Luhn algorithm with the broken Syncthing implementation

luhn.rb

  # Implemented because Syncthing (kind of) uses it
  # @example
  #   @luhn = Luhn.new Base32.table
  #   @luhn.check_character "P56IOI7MZJNU2"
  #   # => "S"
  class Luhn
    def initialize alphabet
      @alphabet = alphabet
      @table_alpha = Hash[ @alphabet.split(//).zip(0 .. @alphabet.length - 1) ]
      @table_code = Hash[ (0 .. @alphabet.length - 1).zip(@alphabet.split(//))]
    end

    attr_reader :alphabet, :table_alpha, :table_code

    def n
      @n ||= @alphabet.length
    end

    FACTOR = 2

    # Starting from the right and working leftwards is easier since 
    # the initial "factor" will always be "2"
    def check_character input, factor=FACTOR
      sum = input.reverse.each_char.inject(0) do |sum,char|
        addend = factor * @table_alpha[char]
        # Alternate the "factor" that each "codePoint" is multiplied by
        factor = (factor == 2) ? 1 : 2
        # Sum the digits of the "addend" as expressed in base "n"
        sum += Integer(addend / n) + (addend % n)
      end

      # Calculate the number that must be added to the "sum" 
      # to make it divisible by "n"
      remainder = sum % n
      checkcode = (n - remainder) % n

      @table_code[checkcode]
    end


    # @see https://forum.syncthing.net/t/v0-9-0-new-node-id-format/478/7
    # @example
    #   @luhn = Luhn.new Base32.table
    #   @luhn.check_for_syncthing "P56IOI7MZJNU2"
    #   # => "Y"
    def check_for_syncthing input
      check_character input.reverse, 1
    end
  end