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+/*
+Copyright 2017 - 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/**
+ * This is an internal module. See {@link SyncAccumulator} for the public class.
+ */
+
+import { logger } from "./logger";
+import { deepCopy, isSupportedReceiptType, MapWithDefault, recursiveMapToObject } from "./utils";
+import { IContent, IUnsigned } from "./models/event";
+import { IRoomSummary } from "./models/room-summary";
+import { EventType } from "./@types/event";
+import { MAIN_ROOM_TIMELINE, ReceiptContent, ReceiptType } from "./@types/read_receipts";
+import { UNREAD_THREAD_NOTIFICATIONS } from "./@types/sync";
+
+interface IOpts {
+    /**
+     * The ideal maximum number of timeline entries to keep in the sync response.
+     * This is best-effort, as clients do not always have a back-pagination token for each event,
+     * so it's possible there may be slightly *less* than this value. There will never be more.
+     * This cannot be 0 or else it makes it impossible to scroll back in a room.
+     * Default: 50.
+     */
+    maxTimelineEntries?: number;
+}
+
+export interface IMinimalEvent {
+    content: IContent;
+    type: EventType | string;
+    unsigned?: IUnsigned;
+}
+
+export interface IEphemeral {
+    events: IMinimalEvent[];
+}
+
+/* eslint-disable camelcase */
+interface UnreadNotificationCounts {
+    highlight_count?: number;
+    notification_count?: number;
+}
+
+export interface IRoomEvent extends IMinimalEvent {
+    event_id: string;
+    sender: string;
+    origin_server_ts: number;
+    /** @deprecated - legacy field */
+    age?: number;
+}
+
+export interface IStateEvent extends IRoomEvent {
+    prev_content?: IContent;
+    state_key: string;
+}
+
+interface IState {
+    events: IStateEvent[];
+}
+
+export interface ITimeline {
+    events: Array<IRoomEvent | IStateEvent>;
+    limited?: boolean;
+    prev_batch: string | null;
+}
+
+export interface IJoinedRoom {
+    "summary": IRoomSummary;
+    "state": IState;
+    "timeline": ITimeline;
+    "ephemeral": IEphemeral;
+    "account_data": IAccountData;
+    "unread_notifications": UnreadNotificationCounts;
+    "unread_thread_notifications"?: Record<string, UnreadNotificationCounts>;
+    "org.matrix.msc3773.unread_thread_notifications"?: Record<string, UnreadNotificationCounts>;
+}
+
+export interface IStrippedState {
+    content: IContent;
+    state_key: string;
+    type: EventType | string;
+    sender: string;
+}
+
+export interface IInviteState {
+    events: IStrippedState[];
+}
+
+export interface IInvitedRoom {
+    invite_state: IInviteState;
+}
+
+export interface ILeftRoom {
+    state: IState;
+    timeline: ITimeline;
+    account_data: IAccountData;
+}
+
+export interface IRooms {
+    [Category.Join]: Record<string, IJoinedRoom>;
+    [Category.Invite]: Record<string, IInvitedRoom>;
+    [Category.Leave]: Record<string, ILeftRoom>;
+}
+
+interface IPresence {
+    events: IMinimalEvent[];
+}
+
+interface IAccountData {
+    events: IMinimalEvent[];
+}
+
+export interface IToDeviceEvent {
+    content: IContent;
+    sender: string;
+    type: string;
+}
+
+interface IToDevice {
+    events: IToDeviceEvent[];
+}
+
+interface IDeviceLists {
+    changed?: string[];
+    left?: string[];
+}
+
+export interface ISyncResponse {
+    "next_batch": string;
+    "rooms": IRooms;
+    "presence"?: IPresence;
+    "account_data": IAccountData;
+    "to_device"?: IToDevice;
+    "device_lists"?: IDeviceLists;
+    "device_one_time_keys_count"?: Record<string, number>;
+
+    "device_unused_fallback_key_types"?: string[];
+    "org.matrix.msc2732.device_unused_fallback_key_types"?: string[];
+}
+/* eslint-enable camelcase */
+
+export enum Category {
+    Invite = "invite",
+    Leave = "leave",
+    Join = "join",
+}
+
+interface IRoom {
+    _currentState: { [eventType: string]: { [stateKey: string]: IStateEvent } };
+    _timeline: {
+        event: IRoomEvent | IStateEvent;
+        token: string | null;
+    }[];
+    _summary: Partial<IRoomSummary>;
+    _accountData: { [eventType: string]: IMinimalEvent };
+    _unreadNotifications: Partial<UnreadNotificationCounts>;
+    _unreadThreadNotifications?: Record<string, Partial<UnreadNotificationCounts>>;
+    _readReceipts: {
+        [userId: string]: {
+            data: IMinimalEvent;
+            type: ReceiptType;
+            eventId: string;
+        };
+    };
+    _threadReadReceipts: {
+        [threadId: string]: {
+            [userId: string]: {
+                data: IMinimalEvent;
+                type: ReceiptType;
+                eventId: string;
+            };
+        };
+    };
+}
+
+export interface ISyncData {
+    nextBatch: string;
+    accountData: IMinimalEvent[];
+    roomsData: IRooms;
+}
+
+type TaggedEvent = IRoomEvent & { _localTs?: number };
+
+function isTaggedEvent(event: IRoomEvent): event is TaggedEvent {
+    return "_localTs" in event && event["_localTs"] !== undefined;
+}
+
+/**
+ * The purpose of this class is to accumulate /sync responses such that a
+ * complete "initial" JSON response can be returned which accurately represents
+ * the sum total of the /sync responses accumulated to date. It only handles
+ * room data: that is, everything under the "rooms" top-level key.
