From 987c1825a60f1ffd06879a1e350d09ae249f94b5 Mon Sep 17 00:00:00 2001
From: Damien Elmes <gpg@ankiweb.net>
Date: Fri, 3 Nov 2023 10:07:47 +1000
Subject: [PATCH] Fix panic when enabling FSRS with add-on-rescheduled cards

https://forums.ankiweb.net/t/error-upon-fsrs-activation-on-anki-23-10/36488
---
 rslib/src/scheduler/fsrs/memory_state.rs | 58 +++++++++++++++---------
 rslib/src/scheduler/fsrs/weights.rs      | 18 +++++---
 2 files changed, 48 insertions(+), 28 deletions(-)

diff --git a/rslib/src/scheduler/fsrs/memory_state.rs b/rslib/src/scheduler/fsrs/memory_state.rs
index e23982014..9720f1c84 100644
--- a/rslib/src/scheduler/fsrs/memory_state.rs
+++ b/rslib/src/scheduler/fsrs/memory_state.rs
@@ -12,7 +12,6 @@ use itertools::Itertools;
 use crate::card::CardType;
 use crate::prelude::*;
 use crate::revlog::RevlogEntry;
-use crate::revlog::RevlogReviewKind;
 use crate::scheduler::fsrs::weights::single_card_revlog_to_items;
 use crate::scheduler::fsrs::weights::Weights;
 use crate::scheduler::states::fuzz::with_review_fuzz;
@@ -235,27 +234,39 @@ pub(crate) fn single_card_revlog_to_item(
     next_day_at: TimestampSecs,
     sm2_retention: f32,
 ) -> Option<FsrsItemWithStartingState> {
-    let have_learning = entries
+    struct FirstReview {
+        interval: f32,
+        ease_factor: f32,
+    }
+    let first_review = entries
         .iter()
-        .any(|e| e.review_kind == RevlogReviewKind::Learning);
-    if have_learning {
-        let items = single_card_revlog_to_items(entries, next_day_at, false);
-        Some(FsrsItemWithStartingState {
-            item: items.unwrap().pop().unwrap(),
-            starting_state: None,
-        })
-    } else if let Some(first_review) = entries.iter().find(|e| e.button_chosen > 0) {
-        let ease_factor = if first_review.ease_factor == 0 {
-            2500
-        } else {
-            first_review.ease_factor
-        };
-        let interval = first_review.interval.max(1);
-        let starting_state =
-            fsrs.memory_state_from_sm2(ease_factor as f32 / 1000.0, interval as f32, sm2_retention);
-        let items = single_card_revlog_to_items(entries, next_day_at, false);
-        items.and_then(|mut items| {
-            let mut item = items.pop().unwrap();
+        .find(|e| e.button_chosen > 0)
+        .map(|e| FirstReview {
+            interval: e.interval.max(1) as f32,
+            ease_factor: if e.ease_factor == 0 {
+                2500
+            } else {
+                e.ease_factor
+            } as f32
+                / 1000.0,
+        });
+    if let Some((mut items, found_learning)) =
+        single_card_revlog_to_items(entries, next_day_at, false)
+    {
+        let mut item = items.pop().unwrap();
+        if found_learning {
+            // we assume the revlog is complete
+            Some(FsrsItemWithStartingState {
+                item,
+                starting_state: None,
+            })
+        } else if let Some(first_review) = first_review {
+            // the revlog has been truncated, but not fully
+            let starting_state = fsrs.memory_state_from_sm2(
+                first_review.ease_factor,
+                first_review.interval,
+                sm2_retention,
+            );
             item.reviews.remove(0);
             if item.reviews.is_empty() {
                 None
@@ -265,7 +276,10 @@ pub(crate) fn single_card_revlog_to_item(
                     starting_state: Some(starting_state),
                 })
             }
-        })
+        } else {
+            // only manual rescheduling; treat like empty
+            None
+        }
     } else {
         None
     }
diff --git a/rslib/src/scheduler/fsrs/weights.rs b/rslib/src/scheduler/fsrs/weights.rs
index c9e771c52..ff066e4b3 100644
--- a/rslib/src/scheduler/fsrs/weights.rs
+++ b/rslib/src/scheduler/fsrs/weights.rs
@@ -101,7 +101,7 @@ fn fsrs_items_for_training(revlogs: Vec<RevlogEntry>, next_day_at: TimestampSecs
         .filter_map(|(_cid, entries)| {
             single_card_revlog_to_items(entries.collect(), next_day_at, true)
         })
-        .flatten()
+        .flat_map(|i| i.0)
         .collect_vec();
     revlogs.sort_by_cached_key(|r| r.reviews.len());
     revlogs
@@ -111,16 +111,22 @@ fn fsrs_items_for_training(revlogs: Vec<RevlogEntry>, next_day_at: TimestampSecs
 /// expects multiple items for a given card when training - for revlog
 /// `[1,2,3]`, we create FSRSItems corresponding to `[1,2]` and `[1,2,3]`
 /// in training, and `[1]`, [1,2]` and `[1,2,3]` when calculating memory
-/// state.
+/// state. Returns (items, found_learn_entry), the latter of which is used
+/// to determine whether the revlogs have been truncated when not training.
 pub(crate) fn single_card_revlog_to_items(
     mut entries: Vec<RevlogEntry>,
     next_day_at: TimestampSecs,
     training: bool,
-) -> Option<Vec<FSRSItem>> {
+) -> Option<(Vec<FSRSItem>, bool)> {
     let mut last_learn_entry = None;
+    let mut found_learn_entry = false;
     for (index, entry) in entries.iter().enumerate().rev() {
-        if entry.review_kind == RevlogReviewKind::Learning {
+        if matches!(
+            (entry.review_kind, entry.button_chosen),
+            (RevlogReviewKind::Learning, 1..=4)
+        ) {
             last_learn_entry = Some(index);
+            found_learn_entry = true;
         } else if last_learn_entry.is_some() {
             break;
         }
@@ -199,7 +205,7 @@ pub(crate) fn single_card_revlog_to_items(
     if items.is_empty() {
         None
     } else {
-        Some(items)
+        Some((items, found_learn_entry))
     }
 }
 
@@ -229,7 +235,7 @@ pub(crate) mod tests {
     }
 
     pub(crate) fn convert(revlog: &[RevlogEntry], training: bool) -> Option<Vec<FSRSItem>> {
-        single_card_revlog_to_items(revlog.to_vec(), NEXT_DAY_AT, training)
+        single_card_revlog_to_items(revlog.to_vec(), NEXT_DAY_AT, training).map(|i| i.0)
     }
 
     #[macro_export]