Feat/support load balance and easy days in rescheduling

[L-M-Sherlock]

It supports load balance and easy days natively in rescheduling. For the sake of performance, I didn't reuse the LoadBalancer and LoadBalancerContext.

Preview:

25.02:

The PR:

The PR with easy days:

[jakeprobst]

For the sake of performance, I didn't reuse the LoadBalancer and LoadBalancerContext.

this feels like a setup for future sadness, how bad was the perf hit that made this the preferable choice?

At the very least, I'd say move everything that is common to both into their own functions so the logic will be consistent across them both. this will also make it easier to see the ways in which these differ, like I can tell they are sliiightly different but its hard to determine where without also pulling up the other code, if all common logic is in the same spot it'll be clear what the differences are.

[L-M-Sherlock]

this feels like a setup for future sadness, how bad was the perf hit that made this the preferable choice?

This function is I/O bound:

anki/rslib/src/scheduler/states/load_balancer.rs

Lines 120 to 128 in 038d85b

	impl LoadBalancer {
	pub fn new(
	today: u32,
	did_to_dcid: HashMap<DeckId, DeckConfigId>,
	next_day_at: TimestampSecs,
	storage: &SqliteStorage,
	) -> Result<LoadBalancer> {
	let cards_on_each_day =
	storage.get_all_cards_due_in_range(today, today + LOAD_BALANCE_DAYS as u32)?;

For rescheduling, the today is the last review date, so I have to re-create load balancer for each card. It's very slow. It would take 1 minute to reschedule thousands of cards.

[dae]

For future maintenance, we definitely want as much code shared between the paths as possible - whether that's by reusing old code and doing things like mutating an existing context for each card instead of creating a new one (which may not be practical here), or by introducing this new code and having the old code use it.

[YukiNagat0]

Does this change respect the mw.col.load_balancer_enabled option?

[L-M-Sherlock]

Does this change respect the mw.col.load_balancer_enabled option?

It does now.

For future maintenance, we definitely want as much code shared between the paths as possible - whether that's by reusing old code and doing things like mutating an existing context for each card instead of creating a new one (which may not be practical here), or by introducing this new code and having the old code use it.

I'm reusing some code snippet now.