roman.rb

DIGIT_VALUES = {
  'I'  => 1,
  'IV' => 4,
  'V'  => 5,
  'IX' => 9,
  'X'  => 10,
  'XL' => 40,
  'L'  => 50,
  'XC' => 90,
  'C'  => 100,
  'CD' => 400,
  'D'  => 500,
  'CM' => 900,
  'M'  => 1000,
}

SORTED_DIGIT_VALUES = DIGIT_VALUES.sort_by { |_roman, arabic| arabic }.reverse

TOKENIZER = Regexp.union(*SORTED_DIGIT_VALUES.map(&:first))

def tokenize(roman)
  roman.scan TOKENIZER
end

def roman2arabic(roman)
  tokenize(roman).map { |r| DIGIT_VALUES[r] }.reduce(0, :+)
end

def arabic2roman(arabic)
  ''.tap { |roman|
    SORTED_DIGIT_VALUES.each do |symbol, value|
      while arabic >= value
        roman << symbol
        arabic -= value
      end
    end
  }
end

def multiply_roman(*args)
  arabic2roman args.map { |a| roman2arabic a }.reduce(1, :*)
end