My implementation of the roman numerals kata (just the roman to arabic part).

roman_numerals_kata.rb

# (Roman to Arabic only)

class RomanNumerals
  def initialize
    @numerals = {
      "I" => 1,
      "V" => 5,
      "X" => 10,
      "L" => 50,
      "C" => 100,
      :eol => 0 }
  end
  def convert(value)
    sum = 0
    glyphs = value.split('').push(:eol)
    previous_value = @numerals[glyphs.shift]
    glyphs.each do |glyph|
      this_value = @numerals[glyph]
      if previous_value >= this_value then
        sum += previous_value
        previous_value = this_value
      else
        previous_value = this_value - previous_value
      end
    end
    sum
  end
end

describe RomanNumerals do
  before :each do
    @rn = RomanNumerals.new
  end
  it 'should have a convert method' do
    @rn.should respond_to(:convert)
  end
  describe 'converting Roman to Arabic' do
    it 'should, given "I", return 1' do
      @rn.convert("I").should == 1
    end
    it 'should, given "II", return 2' do
      @rn.convert("II").should == 2
    end
    it 'should, given "III", return 3' do
      @rn.convert("III").should == 3
    end
    it 'should, given "IV", return 4' do
      @rn.convert("IV").should == 4
    end
    it 'should, given "V", return 5' do
      @rn.convert("V").should == 5
    end
    it 'should, given "VI", return 6' do
      @rn.convert("VI").should == 6
    end
    it 'should, given "VII", return 7' do
      @rn.convert("VII").should == 7
    end
    it 'should, given "VIII", return 8' do
      @rn.convert("VIII").should == 8
    end
    it 'should, given "IX", return 9' do
      @rn.convert("IX").should == 9
    end
    it 'should convert X to 10' do
      @rn.convert("X").should == 10
    end
    it 'should convert XIV to 14' do
      @rn.convert("XIV").should == 14
    end
    it 'should convert XLII to 42' do
      @rn.convert("XLII").should == 42
    end
    it 'should convert XCIX to 99' do
      @rn.convert("XCIX").should == 99
    end
    it 'should convert CCCXCI to 391' do
      @rn.convert("CCCXCI").should == 391
    end
  end
end

Yeah, I started out with "it 'should return Y, given X" which is just all sorts of backwards. I went to the "given X, return Y" format because I wanted the test to describe a basic left-to-right "from -> to" sequence. In later tests I switched to "should convert X to Y" which I actually like better because it has the name of the method in it, and it leaves the "from" closer to the "to".