Prepare utils for local rooms (#9084)

* Prepare utils for local rooms

* Split up direct-messages module

src/utils/dm/findDMRoom.ts

@@ -0,0 +1,42 @@
+/*
+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 { Member } from "../direct-messages";
+import DMRoomMap from "../DMRoomMap";
+import { findDMForUser } from "./findDMForUser";
+
+/**
+ * Tries to find a DM room with some other users.
+ *
+ * @param {MatrixClient} client
+ * @param {Member[]} targets The Members to try to find the room for
+ * @returns {Room | null} Resolved so the room if found, else null
+ */
+export function findDMRoom(client: MatrixClient, targets: Member[]): Room | null {
+    const targetIds = targets.map(t => t.userId);
+    let existingRoom: Room;
+    if (targetIds.length === 1) {
+        existingRoom = findDMForUser(client, targetIds[0]);
+    } else {
+        existingRoom = DMRoomMap.shared().getDMRoomForIdentifiers(targetIds);
+    }
+    if (existingRoom) {
+        return existingRoom;
+    }
+    return null;
+}