Calculates the average document length for this index

def _calculate_average_field_lengths(self):
        """Calculates the average document length for this index"""
        accumulator = defaultdict(int)
        documents_with_field = defaultdict(int)

        for field_ref, length in self.field_lengths.items():
            _field_ref = FieldRef.from_string(field_ref)
            field = _field_ref.field_name

            documents_with_field[field] += 1
            accumulator[field] += length

        for field_name in self._fields:
            accumulator[field_name] /= documents_with_field[field_name]

        self.average_field_length = accumulator