go_for_days(school, days) do
+    go_for_days(increment_day(school), days - 1)
+  end
+
+  def increment_day(school) do
+    school
+    |> Enum.map(&transform_fish/1)
+    |> List.flatten()
+  end
+
+  defp transform_fish(0), do: [6, 8]
+  defp transform_fish(fish), do: fish - 1
+
+  defp retrieve_school(input) do
+    input
+    |> String.split(",", trim: true)
+    |> Enum.map(&String.to_integer/1)
+  end
+end

test/lib/advent_2021/day_06_test.exs

@@ -0,0 +1,25 @@
+defmodule Advent2021.Day06Test do
+  use ExUnit.Case, async: true
+
+  alias Advent2021.Day06
+
+  test "increment_day" do
+    assert Day06.increment_day([3, 4, 3, 1, 2]) == [2, 3, 2, 0, 1]
+    assert Day06.increment_day([2, 3, 2, 0, 1]) == [1, 2, 1, 6, 8, 0]
+    assert Day06.increment_day([6, 0, 6, 4, 5, 6, 0, 1, 1, 2, 6, 7, 8, 8, 8]) == [5, 6, 8, 5, 3, 4, 5, 6, 8, 0, 0, 1, 5, 6, 7, 7, 7]
+  end
+
+  test "fish_count" do
+    assert Day06.fish_count(input(), 18) == 26
+    assert Day06.fish_count(input(), 80) == 5934
+  end
+
+  test "fish_count real" do
+    input = File.read!("test/lib/advent_2021/input/day_06.txt") |> String.trim()
+    assert Day06.fish_count(input, 80) == 373_378
+  end
+
+  defp input do
+    "3,4,3,1,2"
+  end
+end

test/lib/advent_2021/input/day_06.txt

@@ -0,0 +1 @@
+1,1,3,1,3,2,1,3,1,1,3,1,1,2,1,3,1,1,3,5,1,1,1,3,1,2,1,1,1,1,4,4,1,2,1,2,1,1,1,5,3,2,1,5,2,5,3,3,2,2,5,4,1,1,4,4,1,1,1,1,1,1,5,1,2,4,3,2,2,2,2,1,4,1,1,5,1,3,4,4,1,1,3,3,5,5,3,1,3,3,3,1,4,2,2,1,3,4,1,4,3,3,2,3,1,1,1,5,3,1,4,2,2,3,1,3,1,2,3,3,1,4,2,2,4,1,3,1,1,1,1,1,2,1,3,3,1,2,1,1,3,4,1,1,1,1,5,1,1,5,1,1,1,4,1,5,3,1,1,3,2,1,1,3,1,1,1,5,4,3,3,5,1,3,4,3,3,1,4,4,1,2,1,1,2,1,1,1,2,1,1,1,1,1,5,1,1,2,1,5,2,1,1,2,3,2,3,1,3,1,1,1,5,1,1,2,1,1,1,1,3,4,5,3,1,4,1,1,4,1,4,1,1,1,4,5,1,1,1,4,1,3,2,2,1,1,2,3,1,4,3,5,1,5,1,1,4,5,5,1,1,3,3,1,1,1,1,5,5,3,3,2,4,1,1,1,1,1,5,1,1,2,5,5,4,2,4,4,1,1,3,3,1,5,1,1,1,1,1,1