Roman to-from Numeral in Ruby

roman_numerals.rb

class RomanNumerals
  MAX = 4999
  FACTORS = [['M', 1000], ['CM', 900], ['D', 500],
    ['CD', 400], ['C', 100], ['XC', 90], ['L', 50], ['XL', 40],
    ['X', 10], ['IX', 9], ['V', 5], ['IV', 4], ['I', 1]]
  
  def self.to_roman(i)
    if i <= 0 || i > MAX then fail "Roman values must be > 0 and <= #{MAX}" end
    roman = ''
    for code, factor in FACTORS
      count, i = i.divmod(factor)
      roman << (code * count)
    end
    roman
  end
  
  def self.from_roman(r)
    numeral = 0
    for code, factor in FACTORS
      while r.index(code) == 0
        numeral += factor
        r.slice!(code)
      end
    end
    numeral
  end
end