Luhn's algorithm to validate credit card numbers

luhn.ex

defmodule Luhn do
  def validate(num) do
    digits = Integer.digits(num)
    len = length digits

    digits
    |> Stream.with_index
    |> Enum.reverse
    |> Enum.reduce(0, fn {digit, index}, acc ->
      if rem(len - index, 2) == 0 do
        acc + (digit * 2 |> Integer.digits |> Enum.sum)
      else
        acc + digit
      end
    end)
    |> rem(10)
    |> Kernel.==(0)
  end
end