Thanks for sharing some insight into the challenges!
I’ve given this some thought in the past, in the context of working on a small tool for my own personal reviews. Ideally, the way I’d like to see it work as a user (with the caveat that I don’t know how much of a pain this might be based on the current architecture) is:
- Each card can be in in Classic or FSRS mode (presumably driven by some global/deck defaults).
- Immediately assign the FSRS parameters (Stability and Difficulty) to all reviews, or do it lazily as the cards become due for review, even if the card is still in Classic mode.
- Changing a card between Classic and FSRS don’t change any parameters.
- Upon doing any review (FSRS or Classic), the FSRS parameters (S, D) are updated as usual. In Classic mode, assume “Good” for correct. The rationale is that FSRS will have better data to go on when/if the user switches to it. This also means that changing between modes and experimenting is not destructive.
- Upon doing a review in FSRS mode: The next review is simply scheduled based on FSRS. The Classic level of the card is updated up or down based on the new interval - so e.g. if a card went from a 3 day interval to a 15 day interval, the classic level updates from Adept 1 (3 day rounded down to 2) directly to Seasoned 1 (15 rounded down to 14). It’s also possible that a card will remain on the same Classic level.
- Upon doing a review in Classic mode: Schedule the card as per the Classic algorithm.
- From an interface point of view, it’s perfectly acceptable to simply show the Classic level everywhere. IMO it’s more exciting to see the labelled levels rather than just a time period.
- In terms of Grading answers, I’d ideally like it to auto-grade “Again” on failure, then let me choose from “Easy”, “Good”, “Hard” on success (with “Good” selected by default), regardless of whether I’m doing Cloze or Flashcards.
Either way, I’m very much looing forward for a version of this feature to eventually come out on Bunpro. I’ve recently started supplementing my Wanikani reviews with an Anki deck and the difference that FSRS makes for me is unbeliveable! Very excited to see your version, and thanks for all the great work on Bunpro!