GIRO to IBAN converter in ruby

iban.rb

class IBAN
  def self.sanitize(account_number)
    account_number.gsub(/\D*/, '')
  end

  def self.giro2iban(giro, country_code = 'HU')
    giro = sanitize giro
    country_code.upcase!

    unless giro.size == 16 || giro.size == 24
      raise ArgumentError, "giro must be 16 or 24 digits long"
    end

    giro = "#{giro}00000000" if giro.size == 16

    base_str = "#{giro}#{country_code}00"
    str = ""

    base_str.chars.each do |char|
      if char >= 'A' && char <= 'Z'
        char = (char.ord - 55).to_s
      end

      str = str + char
    end

    base_str = str
    str = ""
    i = 0
    n = 9
    m = 0

    while i < base_str.size
      x = (str + base_str[i, n]).to_i
      m = x % 97
      str = m.to_s
      i = i + n
      n = 9 - str.length
    end

    check_digits = (98 - m).to_s

    if check_digits.size == 1
      check_digits = '0' + check_digits
    elsif check_digits.size != 2
      raise "Invalid number of check digits #{check_digits}"
    end

    country_code + check_digits + giro
  end
end