Room List - Store sorted rooms in skip list (#29345)

* Implement a skip list for storing rooms

This data structure stores rooms in a given sorted order and allows for
very fast insertions and deletions.

* Export function to get last timestamp of room

* Write tests for the skip list

* Implement enough of the new store to get a list of rooms

* Make it possible to swap sorting algorithm

* Fix comment

* Don't attach to window object

We don't want the store to be created if the labs flag is off

* Remove the store class

Probably best to include this PR with the minimal vm implmentation

src/stores/room-list-v3/skip-list/Level.ts

@@ -0,0 +1,114 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+import type { RoomNode } from "./RoomNode";
+import { shouldPromote } from "./utils";
+
+/**
+ * Represents one level of the skip list
+ */
+export class Level {
+    public head?: RoomNode;
+    private current?: RoomNode;
+    private _size: number = 0;
+
+    /**
+     * The number of elements in this level
+     */
+    public get size(): number {
+        return this._size;
+    }
+
+    public constructor(public readonly level: number) {}
+
+    /**
+     * Insert node after current
+     */
+    public setNext(node: RoomNode): void {
+        if (!this.head) this.head = node;
+        if (!this.current) {
+            this.current = node;
+        } else {
+            node.previous[this.level] = this.current;
+            this.current.next[this.level] = node;
+            this.current = node;
+        }
+        this._size++;
+    }
+
+    /**
+     * Iterate through the elements in this level and create
+     * a new level above this level by probabilistically determining
+     * whether a given element must be promoted to the new level.
+     */
+    public generateNextLevel(): Level {
+        const nextLevelSentinel = new Level(this.level + 1);
+        let current = this.head;
+        while (current) {
+            if (shouldPromote()) {
+                nextLevelSentinel.setNext(current);
+            }
+            current = current.next[this.level];
+        }
+        return nextLevelSentinel;
+    }
+
+    /**
+     * Removes a given node from this level.
+     * Does nothing if the given node is not present in this level.
+     */
+    public removeNode(node: RoomNode): void {
+        // Let's first see if this node is even in this level
+        const nodeInThisLevel = this.head === node || node.previous[this.level];
+        if (!nodeInThisLevel) {
+            // This node is not in this sentinel level, so nothing to do.
+            return;
+        }
+        const prev = node.previous[this.level];
+        if (prev) {
+            const nextNode = node.next[this.level];
+            prev.next[this.level] = nextNode;
+            if (nextNode) nextNode.previous[this.level] = prev;
+        } else {
+            // This node was the head since it has no back links!
+            // so update the head.
+            const next = node.next[this.level];
+            this.head = next;
+            if (next) next.previous[this.level] = node.previous[this.level];
+        }
+        this._size--;
+    }
+
+    /**
+     * Put newNode after node in this level. No checks are done to ensure
+     * that node is actually present in this level.
+     */
+    public insertAfter(node: RoomNode, newNode: RoomNode): void {
+        const level = this.level;
+        const nextNode = node.next[level];
+        if (nextNode) {
+            newNode.next[level] = nextNode;
+            nextNode.previous[level] = newNode;
+        }
+        node.next[level] = newNode;
+        newNode.previous[level] = node;
+        this._size++;
+    }
+
+    /**
+     *  Insert a given node at the head of this level.
+     */
+    public insertAtHead(newNode: RoomNode): void {
+        const existingNode = this.head;
+        this.head = newNode;
+        if (existingNode) {
+            newNode.next[this.level] = existingNode;
+            existingNode.previous[this.level] = newNode;
+        }
+        this._size++;
+    }
+}

src/stores/room-list-v3/skip-list/RoomNode.ts

@@ -0,0 +1,29 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+import type { Room } from "matrix-js-sdk/src/matrix";
+
+/**
+ * Room skip list stores room nodes.
+ * These hold the actual room object and provides references to other nodes
+ * in different levels.
+ */
+export class RoomNode {
+    public constructor(public readonly room: Room) {}
+
+    /**
+     * This array holds references to the next node in a given level.
+     * eg: next[i] gives the next room node from this room node in level i.
+     */
+    public next: RoomNode[] = [];
+
+    /**
+     * This array holds references to the previous node in a given level.
+     * eg: previous[i] gives the previous room node from this room node in level i.
+     */
+    public previous: RoomNode[] = [];
+}

