module CousinRoman
  module Roman
    extend self

    def valid?(number)
      matches = number.match roman_regex
      matches and !matches[0].empty?
    end

    # Convert Roman +number+ represented as a string to an Integer.
    #
    # It is essential that +numer+ is a valid Roman numeral.
    #
    # The steps are:
    #
    # 1. Replace factors with their numeric values
    # starting from subtractive factors (cause they are compound).
    # 2. Sum this numeric values to get the final answer
    def convert(number)
      intermediate = number.dup.downcase
      compound = braketize_value_of SUBTRACTIVES
      singular = braketize_value_of ONES.merge(FIVES)

      [compound, singular].each { |factors| factors.each { |literal, val| intermediate.gsub!(literal, val) } }

      intermediate.scan(/\((\d*)\)/).reduce(0) do |sum, term|
        sum + term.first.to_i
      end
    end

    def to_arabian(number)
      clean = number.strip
      convert clean if valid? clean
    end

    def to_arabian!(number)
      clean = number.strip
      valid? clean or raise TypeError, 'not a valid roman number'
      convert clean
    end

    def roman_regex
      /^(M{0,3})(D?C{0,3}|C[DM])(L?X{0,3}|X[LC])(V?I{0,3}|I[VX])$/i
    end

    private

    # Make hash of the following structure {'i' => '(1)', 'v' => '(5)'...}
    def braketize_value_of(factors)
      Hash[factors.map { |literal, val| [literal, "(#{val})"] } ]
    end
  end
end