Roman Numerals Manage

roman_numerals.rb

class Fixnum
  VERSION=1
  ROMAN_SYMBOL_MAPPING= {
    1=> "I",
    5=> "V",
    10=>"X",
    50=>"L",
    100=>"C",
    500=>"D",
    1000=>"M"
  }
  SUBTRACTABLE_SYMBOLS = {
  	'V'=>'I',
  	'X'=>'I',
  	'L'=>'X',
  	'C'=>'X',
  	'D'=>'C',
  	'M'=>'C'
  }

  def to_roman
    return "M"*(self/1000)+(self%1000).to_roman if self > 1000
    return "" if self==0
    return ROMAN_SYMBOL_MAPPING[self] if ROMAN_SYMBOL_MAPPING.keys.include? self
  	diff=first_of_next_range-ROMAN_SYMBOL_MAPPING.invert[subtractable_symbol]
  	if diff < self
  		return (diff).to_roman + (self-diff).to_roman
  	elsif diff == self
  		return subtractable_symbol+ROMAN_SYMBOL_MAPPING[first_of_next_range]
  	else
      return range.first.to_roman+(self-range.first).to_roman
    end
  end
  private

  def first_of_next_range
     range.last+1
  end

  def subtractable_symbol
    SUBTRACTABLE_SYMBOLS[ROMAN_SYMBOL_MAPPING[range.last+1]]
  end
  def range
    self.class.ranges.select do |range| range.include? self%1000 end.first
  end
  def self.ranges
    ranges=[]
    6.times do | index|
     ranges << (ROMAN_SYMBOL_MAPPING.keys[index]..ROMAN_SYMBOL_MAPPING.keys[index+1]-1)
    end
    ranges
  end
end