roman_numeral.rb

class RomanNumeral

  def self.to_arabic roman_number
    add_roman roman_number
  end


  private

  def self.add_roman roman_number
    result = 0

    ( 0..roman_number.size - 2 ).each do |i|
      this_char = roman_number[i]
      next_char = roman_number[i+1]

      next_is_greater( this_char, next_char ) ?
        result -= roman_lookup( this_char )  :
        result += roman_lookup( this_char )
    end

    result += roman_lookup last_char_for( roman_number )
  end


  def self.last_char_for str
    str.split('').last
  end


  def self.next_is_greater first,second
    to_arabic( second ) > to_arabic( first )
  end


  def self.roman_lookup(character)
    case character
      when "I" 
        1
      when "V"
        5
      when "X"
        10
      when 'L'
        50
      when "C"
        100
      when 'D'
        500
      when 'M'
        1000
    end
  end
end



describe RomanNumeral do
  it "to arabic" do
    RomanNumeral.to_arabic('MDL').should == 1550
    RomanNumeral.to_arabic('CCXCI').should == 291
  end
end

Thanks Jim for an awesome Vital Testing workshop!