defmodule Cnum.Validator do
  @moduledoc false

  # Validator validates a credit card number through the Luhn algorithm.
  # Test credit card numbers are valid by default even if they would not pass the Luhn check.

  require Integer

  @test_cards [
    2223000048400011,
    2223003122003222,
    2223520043560014,
    30569309025904,
    3530111333300000,
    3566002020360505,
    371449635398431,
    378282246310005,
    378734493671000,
    38520000023237,
    4000056655665556,
    4012888888881881,
    4111111111111111,
    4222222222222,
    4242424242424242,
    5105105105105100,
    5200828282828210,
    5555555555554444,
    5610591081018250,
    6011111111111117,
    6011000990139424,
    6200000000000005
  ]

  @doc false
  def validate(number) do
    if Enum.member?(@test_cards, number),
     do: true, else: luhn_check(number)
  end

  # -------------------------------------------------- #

  defp luhn_check(number) do
    [ check_digit | remaining_digits ] = Integer.digits(number) |> Enum.reverse
    luhn_sum(remaining_digits) |> mod_10_check(check_digit)
  end

  defp luhn_sum(digits, index \\ 0, acc \\ 0)

  defp luhn_sum([], _index, acc), do: acc

  defp luhn_sum([digit | tail], index, acc) when Integer.is_odd(index),
    do: luhn_sum(tail, index + 1, acc + digit)

  defp luhn_sum([digit | tail], index, acc) when Integer.is_even(index),
    do: luhn_sum(tail, index + 1, acc + partial_sum(digit * 2))

  defp mod_10_check(digits_sum, check_digit) do
    Integer.mod((digits_sum + check_digit), 10) == 0
  end

  defp partial_sum(number) when number > 9 do
    Integer.digits(number) |> Enum.sum()
  end

  defp partial_sum(number), do: number
end