class Faker::IDNumber

def south_african_id_checksum_digit(id_number)
  value_parts = id_number.chars
  even_digits = value_parts
                .select
                .with_index { |_, i| (i + 1).even? }
  odd_digits_without_last_character = value_parts[0...-1]
                                      .select
                                      .with_index { |_, i| (i + 1).odd? }
  sum_of_odd_digits = odd_digits_without_last_character.map(&:to_i).reduce(:+)
  even_digits_times_two = (even_digits.join.to_i * 2).to_s
  sum_of_even_digits = even_digits_times_two.chars.map(&:to_i).reduce(:+)
  total_sum = sum_of_odd_digits + sum_of_even_digits
  ((10 - (total_sum % 10)) % 10).to_s
end