Fix/keep two decimals for stability

[L-M-Sherlock]

Reason: some users reported that the rescheduling results are inconsistent between built-in FSRS and the helper add-on. For example, if the memory stability is 8.48, the built-in FSRS will schedule it with 8 days interval. However, it is stored as 8.5 in database. The helper will round it to 9 days. It would also induce the inconsistency during the fuzzing.

[L-M-Sherlock]

@dae, I find that the round_to_places couldn't solve the problem completely. For example, if the stability is 20.49875, round_to_places(s, 2) is 20.5, which still causes the inconsistency.

[dae]

Can't you adjust the helper to use the same precision/rounding as Anki instead?

[L-M-Sherlock]

Can't you adjust the helper to use the same precision/rounding as Anki instead?

Helper reads the value from database. But the value has been rounded before it is stored in database.

[L-M-Sherlock]

For example, if the stability is 20.49875 calculated by built-in FSRS, it will be stored as 20.5. Then, the stability is incorrect in the card info because 20.5 is rounded to 21.

[dae]

Ok, I see. We could potentially solve this by rounding somewhere in fsrs-rs, but since we'll hopefully implement the helper's functions natively in Rust in the future, perhaps it would be more efficient to document this limitation, and wait until then?

[L-M-Sherlock]

I'm OK. It doesn't affect too many cards.