perf(calendar): pre-filter ICS data before parsing

[KristjanESPERANTO]

This adds a pre-filter that drops out-of-window non-recurring events from the raw ICS data before node-ical parses it. Recurring events and anything we cannot classify cheaply are kept - the existing logic handles those as before.

Some calendars accumulate years of old events. Pre-filtering them reduces the amount of work node-ical has to do and keeps event loop lag lower. My benchmarks on a fast machine showed a small but measurable speedup (roughly 10-20% with generated test data); on a Raspberry Pi it should be significantly more noticeable.

For the implementation I ended up using ics-filter, suggested by @rejas. I had a custom version first, but ICS date string parsing has enough complexity that it makes sense to delegate it to a package built specifically for this. I also contributed a couple of fixes to ics-filter along the way, and the maintainer was responsive and open to improvements, so it seems like a good fit.

Solves #4103.

[rejas]

Just a thought: would a library like https://github.com/runely/ics-filter offload some complexity from this PR?

[sdetweil]

@rejas that filters out ALL past events (we want SOME, sometimes)

the filter will only filter out past events

[sdetweil]

i also replied in the issue, not in the PR.

[KristjanESPERANTO]

Thanks @rejas for the pointer to ics-filter! I had a closer look and did some tests - I think the general approach of that package fits well, though there are a few edges. I just opened a first PR there to address one of them (case-insensitive property names): runely/ics-filter#1

I'm setting this to draft for now and waiting for the author to respond. I'd rather improve an existing package than maintain our own implementation here :)

[KristjanESPERANTO]

that filters out ALL past events (we want SOME, sometimes)

We can pass now - maximumNumberOfDays as the min date to icsFilter(), which is the same window filterEvents() (in ./defaultmodules/calendar/calendarfetcherutils.js) already uses for past events.

[KristjanESPERANTO]

Status update: The maintainer of ics-filter was open to my pull requests and has merged them :) From my view, ics-filter is now suitable for our use case.

[KristjanESPERANTO]

I've switched the implementation to ics-filter and updated the PR text. The PR is ready for review 🙂

[khassel]

@KristjanESPERANTO let me know if you want to wait for others to approve, otherwise I will merge ...

[KristjanESPERANTO]

I think one reviewer is enough here, even though I might be biased since this is my own PR 😅