class CursorPagination

    def _get_position_from_instance(self, instance, ordering):
        field_name = ordering[0].lstrip('-')
        if isinstance(instance, dict):
            attr = instance[field_name]
        else:
            attr = getattr(instance, field_name)
        return str(attr)

    def decode_cursor(self, request):
        """
        Given a request with a cursor, return a `Cursor` instance.
        """
        # Determine if we have a cursor, and if so then decode it.
        encoded = request.query_params.get(self.cursor_query_param)
        if encoded is None:
            return None

        try:
            querystring = b64decode(encoded.encode('ascii')).decode('ascii')
            tokens = parse.parse_qs(querystring, keep_blank_values=True)

            offset = tokens.get('o', ['0'])[0]
            offset = _positive_int(offset, cutoff=self.offset_cutoff)

            reverse = tokens.get('r', ['0'])[0]
            reverse = bool(int(reverse))

            position = tokens.get('p', [None])[0]
        except (TypeError, ValueError):
            raise NotFound(self.invalid_cursor_message)

        return Cursor(offset=offset, reverse=reverse, position=position)

    def encode_cursor(self, cursor):
        """
        Given a Cursor instance, return an url with encoded cursor.
        """
        tokens = {}
        if cursor.offset != 0:
            tokens['o'] = str(cursor.offset)
        if cursor.reverse:
            tokens['r'] = '1'
        if cursor.position is not None:
            tokens['p'] = cursor.position

        querystring = parse.urlencode(tokens, doseq=True)
        encoded = b64encode(querystring.encode('ascii')).decode('ascii')
        return replace_query_param(self.base_url, self.cursor_query_param, encoded)

    def get_html_context(self):
        return {
            'previous_url': self.get_previous_link(),
            'next_url': self.get_next_link()
        }

    def get_next_link(self):
        if not self.has_next:
            return None

        if self.page and self.cursor and self.cursor.reverse and self.cursor.offset != 0:
            # If we're reversing direction and we have an offset cursor
            # then we cannot use the first position we find as a marker.
            compare = self._get_position_from_instance(self.page[-1], self.ordering)
        else:
            compare = self.next_position
        offset = 0

        has_item_with_unique_position = False
        for item in reversed(self.page):
            position = self._get_position_from_instance(item, self.ordering)
            if position != compare:
                # The item in this position and the item following it
                # have different positions. We can use this position as
                # our marker.
                has_item_with_unique_position = True
                break

            # The item in this position has the same position as the item
            # following it, we can't use it as a marker position, so increment
            # the offset and keep seeking to the previous item.
            compare = position
            offset += 1

        if self.page and not has_item_with_unique_position:
            # There were no unique positions in the page.
            if not self.has_previous:
                # We are on the first page.
                # Our cursor will have an offset equal to the page size,
                # but no position to filter against yet.
                offset = self.page_size
                position = None
            elif self.cursor.reverse:
                # The change in direction will introduce a paging artifact,
                # where we end up skipping forward a few extra items.
                offset = 0
                position = self.previous_position
            else:
                # Use the position from the existing cursor and increment
                # it's offset by the page size.
                offset = self.cursor.offset + self.page_size
                position = self.previous_position

        if not self.page:
            position = self.next_position

        cursor = Cursor(offset=offset, reverse=False, position=position)
        return self.encode_cursor(cursor)

    def get_ordering(self, request, queryset, view):
        """
        Return a tuple of strings, that may be used in an `order_by` method.
        """
        ordering_filters = [
            filter_cls for filter_cls in getattr(view, 'filter_backends', [])
            if hasattr(filter_cls, 'get_ordering')
        ]

        if ordering_filters:
            # If a filter exists on the view that implements `get_ordering`
            # then we defer to that filter to determine the ordering.
            filter_cls = ordering_filters[0]
            filter_instance = filter_cls()
            ordering = filter_instance.get_ordering(request, queryset, view)
            assert ordering is not None, (
                'Using cursor pagination, but filter class {filter_cls} '
                'returned a `None` ordering.'.format(
                    filter_cls=filter_cls.__name__
                )
            )
        else:
            # The default case is to check for an `ordering` attribute
            # on this pagination instance.
            ordering = self.ordering
            assert ordering is not None, (
                'Using cursor pagination, but no ordering attribute was declared '
                'on the pagination class.'
            )
            assert '__' not in ordering, (
                'Cursor pagination does not support double underscore lookups '
                'for orderings. Orderings should be an unchanging, unique or '
                'nearly-unique field on the model, such as "-created" or "pk".'
            )

        assert isinstance(ordering, (str, list, tuple)), (
            'Invalid ordering. Expected string or tuple, but got {type}'.format(
                type=type(ordering).__name__
            )
        )

        if isinstance(ordering, str):
            return (ordering,)
        return tuple(ordering)

    def get_page_size(self, request):
        if self.page_size_query_param:
            try:
                return _positive_int(
                    request.query_params[self.page_size_query_param],
                    strict=True,
                    cutoff=self.max_page_size
                )
            except (KeyError, ValueError):
                pass

        return self.page_size

    def get_previous_link(self):
        if not self.has_previous:
            return None

        if self.page and self.cursor and not self.cursor.reverse and self.cursor.offset != 0:
            # If we're reversing direction and we have an offset cursor
            # then we cannot use the first position we find as a marker.
            compare = self._get_position_from_instance(self.page[0], self.ordering)
        else:
            compare = self.previous_position
        offset = 0

        has_item_with_unique_position = False
        for item in self.page:
            position = self._get_position_from_instance(item, self.ordering)
            if position != compare:
                # The item in this position and the item following it
                # have different positions. We can use this position as
                # our marker.
                has_item_with_unique_position = True
                break

            # The item in this position has the same position as the item
            # following it, we can't use it as a marker position, so increment
            # the offset and keep seeking to the previous item.
            compare = position
            offset += 1

        if self.page and not has_item_with_unique_position:
            # There were no unique positions in the page.
            if not self.has_next:
                # We are on the final page.
                # Our cursor will have an offset equal to the page size,
                # but no position to filter against yet.
                offset = self.page_size
                position = None
            elif self.cursor.reverse:
                # Use the position from the existing cursor and increment
                # it's offset by the page size.
                offset = self.cursor.offset + self.page_size
                position = self.next_position
            else:
                # The change in direction will introduce a paging artifact,
                # where we end up skipping back a few extra items.
                offset = 0
                position = self.next_position

        if not self.page:
            position = self.previous_position

        cursor = Cursor(offset=offset, reverse=True, position=position)
        return self.encode_cursor(cursor)

    def get_results(self, data):
        return data['results']