src/stores/room-list-v3/skip-list/RoomSkipList.ts

@@ -0,0 +1,181 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+import type { Room } from "matrix-js-sdk/src/matrix";
+import type { Sorter } from "./sorters";
+import { RoomNode } from "./RoomNode";
+import { shouldPromote } from "./utils";
+import { Level } from "./Level";
+
+/**
+ * Implements a skip list that stores rooms using a given sorting algorithm.
+ * See See https://en.wikipedia.org/wiki/Skip_list
+ */
+export class RoomSkipList implements Iterable<Room> {
+    private levels: Level[] = [new Level(0)];
+    private roomNodeMap: Map<string, RoomNode> = new Map();
+    public initialized: boolean = false;
+
+    public constructor(private sorter: Sorter) {}
+
+    private reset(): void {
+        this.levels = [new Level(0)];
+        this.roomNodeMap = new Map();
+    }
+
+    /**
+     * Seed the list with an initial list of rooms.
+     */
+    public seed(rooms: Room[]): void {
+        // 1. First sort the rooms and create a base sorted linked list
+        const sortedRoomNodes = this.sorter.sort(rooms).map((room) => new RoomNode(room));
+        let currentLevel = this.levels[0];
+        for (const node of sortedRoomNodes) {
+            currentLevel.setNext(node);
+            this.roomNodeMap.set(node.room.roomId, node);
+        }
+
+        // 2. Create the rest of the sub linked lists
+        do {
+            this.levels[currentLevel.level] = currentLevel;
+            currentLevel = currentLevel.generateNextLevel();
+        } while (currentLevel.size > 1);
+        this.initialized = true;
+    }
+
+    /**
+     * Change the sorting algorithm used by the skip list.
+     * This will reset the list and will rebuild from scratch.
+     */
+    public useNewSorter(sorter: Sorter, rooms: Room[]): void {
+        this.reset();
+        this.sorter = sorter;
+        this.seed(rooms);
+    }
+
+    /**
+     * Removes a given room from the skip list.
+     */
+    public removeRoom(room: Room): void {
+        const existingNode = this.roomNodeMap.get(room.roomId);
+        this.roomNodeMap.delete(room.roomId);
+        if (existingNode) {
+            for (const level of this.levels) {
+                level.removeNode(existingNode);
+            }
+        }
+    }
+
+    /**
+     * Adds a given room to the correct sorted position in the list.
+     * If the room is already present in the list, it is first removed.
+     */
+    public addRoom(room: Room): void {
+        /**
+         * Remove this room from the skip list if necessary.
+         */
+        this.removeRoom(room);
+
+        const newNode = new RoomNode(room);
+        this.roomNodeMap.set(room.roomId, newNode);
+
+        /**
+         * This array tracks where the new node must be inserted in a
+         * given level.
+         * The index is the level and the value represents where the
+         * insertion must happen.
+         * If the value is null, it simply means that we need to insert
+         * at the head.
+         * If the value is a RoomNode, simply insert after this node.
+         */
+        const insertionNodes: (RoomNode | null)[] = [];
+
+        /**
+         * Now we'll do the actual work of finding where to insert this
+         * node.
+         *
+         * We start at the top most level and move downwards ...
+         */
+        for (let j = this.levels.length - 1; j >= 0; --j) {
+            const level = this.levels[j];
+
+            /**
+             * If the head is undefined, that means this level is empty.
+             * So mark it as such in insertionNodes and skip over this
+             * level.
+             */
+            if (!level.head) {
+                insertionNodes[j] = null;
+                continue;
+            }
+
+            /**
+             * So there's actually some nodes in this level ...
+             * All we need to do is find the node that is smaller or
+             * equal to the node that we wish to insert.
+             */
+            let current = level.head;
+            let previous: RoomNode | null = null;
+            while (current) {
+                if (this.sorter.comparator(current.room, room) < 0) {
+                    previous = current;
+                    current = current.next[j];
+                } else break;
+            }
+
+            /**
+             * previous will now be null if there's no node in this level
+             * smaller than the node we wish to insert or it will be a
+             * RoomNode.
+             * This is exactly what we need to track in insertionNodes!
+             */
+            insertionNodes[j] = previous;
+        }
+
+        /**
+         * We're done with difficult part, now we just need to do the
+         * actual node insertion.
+         */
+        for (const [level, node] of insertionNodes.entries()) {
+            /**
+             * Whether our new node should be present in a level
+             * is decided by coin toss.
+             */
+            if (level === 0 || shouldPromote()) {
+                const levelObj = this.levels[level];
+                if (node) levelObj.insertAfter(node, newNode);
+                else levelObj.insertAtHead(newNode);
+            } else {
+                break;
+            }
+        }
+    }
+
+    public [Symbol.iterator](): SortedRoomIterator {
+        return new SortedRoomIterator(this.levels[0].head!);
+    }
+
+    /**
+     * The number of rooms currently in the skip list.
+     */
+    public get size(): number {
+        return this.levels[0].size;
+    }
+}
+
+class SortedRoomIterator implements Iterator<Room> {
+    public constructor(private current: RoomNode) {}
+
+    public next(): IteratorResult<Room> {
+        const current = this.current;
+        if (!current) return { value: undefined, done: true };
+        this.current = current.next[0];
+        return {
+            value: current.room,
+        };
+    }
+}

