class Faker::NationalHealthService

def check_digit(number: 0)
  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