luhn.ex

defmodule Luhn do

  @spec is_luhn?(number()) :: boolean()
  def is_luhn?(number) do
    number |>
      Integer.digits |>
      Enum.reverse |>
      sort_and_sum_numbers |>
      verify?
  end

  defp sort_and_sum_numbers(number_list) do
    odds =
      Enum.take_every(number_list, 2) |>
      Enum.sum

    evens =
      Enum.take_every(tl(number_list), 2) |>
      Enum.reduce(0, fn(x, acc) -> luhn_reduction_over_9(x) + acc end)

    evens + odds
  end

  defp luhn_reduction_over_9(number) do
    number = number * 2
    cond do
      number > 9 ->
        1 + rem(number, 10)
      true ->
        number
    end
  end

  defp verify?(number), do: rem(number, 10) == 0
end