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 an
timeline item position. Until this patch, `rfind_event_by_id` was used
and was the bottleneck. The improvement is twofold as 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, 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
https://github.com/matrix-org/matrix-rust-sdk/pull/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%.
Ivan Enderlin
77a67de7df