roman_numeral.rb

class Integer
  def to_roman
    RomanNumeral.for(self)
  end
end

class RomanNumeral
  ARABIC_ROMAN = {
    1000 => 'M',
    500 => 'D',
    100 => 'C',
    50 => 'L',
    10 => 'X',
    5 => 'V',
    1 => 'I'
  }

  def self.for(arabic_num)
    self.new(arabic_num).to_s
  end

  attr_reader :arabic_num
  def initialize(arabic_num)
    @arabic_num = arabic_num
  end

  def to_s
    result = ''.tap do |s|
      num = arabic_num
      loop do
        break if num == 0
        next_arabic = next_arabic_num(num)
        s << ARABIC_ROMAN[next_arabic]
        num -= next_arabic
      end
    end
    transform_roman(result)
  end

  def next_arabic_num(num)
    ARABIC_ROMAN.keys.select { |n| n <= num }.max
  end

  def transform_roman(roman_numeral)
    roman_numeral.tap do |s|
      transformations = {
        'DCCCC' => 'CM', # 900
        'CCCC' => 'CD', # 400
        'LXXXX' => 'XC', # 90
        'XXXX' => 'XL', # 40
        'VIIII' => 'IX', # 9
        'IIII' => 'IV' # 4
      }.each { |from, to| s.gsub!(from, to) }
    end
  end
end