fix(ui): Fix performance of ReadReceiptTimelineUpdate::apply

[Hywan]

This patch improves the performance of
ReadReceiptTimelineUpdate::apply, which does 2 things: it calls remove_old_receipt and add_new_receipt. Both of them need a timeline item position. Until this patch, rfind_event_by_id was used and was the bottleneck. The improvement is twofold and is as follows.

First off, when collecting data to create ReadReceiptTimelineUpdate, the timeline item position can be known ahead of time by using EventMeta::timeline_item_index. This data is not always available though, for example if the timeline item isn't created yet. But let's try to collect these data if there are some.

Second, inside ReadReceiptTimelineUpdate::remove_old_receipt, we use the timeline item position collected from EventMeta if it exists. Otherwise, let's fallback to a similar rfind_event_by_id pattern, without using intermediate types. It's more straightforward here: we don't need an EventTimelineItemWithId, we only need the position. Once the position is known, it is stored in Self (!), this is the biggest improvement here. Le't see why.

Finally, inside ReadReceiptTimelineUpdate::add_new_receipt, we use the timeline item position collected from EventMeta if it exists, similarly to what remove_old_receipt does. Otherwise, let's fallback to an iterator to find the position. However, instead of iterating over all items, we can skip the first ones, up to the position of the timeline item holding the old receipt, so up to the position found by remove_old_receipt.

I'm testing this patch with the test_lazy_back_pagination test in #4594. With 10_000 events in the sync, the ReadReceipts::maybe_update_read_receipt method was taking 52% of the whole execution time. With this patch, it takes 8.1%.

The call tree of ReadReceipts::maybe_update_read_receipt and its flamegraph before this patch…

and after…

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• Address [meta] EventCache storage #3280

[codecov]

Codecov Report

Attention: Patch coverage is 82.35294% with 6 lines in your changes missing coverage. Please review.

Project coverage is 85.73%. Comparing base (9ab5547) to head (b59b0ea).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
...ix-sdk-ui/src/timeline/controller/read_receipts.rs	82.35%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4612      +/-   ##
==========================================
- Coverage   85.74%   85.73%   -0.02%     
==========================================
  Files         291      291              
  Lines       33306    33329      +23     
==========================================
+ Hits        28558    28574      +16     
- Misses       4748     4755       +7     

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[bnjbvr]

Nice improvement!

[bnjbvr]

Is this change spurious? Doesn't seem required according to the rest of the diff…

[Hywan]

It is necessary because remove_old_receipt takes a &mut self.