Convert Fixnum to Roman Numeral String

roman_numeral.rb

class Fixnum
  VERSION = 1

  def to_roman
    numeral = ""
    numeral << numeral_thousands
    numeral << numeral_hundreds
    numeral << numeral_tens
    numeral << numeral_ones
  end

  private

  def numeral_thousands
    "M" * (self / 1000)
  end

  def numeral_hundreds
    hundreds = (self % 1000) / 100
    if hundreds == 9
      "CM"
    elsif hundreds >= 5
      hundreds_numeral = "D"
      hundreds_numeral << "C" * (hundreds - 5)
    elsif hundreds == 4
      "CD"
    else
      "C" * hundreds
    end
  end

  def numeral_tens
    tens = (self % 100) / 10
    if tens == 9
      "XC"
    elsif tens >= 5
      tens_numeral = "L"
      tens_numeral << "X" * (tens - 5)
    elsif tens == 4
      "XL"
    else
      "X" * tens
    end
  end

  def numeral_ones
    ones = self % 10
    if ones == 9
      "IX"
    elsif ones >= 5
      ones_numeral = "V"
      ones_numeral << "I" * (ones - 5)
    elsif ones == 4
      "IV"
    else
      "I" * ones
    end
  end
end