Cache the joined member count for a room state

Pushrule evaluation needs the count of joined room members and
filtering the list for joined members takes a nontrivial amount
of time, but caching it is trivial, both code and memory wise.

src/models/room-state.js

@@ -69,9 +69,24 @@ function RoomState(roomId) {
     this._displayNameToUserIds = {};
     this._userIdsToDisplayNames = {};
     this._tokenToInvite = {}; // 3pid invite state_key to m.room.member invite
+    this._joinedMemberCount = null; // cache of the number of joined members
 }
 utils.inherits(RoomState, EventEmitter);
 
+/**
+ * Returns the number of joined members in this room
+ * This method caches the result.
+ * @return {integer} The number of members in this room whose membership is 'join'
+ */
+RoomState.prototype.getJoinedMemberCount = function() {
+    if (this._joinedMemberCount === null) {
+        this._joinedMemberCount = this.getMembers().filter((m) => {
+            return m.membership === 'join';
+        }).length;
+    }
+    return this._joinedMemberCount;
+};
+
 /**
  * Get all RoomMembers in this room.
  * @return {Array<RoomMember>} A list of RoomMembers.
@@ -218,6 +233,7 @@ RoomState.prototype.setStateEvents = function(stateEvents) {
             delete self._sentinels[userId];
 
             self.members[userId] = member;
+            self._joinedMemberCount = null;
             self.emit("RoomState.members", event, self, member);
         } else if (event.getType() === "m.room.power_levels") {
             const members = utils.values(self.members);

src/pushprocessor.js

@@ -156,9 +156,7 @@ function PushProcessor(client) {
             return false;
         }
 
-        const memberCount = Object.keys(room.currentState.members).filter(function(m) {
-            return room.currentState.members[m].membership == 'join';
-        }).length;
+        const memberCount = room.currentState.getJoinedMemberCount();
 
         const m = cond.is.match(/^([=<>]*)([0-9]*)$/);
         if (!m) {