cc_validation

defmodule CcValidation do
  @moduledoc """
  CcValidation implements methods to verify validity of a credit card number
  """

  @doc """
  Validate a credit card number using Luhn Algorithm

  ## Examples

  Valid credit card
  iex> CcValidation.validate("4716892095589823")
  {:ok, true}

  Invalid credit card
  iex> CcValidation.validate("4024007106963124")
  {:error, false}

  Test card returns false
  iex> CcValidation.validate("4111111111111111")
  {:error, false}

  ### Validate test card numbers when check_test_numbers is true

  A valid test card
  iex> CcValidation.validate("4111111111111111", true)
  {:ok, true, test_number: true}

  An invalid test card
  iex> CcValidation.validate("4212121212121212", true)
  {:error, false, test_number: false}

  A valid card when checking for test numbers just passes through
  to the validate function but will also return that it is not
  a test card
  iex> CcValidation.validate("4716892095589823", true)
  {:ok, true, test_number: false}

  ### Card numbers that are of length less than 13 or greater than 19 are
  invalid.

  iex> CcValidation.validate("123")
  {:error, false}

  iex> CcValidation.validate("12345678901234567890")
  {:error, false}
  """
  def validate(number, check_test_numbers \\ false)
  def validate(number, true) do
    case CcValidation.TestNumbers.exists?(number) do
      true ->
        {:ok, true, test_number: true}
      false ->
        Tuple.append(validate(number), test_number: false)
    end
  end
  def validate(number, false)
  when byte_size(number) < 13 or byte_size(number) > 19 do
    {:error, false}
  end
  def validate(number, false) do
    case CcValidation.TestNumbers.exists?(number) do
      true ->
        {:error, false}
      _ ->
        total = number
        |> String.codepoints
        |> Enum.reverse
        |> Stream.with_index
        |> Enum.map(fn {num, i} ->
          case rem(i + 1, 2) do
            0 ->
              doubled = String.to_integer(num) * 2
              if doubled > 9 do
                doubled - 9
              else
                doubled
              end
            _ ->
              String.to_integer(num)
          end
        end)
        |> Enum.sum

        case rem(total, 10) do
          0 ->
            {:ok, true}
          _ ->
            {:error, false}
        end
    end
  end
end