Simplify Revlog->FSRSItem conversion

[dae]

Depends on #12.

I wasn't entirely sure what the current code was trying to do, so please double-check it still performs as you expect / let me know if it's changed the behavior.

[dae]

My earlier code was quite wrong, as I'd assumed there'd be only one FSRSItem for each card. The new code is passing the tests - does it look ok to you?

One remaining question - if we look at the shape of FSRSItem and FSRSReview, the delta_t and rating are now the same:

/// Represents a single review on a card, and contains the previous reviews for that card.
#[derive(Debug, Clone, Deserialize, Serialize, PartialEq)]
pub struct FSRSItem {
    /// The previous reviews done to the card before the current one
    pub reviews: Vec<FSRSReview>,
    /// 1-4
    pub rating: i32,
    /// The number of days that passed
    pub delta_t: i32,
}

#[derive(Debug, Clone, Deserialize, Serialize, PartialEq)]
pub struct FSRSReview {
    /// 1-4
    pub rating: i32,
    /// The number of days that passed
    pub delta_t: i32,
}

Would it make sense to remove the rating/delta_t from FSRSItem, and instead place one more review in the vec instead? Eg have it store current+previous reviews, and not just previous reviews.

[L-M-Sherlock]

Would it make sense to remove the rating/delta_t from FSRSItem, and instead place one more review in the vec instead?

Yeah, it's OK. I have read the code and found that you convert rating to label in FSRSBatcher. After insert the last rating/delta_t into vec, we need pop it and convert it into label in FSRSBatcher.

[L-M-Sherlock]

Nice work! I have reviewed the code and trained FSRS with the test collection.