+ *
+ * This class is used when persisting room data so a complete /sync response can
+ * be loaded from disk and incremental syncs can be performed on the server,
+ * rather than asking the server to do an initial sync on startup.
+ */
+export class SyncAccumulator {
+    private accountData: Record<string, IMinimalEvent> = {}; // $event_type: Object
+    private inviteRooms: Record<string, IInvitedRoom> = {}; // $roomId: { ... sync 'invite' json data ... }
+    private joinRooms: { [roomId: string]: IRoom } = {};
+    // the /sync token which corresponds to the last time rooms were
+    // accumulated. We remember this so that any caller can obtain a
+    // coherent /sync response and know at what point they should be
+    // streaming from without losing events.
+    private nextBatch: string | null = null;
+
+    public constructor(private readonly opts: IOpts = {}) {
+        this.opts.maxTimelineEntries = this.opts.maxTimelineEntries || 50;
+    }
+
+    public accumulate(syncResponse: ISyncResponse, fromDatabase = false): void {
+        this.accumulateRooms(syncResponse, fromDatabase);
+        this.accumulateAccountData(syncResponse);
+        this.nextBatch = syncResponse.next_batch;
+    }
+
+    private accumulateAccountData(syncResponse: ISyncResponse): void {
+        if (!syncResponse.account_data || !syncResponse.account_data.events) {
+            return;
+        }
+        // Clobbers based on event type.
+        syncResponse.account_data.events.forEach((e) => {
+            this.accountData[e.type] = e;
+        });
+    }
+
+    /**
+     * Accumulate incremental /sync room data.
+     * @param syncResponse - the complete /sync JSON
+     * @param fromDatabase - True if the sync response is one saved to the database
+     */
+    private accumulateRooms(syncResponse: ISyncResponse, fromDatabase = false): void {
+        if (!syncResponse.rooms) {
+            return;
+        }
+        if (syncResponse.rooms.invite) {
+            Object.keys(syncResponse.rooms.invite).forEach((roomId) => {
+                this.accumulateRoom(roomId, Category.Invite, syncResponse.rooms.invite[roomId], fromDatabase);
+            });
+        }
+        if (syncResponse.rooms.join) {
+            Object.keys(syncResponse.rooms.join).forEach((roomId) => {
+                this.accumulateRoom(roomId, Category.Join, syncResponse.rooms.join[roomId], fromDatabase);
+            });
+        }
+        if (syncResponse.rooms.leave) {
+            Object.keys(syncResponse.rooms.leave).forEach((roomId) => {
+                this.accumulateRoom(roomId, Category.Leave, syncResponse.rooms.leave[roomId], fromDatabase);
+            });
+        }
+    }
+
+    private accumulateRoom(roomId: string, category: Category.Invite, data: IInvitedRoom, fromDatabase: boolean): void;
+    private accumulateRoom(roomId: string, category: Category.Join, data: IJoinedRoom, fromDatabase: boolean): void;
+    private accumulateRoom(roomId: string, category: Category.Leave, data: ILeftRoom, fromDatabase: boolean): void;
+    private accumulateRoom(roomId: string, category: Category, data: any, fromDatabase = false): void {
+        // Valid /sync state transitions
+        //       +--------+ <======+            1: Accept an invite
+        //   +== | INVITE |        | (5)        2: Leave a room
+        //   |   +--------+ =====+ |            3: Join a public room previously
+        //   |(1)            (4) | |               left (handle as if new room)
+        //   V         (2)       V |            4: Reject an invite
+        // +------+ ========> +--------+         5: Invite to a room previously
+        // | JOIN |    (3)    | LEAVE* |            left (handle as if new room)
+        // +------+ <======== +--------+
+        //
+        // * equivalent to "no state"
+        switch (category) {
+            case Category.Invite: // (5)
+                this.accumulateInviteState(roomId, data as IInvitedRoom);
+                break;
+
+            case Category.Join:
+                if (this.inviteRooms[roomId]) {
+                    // (1)
+                    // was previously invite, now join. We expect /sync to give
+                    // the entire state and timeline on 'join', so delete previous
+                    // invite state
