From d16faacd0f37e14386b1980cca7e1483686a9164 Mon Sep 17 00:00:00 2001
From: Damien Elmes <gpg@ankiweb.net>
Date: Sat, 30 Sep 2023 13:39:22 +1000
Subject: [PATCH] Allow cards with no learning history when not training

---
 rslib/src/scheduler/fsrs/memory_state.rs | 55 ++++++++++++++++++++++++
 rslib/src/scheduler/fsrs/weights.rs      | 15 ++++---
 2 files changed, 64 insertions(+), 6 deletions(-)

diff --git a/rslib/src/scheduler/fsrs/memory_state.rs b/rslib/src/scheduler/fsrs/memory_state.rs
index 8deb78f17..8e5b64054 100644
--- a/rslib/src/scheduler/fsrs/memory_state.rs
+++ b/rslib/src/scheduler/fsrs/memory_state.rs
@@ -125,3 +125,58 @@ pub(crate) fn single_card_revlog_to_item(
     let items = single_card_revlog_to_items(entries, next_day_at, false);
     items.and_then(|mut i| i.pop())
 }
+
+#[cfg(test)]
+mod tests {
+    use fsrs::MemoryState;
+
+    use super::super::weights::tests::fsrs_items;
+    use super::*;
+    use crate::revlog::RevlogReviewKind;
+    use crate::scheduler::fsrs::weights::tests::convert;
+    use crate::scheduler::fsrs::weights::tests::review;
+    use crate::scheduler::fsrs::weights::tests::revlog;
+
+    #[test]
+    fn bypassed_learning_is_handled() {
+        // cards without any learning steps due to truncated history still have memory
+        // state calculated
+        assert_eq!(
+            convert(
+                &[
+                    RevlogEntry {
+                        ease_factor: 2500,
+                        ..revlog(RevlogReviewKind::Manual, 7)
+                    },
+                    revlog(RevlogReviewKind::Review, 6),
+                ],
+                false,
+            ),
+            fsrs_items!([review(0)])
+        );
+    }
+
+    #[test]
+    fn zero_history_is_handled() {
+        // when the history is empty, no items are produced
+        assert_eq!(convert(&[], false), None);
+        // but memory state should still be inferred, by using the card's current state
+        let mut card = Card {
+            ctype: CardType::Review,
+            interval: 100,
+            ease_factor: 1300,
+            ..Default::default()
+        };
+        card.set_memory_state(&FSRS::new(Some(&[])).unwrap(), None);
+        assert_eq!(
+            card.memory_state,
+            Some(
+                MemoryState {
+                    stability: 100.0,
+                    difficulty: 9.692858
+                }
+                .into()
+            )
+        );
+    }
+}
diff --git a/rslib/src/scheduler/fsrs/weights.rs b/rslib/src/scheduler/fsrs/weights.rs
index 7464e40b3..c069c02ff 100644
--- a/rslib/src/scheduler/fsrs/weights.rs
+++ b/rslib/src/scheduler/fsrs/weights.rs
@@ -135,8 +135,8 @@ pub(crate) fn single_card_revlog_to_items(
         if idx > 0 {
             entries.drain(..idx);
         }
-    } else {
-        // we ignore cards that don't have any learning steps
+    } else if training {
+        // when training, we ignore cards that don't have any learning steps
         return None;
     }
 
@@ -210,12 +210,12 @@ impl RevlogEntry {
 }
 
 #[cfg(test)]
-mod tests {
+pub(crate) mod tests {
     use super::*;
 
     const NEXT_DAY_AT: TimestampSecs = TimestampSecs(86400 * 100);
 
-    fn revlog(review_kind: RevlogReviewKind, days_ago: i64) -> RevlogEntry {
+    pub(crate) fn revlog(review_kind: RevlogReviewKind, days_ago: i64) -> RevlogEntry {
         RevlogEntry {
             review_kind,
             id: ((NEXT_DAY_AT.0 - days_ago * 86400) * 1000).into(),
@@ -224,14 +224,15 @@ mod tests {
         }
     }
 
-    fn review(delta_t: u32) -> FSRSReview {
+    pub(crate) fn review(delta_t: u32) -> FSRSReview {
         FSRSReview { rating: 3, delta_t }
     }
 
-    fn convert(revlog: &[RevlogEntry], training: bool) -> Option<Vec<FSRSItem>> {
+    pub(crate) fn convert(revlog: &[RevlogEntry], training: bool) -> Option<Vec<FSRSItem>> {
         single_card_revlog_to_items(revlog.to_vec(), NEXT_DAY_AT, training)
     }
 
+    #[macro_export]
     macro_rules! fsrs_items {
         ($($reviews:expr),*) => {
             Some(vec![
@@ -244,6 +245,8 @@ mod tests {
         };
     }
 
+    pub(crate) use fsrs_items;
+
     #[test]
     fn delta_t_is_correct() -> Result<()> {
         assert_eq!(