class Faker::NationalHealthService

def british_number
  base_number = rand(999_000_001...999_999_999)
  # If the check digit is equivalent to 10, the number is invalid.
  # See https://en.wikipedia.org/wiki/NHS_number
  base_number -= 1 if check_digit(number: base_number) == 10
  "#{base_number}#{check_digit(number: base_number)}".chars
                                                     .insert(3, ' ')
                                                     .insert(7, ' ')
                                                     .join
end