Pick servers for ?via on matrix.to links based on some heuristics

src/matrix-to.js

@@ -14,11 +14,14 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import MatrixClientPeg from "./MatrixClientPeg";
+
 export const host = "matrix.to";
 export const baseUrl = `https://${host}`;
 
 export function makeEventPermalink(roomId, eventId) {
-    return `${baseUrl}/#/${roomId}/${eventId}`;
+    const serverCandidates = pickServerCandidates(roomId);
+    return `${baseUrl}/#/${roomId}/${eventId}?${encodeServerCandidates(serverCandidates)}`;
 }
 
 export function makeUserPermalink(userId) {
@@ -26,9 +29,86 @@ export function makeUserPermalink(userId) {
 }
 
 export function makeRoomPermalink(roomId) {
-    return `${baseUrl}/#/${roomId}`;
+    const serverCandidates = pickServerCandidates(roomId);
+    return `${baseUrl}/#/${roomId}?${encodeServerCandidates(serverCandidates)}`;
 }
 
 export function makeGroupPermalink(groupId) {
     return `${baseUrl}/#/${groupId}`;
 }
+
+export function encodeServerCandidates(candidates) {
+    if (!candidates) return '';
+    return `via=${candidates.map(c => encodeURIComponent(c)).join("&via=")}`
+}
+
+export function pickServerCandidates(roomId) {
+    const client = MatrixClientPeg.get();
+    const room = client.getRoom(roomId);
+    if (!room) return [];
+
+    // Permalinks can have servers appended to them so that the user
+    // receiving them can have a fighting chance at joining the room.
+    // These servers are called "candidates" at this point because
+    // it is unclear whether they are going to be useful to actually
+    // join in the future.
+    //
+    // We pick 3 servers based on the following criteria:
+    //
+    //   Server 1: The highest power level user in the room, provided
+    //   they are at least PL 50. We don't calculate "what is a moderator"
+    //   here because it is less relevant for the vast majority of rooms.
+    //   We also want to ensure that we get an admin or high-ranking mod
+    //   as they are less likely to leave the room. If no user happens
+    //   to meet this criteria, we'll pick the most popular server in the
+    //   room.
+    //
+    //   Server 2: The next most popular server in the room (in user
+    //   distribution). This will probably be matrix.org in most cases
+    //   although it is certainly possible to be some other server. This
+    //   cannot be the same as Server 1. If no other servers are available
+    //   then we'll only return Server 1.
+    //
+    //   Server 3: The next most popular server by user distribution. This
+    //   has the same rules as Server 2, with the added exception that it
+    //   must be unique from Server 1 and 2.
+
+    // Rationale for popular servers: It's hard to get rid of people when
+    // they keep flocking in from a particular server. Sure, the server could
+    // be ACL'd in the future or for some reason be evicted from the room
+    // however an event like that is unlikely the larger the room gets.
+
+    // Note: Users receiving permalinks that happen to have all 3 potential
+    // servers fail them (in terms of joining) are somewhat expected to hunt
+    // down the person who gave them the link to ask for a participating server.
+    // The receiving user can then manually append the known-good server to
+    // the list and magically have the link work.
+
+    const populationMap: {[server:string]:number} = {};
+    const highestPlUser = {userId:null, powerLevel: 0, serverName: null};
+
+    for (const member of room.getJoinedMembers()) {
+        const serverName = member.userId.split(":").splice(1).join(":");
+        if (member.powerLevel > highestPlUser.powerLevel) {
+            highestPlUser.userId = member.userId;
+            highestPlUser.powerLevel = member.powerLevel;
+            highestPlUser.serverName = serverName;
+        }
+
+        if (!populationMap[serverName]) populationMap[serverName] = 0;
+        populationMap[serverName]++;
+    }
+
+    const candidates = [];
+    if (highestPlUser.powerLevel >= 50) candidates.push(highestPlUser.serverName);
+
+    const maxCandidates = 3;
+    const serversByPopulation = Object.keys(populationMap)
+        .sort((a, b) => populationMap[a] - populationMap[b])
+        .filter(a => !candidates.includes(a));
+    while(candidates.length < maxCandidates && candidates.length <= serversByPopulation.length) {
+        candidates.push(serversByPopulation[Math.max(0, candidates.length - 1)]);
+    }
+
+    return candidates;
+}