You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: code.rb
    path of file: ./repos/faker/lib/faker/default
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

module Faker
  class Code < Base
    flexible :code
    class << self
      ##
      # Produces a random NPI (National Provider Identifier) code.
      #
      # @return [String]
      #
      # @example
      #   Faker::Code.npi #=> "9804062802"
      #
      # @
    
      def generate_sin_check_digit(digits)
        # Fast Luhn checksum code from luhn.js:
        # https://gist.github.com/ShirtlessKirk/2134376

        len = 9
        mul = 0

        luhn_arr = [
          [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
          [0, 2, 4, 6, 8, 1, 3, 5, 7, 9]
        ]
        sum = 0

        while len.positive?
          len -= 1
          num = digits[len].to_i
          sum += luhn_arr[mul][num]
          mul ^= 1
        end