Ruby Roman numeral converter.

roman.rb

require "test/unit"

InvalidRomanChars = Class.new(StandardError)
OutOfRange        = Class.new(StandardError)
InvalidInteger    = Class.new(StandardError)

class Roman
  I,IV,V,IX,X,XL,L,XC,C,CD,D,CM,M = 1,4,5,9,10,40,50,90,100,400,500,900,1000
  LETTERS = %w(IV V IX I XL L XC X CD D CM C M)
  
  def initialize(value)
    @value = value
  end
  
  def to_roman
    @value = @value.to_i
    raise InvalidInteger.new('Given value should be integer and greater than zero.') if @value.zero? 
    raise OutOfRange.new('Given value is greater than 9999') if @value > 9999
    roman = ''
    until @value.zero? 
      int = highest_integer
      roman << correspondind_roman_letter(int).to_s
      @value -= int
    end 
    roman
  end 
  
  def to_number
    @value.strip!
    return 0 if @value.size.zero? 
    @value.upcase!
    sum = 0

    raise InvalidRomanChars if (@value.chars - LETTERS).any?
    
    until @value.empty?
      LETTERS.each do |letter|
        if @value.start_with?(letter)
          sum += self.class.const_get(letter)
          @value[0...letter.size] = ''
          break
        else 
        end
      end
    end
    sum 
  end
  
  private 
  
  def highest_integer
    digits = @value.digits
    digit = digits.last
    2.upto(digits.size) do
      digit *= 10
    end
    
    if correspondind_roman_letter(digit)
      digit 
    else 
      found = false 
      digit.downto(1) do |i|
        if correspondind_roman_letter(i)
          digit = i
          found = true 
          break 
        end
      end
      found ? digit : digit.digits.last
    end 
  end
  
  def correspondind_roman_letter(int)
    self.class.constants.detect do |cons|
      self.class.const_get(cons) == int 
    end
  end 
  
  class << self 
    def to_number(roman_str)
      Roman.new(roman_str).to_number
    end
    
    def to_roman(int)
      Roman.new(int).to_roman
    end
  end
end


class TestRomanClass < Test::Unit::TestCase
  
  def test_to_roman
    assert_equal('I', Roman.to_roman(1))
    assert_equal('X', Roman.to_roman(10))
    assert_equal('XX', Roman.to_roman(20))
    assert_equal('XIX', Roman.to_roman(19))
    assert_equal('MCMXC', Roman.to_roman(1990))
    assert_equal('MMVIII', Roman.to_roman(2008))
    assert_equal('MDCLXVI', Roman.to_roman(1666))
    assert_equal('MMMMMMMI', Roman.to_roman(7001))
  end 
  
  def test_to_roman_with_string_integer
    assert_equal('I', Roman.to_roman('1'))
    assert_equal('X', Roman.to_roman('10'))
    assert_equal('XX', Roman.to_roman('20'))
    assert_equal('XIX', Roman.to_roman('19'))
    assert_equal('MCMXC', Roman.to_roman('1990'))
    assert_equal('MMVIII', Roman.to_roman('2008'))
    assert_equal('MDCLXVI', Roman.to_roman('1666'))
    assert_equal('MMMMMMMI', Roman.to_roman('7001'))
  end
  
  def test_to_roman_with_zero
    assert_raise(InvalidInteger) do 
      Roman.to_roman('0')
    end
  end
  
  def test_to_roman_with_string_litterals
    assert_raise(InvalidInteger) do 
      Roman.to_roman('test')
    end
  end 
  
  def test_to_roman_out_of_range
    assert_raise(OutOfRange) do 
      Roman.to_roman(10_000_00)
    end
  end
  
  def test_to_number
    assert_equal(0, Roman.to_number(''))
    assert_equal(10, Roman.to_number('X'))
    assert_equal(20, Roman.to_number('XX'))
    assert_equal(19, Roman.to_number('XIX'))
    assert_equal(1990, Roman.to_number('MCMXC'))
    assert_equal(2008, Roman.to_number('MMVIII'))
    assert_equal(1666, Roman.to_number('MDCLXVI'))
  end
  
  def test_to_number_when_invalid_roman_chars
    assert_raise(InvalidRomanChars) do 
      Roman.to_number('invalid')
      Roman.to_number('1234')
      Roman.to_number('xxxa')
    end
  end
  
  def test_to_numeber_with_lowercase
    assert_equal(10, Roman.to_number('x'))
    assert_equal(20, Roman.to_number('xx'))
    assert_equal(19, Roman.to_number('xix'))
    assert_equal(1990, Roman.to_number('mcmxc'))
    assert_equal(2008, Roman.to_number('mmviii'))
    assert_equal(1666, Roman.to_number('mdclxvi'))
  end 
end

Ruby Roman numeral converter either from Roman letters to integer or from integers to Roman letters.

Usage:

Roman.to_number('XX') # => 20
Roman.to_roman(20) # => XX