convert.rb

require 'number_converter'

ARGV.each do |input_number|
  result = NumberConverter.new(input_number).convert
  puts "#{input_number} = #{result}"
end

number_converter.rb

class NumberConverter
  attr_accessor :number, :type
  ROMAN_NUMERALS_VALUES = {
    'I' => 1,
    'V' => 5,
    'X' => 10,
    'L' => 50,
    'C' => 100,
    'D' => 500,
    'M' => 1000
  }

  SUBTRACTION_RULES = {
    'I' => ['V', 'X'],
    'X' => ['L', 'C'],
    'C' => ['D', 'M']
  }

  ARABIAN_REGEX = /\d+/
  ROMAN_REGEX = /[MDCLXVI]+/

  def initialize(input_number)
    @number = input_number.to_s
    if number.match(ARABIAN_REGEX)
      @type = :arabian
    elsif number.match(ROMAN_REGEX)
      @type = :roman
    else
      raise 'Incorrect input number'
    end
  end

  def convert
    case type
    when :arabian
      to_roman
    when :roman
      to_arabic
    end
  end

  def roman_numerals_extension
    @roman_numerals_extension ||= {}.tap do |extension_hash|
      SUBTRACTION_RULES.each do |subtrahend, rule|
        rule.each do |minuend|
          min = ROMAN_NUMERALS_VALUES[minuend]
          sub = ROMAN_NUMERALS_VALUES[subtrahend]
          extension_hash[subtrahend + minuend] = min - sub
        end
      end
    end
  end

  def to_arabic
    tmp_number = number.dup
    roman_numerals_extension.each do |extension, value|
      tmp_number.gsub!(extension, '+' + value.to_s)
    end
    ROMAN_NUMERALS_VALUES.each do |numeral, value|
      tmp_number.gsub!(numeral, '+' + value.to_s)
    end
    eval tmp_number
  end

  def to_roman
    decimal_number = number.to_i
    roman_numerals_extended = ROMAN_NUMERALS_VALUES.merge(roman_numerals_extension)
    sorted_numerals = roman_numerals_extended.keys.sort{
      |a,b| roman_numerals_extended[b] <=> roman_numerals_extended[a]
    }
    sorted_numerals.inject("") do |roman_number, numeral|
      value = roman_numerals_extended[numeral]
      while decimal_number - value >= 0
        roman_number << numeral
        decimal_number -= value
      end
      roman_number
    end
  end
end

number_converter_test.rb

require 'test/unit'
require 'number_converter'

class NumberConverterTest < Test::Unit::TestCase
  def test_should_convert_to_roman
    assert_equal 'III', NumberConverter.new(3).convert
    assert_equal 'IV', NumberConverter.new(4).convert
    assert_equal 'VIII',  NumberConverter.new(8).convert
    assert_equal 'XXIX', NumberConverter.new(29).convert
    assert_equal 'XXXVIII', NumberConverter.new(38).convert
    assert_equal 'MCMXCIX', NumberConverter.new(1999).convert
    assert_equal 'CCXCI', NumberConverter.new(291).convert
    assert_equal 'XXXI', NumberConverter.new(31).convert
    assert_equal 'CM',  NumberConverter.new(900).convert
    assert_equal 'MCMLIV',  NumberConverter.new(1954).convert
    assert_equal 'MMVI',  NumberConverter.new(2006).convert
  end

  def test_should_convert_to_arabic
    assert_equal 3, NumberConverter.new('III').convert
    assert_equal 4, NumberConverter.new('IV').convert
    assert_equal 8, NumberConverter.new('VIII').convert
    assert_equal 29, NumberConverter.new('XXIX').convert
    assert_equal 38, NumberConverter.new('XXXVIII').convert
    assert_equal 1999, NumberConverter.new('MCMXCIX').convert
    assert_equal 291, NumberConverter.new('CCXCI').convert
    assert_equal 31, NumberConverter.new('XXXI').convert
    assert_equal 900,  NumberConverter.new('CM').convert
    assert_equal 1954,  NumberConverter.new('MCMLIV').convert
    assert_equal 2006,  NumberConverter.new('MMVI').convert
  end
end