Fixnum convert to Roman Number String

roman_number.rb

class Fixnum
  def to_roman
    return "" unless (1..3999).include?(self)

    digit_count = Math.log(self, 10).to_i

    case digit_count
    when 0
      minor = "I"
      major = "V"
      next_minor = "X"
    when 1
      minor = "X"
      major = "L"
      next_minor = "C"
    when 2
      minor = "C"
      major = "D"
      next_minor = "M"
    when 3
      minor = "M"
      major = "?"
      next_minor = "?"
    end

    if digit_count > 0
      first_digit = self / 10 ** digit_count
      next_number = self - first_digit * 10 ** digit_count
    else
      first_digit = self
      next_number = 0
    end

    mod = first_digit % 10

    if mod == 9
      roman = "#{minor}#{next_minor}"
    elsif mod == 4
      roman = "#{minor}#{major}"
    elsif mod >= 5
      roman = "#{major}" + "#{minor}" * (mod - 5)
    else
      roman = "#{minor}" * mod
    end

    return roman + next_number.to_roman
  end
end

describe "Fixnum#to_roman" do
  shared_examples "should return roman string" do |num, roman|
    context "#{num}" do
      subject {num}
      its(:to_roman) { should == roman }
    end
  end

  it_behaves_like "should return roman string", 1, "I"
  it_behaves_like "should return roman string", 2, "II"
  it_behaves_like "should return roman string", 5, "V"
  it_behaves_like "should return roman string", 4, "IV"
  it_behaves_like "should return roman string", 9, "IX"
  it_behaves_like "should return roman string", 10, "X"
  it_behaves_like "should return roman string", 12, "XII"
  it_behaves_like "should return roman string", 801, "DCCCI"
  it_behaves_like "should return roman string", 1999, "MCMXCIX"
end