+                    delete this.inviteRooms[roomId];
+                }
+                // (3)
+                this.accumulateJoinState(roomId, data as IJoinedRoom, fromDatabase);
+                break;
+
+            case Category.Leave:
+                if (this.inviteRooms[roomId]) {
+                    // (4)
+                    delete this.inviteRooms[roomId];
+                } else {
+                    // (2)
+                    delete this.joinRooms[roomId];
+                }
+                break;
+
+            default:
+                logger.error("Unknown cateogory: ", category);
+        }
+    }
+
+    private accumulateInviteState(roomId: string, data: IInvitedRoom): void {
+        if (!data.invite_state || !data.invite_state.events) {
+            // no new data
+            return;
+        }
+        if (!this.inviteRooms[roomId]) {
+            this.inviteRooms[roomId] = {
+                invite_state: data.invite_state,
+            };
+            return;
+        }
+        // accumulate extra keys for invite->invite transitions
+        // clobber based on event type / state key
+        // We expect invite_state to be small, so just loop over the events
+        const currentData = this.inviteRooms[roomId];
+        data.invite_state.events.forEach((e) => {
+            let hasAdded = false;
+            for (let i = 0; i < currentData.invite_state.events.length; i++) {
+                const current = currentData.invite_state.events[i];
+                if (current.type === e.type && current.state_key == e.state_key) {
+                    currentData.invite_state.events[i] = e; // update
+                    hasAdded = true;
+                }
+            }
+            if (!hasAdded) {
+                currentData.invite_state.events.push(e);
+            }
+        });
+    }
+
+    // Accumulate timeline and state events in a room.
+    private accumulateJoinState(roomId: string, data: IJoinedRoom, fromDatabase = false): void {
+        // We expect this function to be called a lot (every /sync) so we want
+        // this to be fast. /sync stores events in an array but we often want
+        // to clobber based on type/state_key. Rather than convert arrays to
+        // maps all the time, just keep private maps which contain
+        // the actual current accumulated sync state, and array-ify it when
+        // getJSON() is called.
+
+        // State resolution:
+        // The 'state' key is the delta from the previous sync (or start of time
+        // if no token was supplied), to the START of the timeline. To obtain
+        // the current state, we need to "roll forward" state by reading the
+        // timeline. We want to store the current state so we can drop events
+        // out the end of the timeline based on opts.maxTimelineEntries.
+        //
+        //      'state'     'timeline'     current state
+        // |-------x<======================>x
+        //          T   I   M   E
+        //
+        // When getJSON() is called, we 'roll back' the current state by the
+        // number of entries in the timeline to work out what 'state' should be.
+
+        // Back-pagination:
+        // On an initial /sync, the server provides a back-pagination token for
+        // the start of the timeline. When /sync deltas come down, they also
+        // include back-pagination tokens for the start of the timeline. This
+        // means not all events in the timeline have back-pagination tokens, as
+        // it is only the ones at the START of the timeline which have them.
+        // In order for us to have a valid timeline (and back-pagination token
+        // to match), we need to make sure that when we remove old timeline
+        // events, that we roll forward to an event which has a back-pagination
+        // token. This means we can't keep a strict sliding-window based on
+        // opts.maxTimelineEntries, and we may have a few less. We should never
+        // have more though, provided that the /sync limit is less than or equal
+        // to opts.maxTimelineEntries.
+
+        if (!this.joinRooms[roomId]) {
+            // Create truly empty objects so event types of 'hasOwnProperty' and co
+            // don't cause this code to break.
