https://medium.com/@sashang/no-debugging-programming-in-elixir-e26f3d3fd243

gistfile1.txt

defmodule Inversion do
  def run(filename) do
    {sorted, count} = File.read!(filename)
      |> String.split("\r\n", trim: true)
      |> Enum.map(fn(x) -> String.to_integer(x) end)
      |> devide_and_conquer
    count
  end

  defp devide_and_conquer(data) do
    case data do
      [] -> {[], 0}
      [x] -> {[x], 0}
      [x,y] when x > y -> {[y,x], 1}
      [x,y] when x <= y -> {[x,y], 0}
      list ->
        count = length(list)
        half_index = div(count, 2)
        {sorted_x, x} = devide_and_conquer(Enum.slice(list, (0..half_index-1)))
        {sorted_y, y} = devide_and_conquer(Enum.slice(list, half_index..count-1))
        {sorted_z, z} = merge(sorted_x, sorted_y, [], 0)
        {Enum.reverse(sorted_z), x + y + z}
    end
  end

  defp merge(a, b, acc_list, acc_inversions) do
    case {a, b} do
      {[], []} -> {acc_list, acc_inversions}
      {a, []} -> {Enum.reverse(a) ++ acc_list, acc_inversions}
      {[], b} -> {Enum.reverse(b) ++ acc_list, acc_inversions}
      {a, b} when hd(a) > hd(b) -> merge(a, tl(b), [hd(b) | acc_list], length(a) + acc_inversions)
      {a, b} when hd(a) <= hd(b) -> merge(tl(a), b, [hd(a) | acc_list], acc_inversions)
    end
  end
end