Stop doing O(n^2) work to find event's home (#3227)

* Stop doing O(n^2) work to find event's home

In certain rooms (e.g. with many state changes hidden via user preferences), the
events array presented to `eventShouldLiveIn` may contain 100s of events. As
part of its various checks, `eventShouldLiveIn` would get an event's associated
ID (reply / relation / redaction parent). It would then use `events.find` to
search the entire (possibly large) `events` array to look for the parent. (This
by itself seems sub-optimal and should probably change to use a map.)

For many events in a room, there is no associated ID. Unfortunately,
`eventShouldLiveIn` did not check whether the associated ID actually exists
before running off to search all of `events`, resulting in O(n^2) work.

This changes `eventShouldLiveIn` to first check that there is an associated ID
before proceeding with its (slow) search. For some rooms, this change
drastically improves performance from ~100% CPU usage to nearly idle.

Signed-off-by: J. Ryan Stinnett <jryans@gmail.com>

* Add type to `parentEvent`

Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>

---------

Signed-off-by: J. Ryan Stinnett <jryans@gmail.com>
Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>

src/models/room.ts

@@ -2028,8 +2028,11 @@ export class Room extends ReadReceipt<RoomEmittedEvents, RoomEventHandlerMap> {
             };
         }
 
-        const parentEventId = event.getAssociatedId()!;
-        const parentEvent = this.findEventById(parentEventId) ?? events?.find((e) => e.getId() === parentEventId);
+        const parentEventId = event.getAssociatedId();
+        let parentEvent: MatrixEvent | undefined;
+        if (parentEventId) {
+            parentEvent = this.findEventById(parentEventId) ?? events?.find((e) => e.getId() === parentEventId);
+        }
 
         // Treat relations and redactions as extensions of their parents so evaluate parentEvent instead
         if (parentEvent && (event.isRelation() || event.isRedaction())) {