Convert roman numerals into arabic numerals in place

roman_numerals.rb

# frozen_string_literal: true

# Enhance string class with roman numeral conversion methods
# To use, put `using RomanNumerals`
# @see https://docs.ruby-lang.org/en/master/syntax/refinements_rdoc.html
module RomanNumerals
  refine String do
    # Convert a roman numeral string into an arabic numeral
    def to_arabic(zero_pad: 0)
      result = 0
      str = self
      roman_mapping.each_value do |roman|
        while str.start_with?(roman)
          result += roman_mapping.invert[roman]
          str = str.slice(roman.length, str.length)
        end
      end
      result.zero? ? '' : result.to_s.rjust(zero_pad, '0')
    end

    # Find and replace roman numerals in a string with (zero-padded) arabic numerals
    def convert_roman_in_place(zero_pad: 0)
      gsub(/\bM{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})\b/m) { |num| num.to_arabic(zero_pad:) }
    end

    private

    # Roman numeral values
    def roman_mapping
      {
        '1000': 'M',
        '900': 'CM',
        '500': 'D',
        '400': 'CD',
        '100': 'C',
        '90': 'XC',
        '50': 'L',
        '40': 'XL',
        '10': 'X',
        '9': 'IX',
        '5': 'V',
        '4': 'IV',
        '1': 'I'
      }
    end
  end
end