Parallel test runner

ntest.ex

defmodule Mix.Tasks.NTest do
  use Mix.Task

  @wildcard "apps/*/test/**/*_test.exs"

  def run([i, n | args]) do
    {i, _} = Integer.parse(i)
    {n, _} = Integer.parse(n)

    test_paths =
      @wildcard
      |> Path.wildcard()
      |> Enum.map(&Path.split/1)
      |> Enum.map(&Enum.drop(&1, 2))
      |> Enum.map(&Path.join/1)
      |> Enum.filter(fn path ->
        path |> hash |> rem(n) == i - 1
      end)

    {stdout, exit_code} = System.cmd("mix", ["test"] ++ args ++ test_paths)
    IO.puts(stdout)
    exit({:shutdown, exit_code})
  end

  defp hash(s) do
    :md5
    |> :crypto.hash(s)
    |> Base.encode16()
    |> Integer.parse(16)
    |> elem(0)
  end
end