class ActiveSupport::Duration::ISO8601Parser

def parse!
  while !finished?
    case mode
    when :start
      if scan(SIGN_MARKER)
        self.sign = (scanner.matched == "-") ? -1 : 1
        self.mode = :sign
      else
        raise_parsing_error
      end
    when :sign
      if scan(DATE_MARKER)
        self.mode = :date
      else
        raise_parsing_error
      end
    when :date
      if scan(TIME_MARKER)
        self.mode = :time
      elsif scan(DATE_COMPONENT)
        parts[DATE_TO_PART[scanner[2]]] = number * sign
      else
        raise_parsing_error
      end
    when :time
      if scan(TIME_COMPONENT)
        parts[TIME_TO_PART[scanner[2]]] = number * sign
      else
        raise_parsing_error
      end
    end
  end
  validate!
  parts
end