+            this.joinRooms[roomId] = {
+                _currentState: Object.create(null),
+                _timeline: [],
+                _accountData: Object.create(null),
+                _unreadNotifications: {},
+                _unreadThreadNotifications: {},
+                _summary: {},
+                _readReceipts: {},
+                _threadReadReceipts: {},
+            };
+        }
+        const currentData = this.joinRooms[roomId];
+
+        if (data.account_data && data.account_data.events) {
+            // clobber based on type
+            data.account_data.events.forEach((e) => {
+                currentData._accountData[e.type] = e;
+            });
+        }
+
+        // these probably clobber, spec is unclear.
+        if (data.unread_notifications) {
+            currentData._unreadNotifications = data.unread_notifications;
+        }
+        currentData._unreadThreadNotifications =
+            data[UNREAD_THREAD_NOTIFICATIONS.stable!] ?? data[UNREAD_THREAD_NOTIFICATIONS.unstable!] ?? undefined;
+
+        if (data.summary) {
+            const HEROES_KEY = "m.heroes";
+            const INVITED_COUNT_KEY = "m.invited_member_count";
+            const JOINED_COUNT_KEY = "m.joined_member_count";
+
+            const acc = currentData._summary;
+            const sum = data.summary;
+            acc[HEROES_KEY] = sum[HEROES_KEY] || acc[HEROES_KEY];
+            acc[JOINED_COUNT_KEY] = sum[JOINED_COUNT_KEY] || acc[JOINED_COUNT_KEY];
+            acc[INVITED_COUNT_KEY] = sum[INVITED_COUNT_KEY] || acc[INVITED_COUNT_KEY];
+        }
+
+        data.ephemeral?.events?.forEach((e) => {
+            // We purposefully do not persist m.typing events.
+            // Technically you could refresh a browser before the timer on a
+            // typing event is up, so it'll look like you aren't typing when
+            // you really still are. However, the alternative is worse. If
+            // we do persist typing events, it will look like people are
+            // typing forever until someone really does start typing (which
+            // will prompt Synapse to send down an actual m.typing event to
+            // clobber the one we persisted).
+            if (e.type !== EventType.Receipt || !e.content) {
+                // This means we'll drop unknown ephemeral events but that
+                // seems okay.
+                return;
+            }
+            // Handle m.receipt events. They clobber based on:
+            //   (user_id, receipt_type)
+            // but they are keyed in the event as:
+            //   content:{ $event_id: { $receipt_type: { $user_id: {json} }}}
+            // so store them in the former so we can accumulate receipt deltas
+            // quickly and efficiently (we expect a lot of them). Fold the
+            // receipt type into the key name since we only have 1 at the
+            // moment (m.read) and nested JSON objects are slower and more
+            // of a hassle to work with. We'll inflate this back out when
+            // getJSON() is called.
+            Object.keys(e.content).forEach((eventId) => {
+                Object.entries<ReceiptContent>(e.content[eventId]).forEach(([key, value]) => {
+                    if (!isSupportedReceiptType(key)) return;
+
+                    for (const userId of Object.keys(value)) {
+                        const data = e.content[eventId][key][userId];
+
+                        const receipt = {
+                            data: e.content[eventId][key][userId],
+                            type: key as ReceiptType,
+                            eventId: eventId,
+                        };
+
+                        if (!data.thread_id || data.thread_id === MAIN_ROOM_TIMELINE) {
+                            currentData._readReceipts[userId] = receipt;
+                        } else {
+                            currentData._threadReadReceipts = {
+                                ...currentData._threadReadReceipts,
+                                [data.thread_id]: {
+                                    ...(currentData._threadReadReceipts[data.thread_id] ?? {}),
+                                    [userId]: receipt,
+                                },
+                            };
+                        }
+                    }
+                });
+            });
+        });
+
+        // if we got a limited sync, we need to remove all timeline entries or else
+        // we will have gaps in the timeline.
+        if (data.timeline && data.timeline.limited) {
+            currentData._timeline = [];
+        }
+
+        // Work out the current state. The deltas need to be applied in the order:
+        // - existing state which didn't come down /sync.
+        // - State events under the 'state' key.
+        // - State events in the 'timeline'.