src/stores/room-list-v3/skip-list/sorters/AlphabeticSorter.ts

@@ -0,0 +1,23 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+import type { Room } from "matrix-js-sdk/src/matrix";
+import type { Sorter } from ".";
+
+export class AlphabeticSorter implements Sorter {
+    private readonly collator = new Intl.Collator();
+
+    public sort(rooms: Room[]): Room[] {
+        return [...rooms].sort((a, b) => {
+            return this.comparator(a, b);
+        });
+    }
+
+    public comparator(roomA: Room, roomB: Room): number {
+        return this.collator.compare(roomA.name, roomB.name);
+    }
+}

src/stores/room-list-v3/skip-list/sorters/RecencySorter.ts

@@ -0,0 +1,33 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+import type { Room } from "matrix-js-sdk/src/matrix";
+import type { Sorter } from ".";
+import { getLastTs } from "../../../room-list/algorithms/tag-sorting/RecentAlgorithm";
+
+export class RecencySorter implements Sorter {
+    public constructor(private myUserId: string) {}
+
+    public sort(rooms: Room[]): Room[] {
+        const tsCache: { [roomId: string]: number } = {};
+        return [...rooms].sort((a, b) => this.comparator(a, b, tsCache));
+    }
+
+    public comparator(roomA: Room, roomB: Room, cache?: any): number {
+        const roomALastTs = this.getTs(roomA, cache);
+        const roomBLastTs = this.getTs(roomB, cache);
+        return roomBLastTs - roomALastTs;
+    }
+
+    private getTs(room: Room, cache?: { [roomId: string]: number }): number {
+        const ts = cache?.[room.roomId] ?? getLastTs(room, this.myUserId);
+        if (cache) {
+            cache[room.roomId] = ts;
+        }
+        return ts;
+    }
+}

src/stores/room-list-v3/skip-list/sorters/index.ts

@@ -0,0 +1,13 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+import type { Room } from "matrix-js-sdk/src/matrix";
+
+export interface Sorter {
+    sort(rooms: Room[]): Room[];
+    comparator(roomA: Room, roomB: Room): number;
+}

src/stores/room-list-v3/skip-list/utils.ts

@@ -0,0 +1,10 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+export function shouldPromote(): boolean {
+    return Math.random() < 0.5;
+}

src/stores/room-list/algorithms/tag-sorting/RecentAlgorithm.ts

@@ -62,7 +62,7 @@ export const sortRooms = (rooms: Room[]): Room[] => {
     });
 };
 
-const getLastTs = (r: Room, userId: string): number => {
+export const getLastTs = (r: Room, userId: string): number => {
     const mainTimelineLastTs = ((): number => {
         // Apparently we can have rooms without timelines, at least under testing
         // environments. Just return MAX_INT when this happens.