Prepare utils for local rooms (#9084)

* Prepare utils for local rooms

* Split up direct-messages module

src/utils/dm/findDMForUser.ts

@@ -0,0 +1,55 @@
+/*
+Copyright 2022 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.
+*/
+
+import { MatrixClient, Room } from "matrix-js-sdk/src/matrix";
+
+import DMRoomMap from "../DMRoomMap";
+import { isLocalRoom } from "../localRoom/isLocalRoom";
+import { isJoinedOrNearlyJoined } from "../membership";
+
+/**
+ * Tries to find a DM room with a specific user.
+ *
+ * @param {MatrixClient} client
+ * @param {string} userId ID of the user to find the DM for
+ * @returns {Room} Room if found
+ */
+export function findDMForUser(client: MatrixClient, userId: string): Room {
+    const roomIds = DMRoomMap.shared().getDMRoomsForUserId(userId);
+    const rooms = roomIds.map(id => client.getRoom(id));
+    const suitableDMRooms = rooms.filter(r => {
+        // Validate that we are joined and the other person is also joined. We'll also make sure
+        // that the room also looks like a DM (until we have canonical DMs to tell us). For now,
+        // a DM is a room of two people that contains those two people exactly. This does mean
+        // that bots, assistants, etc will ruin a room's DM-ness, though this is a problem for
+        // canonical DMs to solve.
+        if (r && r.getMyMembership() === "join") {
+            if (isLocalRoom(r)) return false;
+
+            const members = r.currentState.getMembers();
+            const joinedMembers = members.filter(m => isJoinedOrNearlyJoined(m.membership));
+            const otherMember = joinedMembers.find(m => m.userId === userId);
+            return otherMember && joinedMembers.length === 2;
+        }
+        return false;
+    }).sort((r1, r2) => {
+        return r2.getLastActiveTimestamp() -
+            r1.getLastActiveTimestamp();
+    });
+    if (suitableDMRooms.length) {
+        return suitableDMRooms[0];
+    }
+}