roman_number.rb

class RomanNumber
  ROMAN_NUMBER = { I: 1,
                   V: 5,
                   X: 10,
                   L: 50,
                   C: 100,
                   D: 500,
                   M: 1000 }
  
  def initialize(roman)
    @roman = roman
    fail(NotValidRomanNumber, 'String is not a valid roman number') unless valid?
    @syms = @roman.split('').collect(&:to_sym)
  end
  
  def valid?
    @roman =~ /^M{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$/
  end
  
  def to_integer
    total = 0
    @syms.each_with_index do |sym, index|
      @current_sym = sym
      @index = index
      if sym != @syms.last && next_number_greater?
        total -= convert_to_integer
      else
        total += convert_to_integer
      end
    end
    total
  end
  
  private
  #converts the single roman symbol to integer
  def convert_to_integer(sym = @current_sym)
    ROMAN_NUMBER[sym]
  end

  def next_number_greater?(index=@index)
    @grtr = convert_to_integer < convert_to_integer(next_symbol(index))
  end
  
  def next_symbol(index)
    @syms[index + 1]
  end
end

class NotValidRomanNumber < StandardError; end

roman_number_spec.rb

require './roman_number'
require 'test/unit'
class TestRoman < Test::Unit::TestCase
  def test_single_roman_number
    assert_equal RomanNumber.new('I').to_integer, 1
    assert_equal RomanNumber.new('V').to_integer, 5
    assert_equal RomanNumber.new('X').to_integer, 10
    assert_equal RomanNumber.new('L').to_integer, 50
    assert_equal RomanNumber.new('C').to_integer, 100
    assert_equal RomanNumber.new('D').to_integer, 500
    assert_equal RomanNumber.new('M').to_integer, 1000
  end
  
  def test_multiple_identical
    assert_equal RomanNumber.new('II').to_integer, 2
    assert_equal RomanNumber.new('III').to_integer, 3
    assert_equal RomanNumber.new('XX').to_integer, 20
    assert_equal RomanNumber.new('CC').to_integer, 200
    assert_equal RomanNumber.new('MMM').to_integer, 3000
  end

  def test_multiple_different
    assert_equal RomanNumber.new('IV').to_integer, 4
    assert_equal RomanNumber.new('XXXI').to_integer, 31
    assert_equal RomanNumber.new('MCMXCVIII').to_integer, 1998
    assert_equal RomanNumber.new('MMMDCCCLXXXVIII').to_integer, 3888
  end

  def test_invalid
    assert_raise(NotValidRomanNumber, 'String is not a valid roman number') do
      RomanNumber.new('232').to_integer
    end
    assert_raise(NotValidRomanNumber, 'String is not a valid roman number') do
      RomanNumber.new('KJHDHKDH').to_integer
    end
    assert_raise(NotValidRomanNumber, 'String is not a valid roman number') do 
      RomanNumber.new('MMMMMM').to_integer
    end
    assert_raise(NotValidRomanNumber, 'String is not a valid roman number') do
      RomanNumber.new('IIIII').to_integer
    end
  end

end