roman.rb

class Integer
 def roman(s=self)
  return "-" if 3999 < s
  
  v = 1
  r = {1 => 'I', 4 => 'IV', 5 => 'V', 9=> 'IX', 10 =>  'X',
   40 => 'XL', 50 => 'L', 90 => 'XC', 100 => 'C', 400 => 'CD',
   500 => 'D', 900=> 'CM', 1000 => 'M', 9999 => "" }
  
  r.keys.sort.each{|k|
   if k > s && s > 0
    return r[v] + roman(s - v); break
   end
   v = k }
  return ""
 end
end