+        data.state?.events?.forEach((e) => {
+            setState(currentData._currentState, e);
+        });
+        data.timeline?.events?.forEach((e, index) => {
+            // this nops if 'e' isn't a state event
+            setState(currentData._currentState, e);
+            // append the event to the timeline. The back-pagination token
+            // corresponds to the first event in the timeline
+            let transformedEvent: TaggedEvent;
+            if (!fromDatabase) {
+                transformedEvent = Object.assign({}, e);
+                if (transformedEvent.unsigned !== undefined) {
+                    transformedEvent.unsigned = Object.assign({}, transformedEvent.unsigned);
+                }
+                const age = e.unsigned ? e.unsigned.age : e.age;
+                if (age !== undefined) transformedEvent._localTs = Date.now() - age;
+            } else {
+                transformedEvent = e;
+            }
+
+            currentData._timeline.push({
+                event: transformedEvent,
+                token: index === 0 ? data.timeline.prev_batch ?? null : null,
+            });
+        });
+
+        // attempt to prune the timeline by jumping between events which have
+        // pagination tokens.
+        if (currentData._timeline.length > this.opts.maxTimelineEntries!) {
+            const startIndex = currentData._timeline.length - this.opts.maxTimelineEntries!;
+            for (let i = startIndex; i < currentData._timeline.length; i++) {
+                if (currentData._timeline[i].token) {
+                    // keep all events after this, including this one
+                    currentData._timeline = currentData._timeline.slice(i, currentData._timeline.length);
+                    break;
+                }
+            }
+        }
+    }
+
+    /**
+     * Return everything under the 'rooms' key from a /sync response which
+     * represents all room data that should be stored. This should be paired
+     * with the sync token which represents the most recent /sync response
+     * provided to accumulate().
+     * @param forDatabase - True to generate a sync to be saved to storage
+     * @returns An object with a "nextBatch", "roomsData" and "accountData"
+     * keys.
+     * The "nextBatch" key is a string which represents at what point in the
+     * /sync stream the accumulator reached. This token should be used when
+     * restarting a /sync stream at startup. Failure to do so can lead to missing
+     * events. The "roomsData" key is an Object which represents the entire
+     * /sync response from the 'rooms' key onwards. The "accountData" key is
+     * a list of raw events which represent global account data.
+     */
+    public getJSON(forDatabase = false): ISyncData {
+        const data: IRooms = {
+            join: {},
+            invite: {},
+            // always empty. This is set by /sync when a room was previously
+            // in 'invite' or 'join'. On fresh startup, the client won't know
+            // about any previous room being in 'invite' or 'join' so we can
+            // just omit mentioning it at all, even if it has previously come
+            // down /sync.
+            // The notable exception is when a client is kicked or banned:
+            // we may want to hold onto that room so the client can clearly see
+            // why their room has disappeared. We don't persist it though because
+            // it is unclear *when* we can safely remove the room from the DB.
+            // Instead, we assume that if you're loading from the DB, you've
+            // refreshed the page, which means you've seen the kick/ban already.
+            leave: {},
+        };
+        Object.keys(this.inviteRooms).forEach((roomId) => {
+            data.invite[roomId] = this.inviteRooms[roomId];
+        });
+        Object.keys(this.joinRooms).forEach((roomId) => {
+            const roomData = this.joinRooms[roomId];
+            const roomJson: IJoinedRoom = {
+                ephemeral: { events: [] },
+                account_data: { events: [] },
+                state: { events: [] },
+                timeline: {
+                    events: [],
+                    prev_batch: null,
+                },
+                unread_notifications: roomData._unreadNotifications,
+                unread_thread_notifications: roomData._unreadThreadNotifications,
+                summary: roomData._summary as IRoomSummary,
+            };
+            // Add account data
+            Object.keys(roomData._accountData).forEach((evType) => {
+                roomJson.account_data.events.push(roomData._accountData[evType]);
+            });
+
+            // Add receipt data
+            const receiptEvent = {
+                type: EventType.Receipt,
+                room_id: roomId,
+                content: {
+                    // $event_id: { "m.read": { $user_id: $json } }
+                } as IContent,
+            };
+
+            const receiptEventContent: MapWithDefault<
+                string,
+                MapWithDefault<ReceiptType, Map<string, object>>
+            > = new MapWithDefault(() => new MapWithDefault(() => new Map()));
+
+            for (const [userId, receiptData] of Object.entries(roomData._readReceipts)) {
+                receiptEventContent
+                    .getOrCreate(receiptData.eventId)
+                    .getOrCreate(receiptData.type)
+                    .set(userId, receiptData.data);
+            }
+
+            for (const threadReceipts of Object.values(roomData._threadReadReceipts)) {
+                for (const [userId, receiptData] of Object.entries(threadReceipts)) {
+                    receiptEventContent
+                        .getOrCreate(receiptData.eventId)
+                        .getOrCreate(receiptData.type)
+                        .set(userId, receiptData.data);
+                }
+            }
+
+            receiptEvent.content = recursiveMapToObject(receiptEventContent);
+
+            // add only if we have some receipt data
+            if (receiptEventContent.size > 0) {
+                roomJson.ephemeral.events.push(receiptEvent as IMinimalEvent);
+            }
+
+            // Add timeline data
+            roomData._timeline.forEach((msgData) => {
+                if (!roomJson.timeline.prev_batch) {
+                    // the first event we add to the timeline MUST match up to
+                    // the prev_batch token.
