Integer.class_eval do
  def to_words
    return '?' unless (1..999_999_999_999).include? self
    numbers = [nil, 'one', 'two', 'three', 'four', 'five', 'six', 'seven',
      'eight', 'nine', 'ten', 'eleven', 'twelve', 'thirteen', 'fourteen',
      'fifteen', 'sixteen', 'seventeen', 'eighteen', 'nineteen']
    decades = [nil, nil, 'twenty', 'thirty', 'forty', 'fifty', 'sixty',
      'seventy', 'eighty', 'ninety']
    case self
      when 1..19
        numbers[self]
      when 20..99
        decades[self/10] + (self%10 == 0 ? '' : ('-' + numbers[self%10]))
      when 100..999
        numbers[self/100] + ' hundred' + (self%100 == 0 ? '' : ' and ' + (self%100).to_words)
      when 1_000..999_999
        words_for_big_numbers 1_000, 'thousand'
      when 1_000_000..999_999_999
        words_for_big_numbers 1_000_000, 'million'
      when 1_000_000_000..999_999_999_999
        words_for_big_numbers 1_000_000_000, 'billion'
    end
  end
  
  private
  def words_for_big_numbers multiplier_value, multiplier_name
    (self/multiplier_value).to_words + ' ' + multiplier_name +
      if self%multiplier_value == 0
        ''
      else
        (self%multiplier_value < 100 ? ' and ' : ', ') + (self%multiplier_value).to_words
      end
  end
end