class ActiveSupport::Duration::ISO8601Parser

def finished?
  scanner.eos?
end

def initialize(string)
  @scanner = StringScanner.new(string)
  @parts = {}
  @mode = :start
  @sign = 1
end

def number
  PERIOD_OR_COMMA.match?(scanner[1]) ? scanner[1].tr(COMMA, PERIOD).to_f : scanner[1].to_i
end

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

def raise_parsing_error(reason = nil)
  raise ParsingError, "Invalid ISO 8601 duration: #{scanner.string.inspect} #{reason}".strip
end

def scan(pattern)
  scanner.scan(pattern)
end

def validate!
  raise_parsing_error("is empty duration") if parts.empty?
  # Mixing any of Y, M, D with W is invalid.
  if parts.key?(:weeks) && (parts.keys & DATE_COMPONENTS).any?
    raise_parsing_error("mixing weeks with other date parts not allowed")
  end
  # Specifying an empty T part is invalid.
  if mode == :time && (parts.keys & TIME_COMPONENTS).empty?
    raise_parsing_error("time part marker is present but time part is empty")
  end
  fractions = parts.values.reject(&:zero?).select { |a| (a % 1) != 0 }
  unless fractions.empty? || (fractions.size == 1 && fractions.last == @parts.values.reject(&:zero?).last)
    raise_parsing_error "(only last part can be fractional)"
  end
  true
end