Solve 2021 Day 6 in elixir

2021/Day6/Elixir/lanternfish.ex

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+ExUnit.start()
+
+defmodule Lanternfish do
+  def load_file() do
+    File.read!("../input.txt")
+    |> String.split(",", trim: true)
+  end
+
+  def parse_swarm(input) do
+    input
+    |> Enum.group_by(& &1)
+    |> Enum.map(fn {i, vals} -> {i, Enum.count(vals)} end)
+    |> Map.new()
+  end
+
+  def give_offspring(fish, 0) do
+    fish |> Map.delete(9)
+  end
+
+  def give_offspring(fish, days_remaining) do
+    safe_fish =
+      fish
+      |> Map.put_new(0, 0)
+      |> Map.put_new(1, 0)
+      |> Map.put_new(2, 0)
+      |> Map.put_new(3, 0)
+      |> Map.put_new(4, 0)
+      |> Map.put_new(5, 0)
+      |> Map.put_new(6, 0)
+      |> Map.put_new(7, 0)
+      |> Map.put_new(8, 0)
+      |> Map.put_new(9, 0)
+
+    safe_fish
+    |> Map.put(7, safe_fish[7] + safe_fish[0])
+    |> Map.put(9, safe_fish[9] + safe_fish[0])
+    |> Enum.map(fn {internal_counter, amount} -> {internal_counter - 1, amount} end)
+    |> Map.new()
+    |> Map.delete(-1)
+    |> Map.delete(9)
+    |> give_offspring(days_remaining - 1)
+  end
+
+  def solve(days) do
+    fish =
+      load_file()
+      |> Enum.map(&Integer.parse/1)
+      |> Enum.map(&elem(&1, 0))
+      |> parse_swarm()
+      |> give_offspring(days)
+
+    Enum.map(fish, fn {_, amount} -> amount end)
+    |> Enum.sum()
+  end
+
+  @spec first() :: Integer
+  def first() do
+    solve(80)
+  end
+
+  @spec second() :: Integer
+  def second() do
+    solve(256)
+  end
+end
+
+defmodule LanternfishTest do
+  use ExUnit.Case
+
+  describe "giveOffsprint/2" do
+    test "Initial value" do
+      swarm = Lanternfish.parse_swarm([3, 4, 3, 1, 2])
+      assert Lanternfish.give_offspring(swarm, 0) == %{1 => 1, 2 => 1, 3 => 2, 4 => 1}
+    end
+
+    test "After 1 day" do
+      swarm = Lanternfish.parse_swarm([3, 4, 3, 1, 2])
+
+      assert Lanternfish.give_offspring(swarm, 1) == %{
+               0 => 1,
+               1 => 1,
+               2 => 2,
+               3 => 1,
+               4 => 0,
+               5 => 0,
+               6 => 0,
+               7 => 0,
+               8 => 0
+             }
+    end
+
+    test "After 2 days" do
+      swarm = Lanternfish.parse_swarm([3, 4, 3, 1, 2])
+
+      assert Lanternfish.give_offspring(swarm, 2) == %{
+               0 => 1,
+               1 => 2,
+               2 => 1,
+               3 => 0,
+               4 => 0,
+               5 => 0,
+               6 => 1,
+               7 => 0,
+               8 => 1
+             }
+    end
+  end
+end

2021/Day6/test.txt

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