romans.rb

class Fixnum
  def to_roman
    # Roman numeral problem described in Sandi Metz's newsletter:
    # http://us3.campaign-archive2.com/?u=1090565ccff48ac602d0a84b4&id=24e2bac263&e=6df8a5006fa
    # You don't need conditionals, and you don't need two steps either.
    # The trick: there's nothing special about the subtractive cases.
    # Treat them as a single numeral that just happens to be two
    # characters rather than one.
    all_numerals = {
      'I'  => 1,
      'IV' => 4,
      'V'  => 5,
      'IX' => 9,
      'X'  => 10,
      'XL' => 40,
      'L'  => 50,
      'XC' => 90,
      'C'  => 100,
      'CD' => 400,
      'D'  => 500,
      'CM' => 900,
      'M'  => 1000
    }

    remainder = self
    result = ''
    all_numerals.reverse_each do |letter_value, integer_value|
      quotient, remainder = remainder.divmod(integer_value)
      result += letter_value * quotient
    end

    return result
  end
end