class Faker::NationalHealthService

def british_number
  base_number = rand(400_000_001...499_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)}".to_s
                                                     .chars
                                                     .insert(3, ' ')
                                                     .insert(7, ' ')
                                                     .join('')
end

def check_digit(legacy_number = NOT_GIVEN, number: 0)
  warn_for_deprecated_arguments do |keywords|
    keywords << :number if legacy_number != NOT_GIVEN
  end
  sum = 0
  number.to_s.chars.each_with_index do |digit, idx|
    position = idx + 1
    sum += (digit.to_i * (11 - position))
  end
  result = 11 - (sum % 11)
  # A value of 11 is considered the same as 0
  # See https://en.wikipedia.org/wiki/NHS_number
  return 0 if result == 11
  result
end