+                    if (!msgData.token) {
+                        return; // this shouldn't happen as we prune constantly.
+                    }
+                    roomJson.timeline.prev_batch = msgData.token;
+                }
+
+                let transformedEvent: (IRoomEvent | IStateEvent) & { _localTs?: number };
+                if (!forDatabase && isTaggedEvent(msgData.event)) {
+                    // This means we have to copy each event, so we can fix it up to
+                    // set a correct 'age' parameter whilst keeping the local timestamp
+                    // on our stored event. If this turns out to be a bottleneck, it could
+                    // be optimised either by doing this in the main process after the data
+                    // has been structured-cloned to go between the worker & main process,
+                    // or special-casing data from saved syncs to read the local timestamp
+                    // directly rather than turning it into age to then immediately be
+                    // transformed back again into a local timestamp.
+                    transformedEvent = Object.assign({}, msgData.event);
+                    if (transformedEvent.unsigned !== undefined) {
+                        transformedEvent.unsigned = Object.assign({}, transformedEvent.unsigned);
+                    }
+                    delete transformedEvent._localTs;
+                    transformedEvent.unsigned = transformedEvent.unsigned || {};
+                    transformedEvent.unsigned.age = Date.now() - msgData.event._localTs!;
+                } else {
+                    transformedEvent = msgData.event;
+                }
+                roomJson.timeline.events.push(transformedEvent);
+            });
+
+            // Add state data: roll back current state to the start of timeline,
+            // by "reverse clobbering" from the end of the timeline to the start.
+            // Convert maps back into arrays.
+            const rollBackState = Object.create(null);
+            for (let i = roomJson.timeline.events.length - 1; i >= 0; i--) {
+                const timelineEvent = roomJson.timeline.events[i];
+                if (
+                    (timelineEvent as IStateEvent).state_key === null ||
+                    (timelineEvent as IStateEvent).state_key === undefined
+                ) {
+                    continue; // not a state event
+                }
+                // since we're going back in time, we need to use the previous
+                // state value else we'll break causality. We don't have the
+                // complete previous state event, so we need to create one.
+                const prevStateEvent = deepCopy(timelineEvent);
+                if (prevStateEvent.unsigned) {
+                    if (prevStateEvent.unsigned.prev_content) {
+                        prevStateEvent.content = prevStateEvent.unsigned.prev_content;
+                    }
+                    if (prevStateEvent.unsigned.prev_sender) {
+                        prevStateEvent.sender = prevStateEvent.unsigned.prev_sender;
+                    }
+                }
+                setState(rollBackState, prevStateEvent);
+            }
+            Object.keys(roomData._currentState).forEach((evType) => {
+                Object.keys(roomData._currentState[evType]).forEach((stateKey) => {
+                    let ev = roomData._currentState[evType][stateKey];
+                    if (rollBackState[evType] && rollBackState[evType][stateKey]) {
+                        // use the reverse clobbered event instead.
+                        ev = rollBackState[evType][stateKey];
+                    }
+                    roomJson.state.events.push(ev);
+                });
+            });
+            data.join[roomId] = roomJson;
+        });
+
+        // Add account data
+        const accData: IMinimalEvent[] = [];
+        Object.keys(this.accountData).forEach((evType) => {
+            accData.push(this.accountData[evType]);
+        });
+
+        return {
+            nextBatch: this.nextBatch!,
+            roomsData: data,
+            accountData: accData,
+        };
+    }
+
+    public getNextBatchToken(): string {
+        return this.nextBatch!;
+    }
+}
+
+function setState(eventMap: Record<string, Record<string, IStateEvent>>, event: IRoomEvent | IStateEvent): void {
+    if ((event as IStateEvent).state_key === null || (event as IStateEvent).state_key === undefined || !event.type) {
+        return;
+    }
+    if (!eventMap[event.type]) {
+        eventMap[event.type] = Object.create(null);
+    }
+    eventMap[event.type][(event as IStateEvent).state_key] = event as IStateEvent;
+}