roman_number.rb

class Fixnum
  def to_roman
    return '' unless self < 5000
    
    roman_thousands(self) << roman_hundreds(self) << roman_tens(self) << roman_units(self)
  end

  private
  
  def roman_units(unit)
    list_unit = ['','I','II','III','IV','V','VI','VII','VIII','IX']
    
    list_unit[unit % 10]
  end

  def roman_tens(ten)
    return '' unless self > 9

    list_ten = ['','X','XX','XXX','XL','L','LX','LXX','LXXX','XC']
    index = (ten % 100) / 10

    list_ten[index]
  end
  
  def roman_hundreds(hundred)
    return '' unless self > 99

    list_hundred = ['','C','CC','CCC','CD','D','DC','DCC','DCCC','CM']
    index = (hundred % 1000) / 100
    
    list_hundred[index]
  end

  def roman_thousands(thousand)
    return '' unless self > 999
    
    list_thousand = ['','M','MM','MMM','MMMM']
    index = (thousand % 10000) / 1000
    
    list_thousand[index]
  end
end