gistfile1.ex

defmodule CheckDigit do

  import Enum

  @doc """
  Determine if a sequence of digits is valid, assuming the last digit is
  a Luhn checksum. (http://en.wikipedia.org/wiki/Luhn_algorithm)
  """

  def valid?(numbers) when is_list(numbers) do
    numbers
      |> reverse 
      |> map(&1 - ?0)
      |> sum
      |> rem(10) == 0
  end

  defp sum(digits), do: _sum(digits, 0)

  defp _sum([], acc), do: acc
  defp _sum([odd], acc), do: acc + odd
  defp _sum([odd, even | tail], acc) when even >= 5 do
    _sum(tail, 2 * (even - 5) + 1 + odd + acc)
  end
  defp _sum([odd, even | tail], acc) do
    _sum(tail, even + odd + acc)
  end

end

IO.puts CheckDigit.valid? '79927398713'