From bf0602085526c45d000e2961a9119e9a45ad0ddd Mon Sep 17 00:00:00 2001
From: Damien Elmes <dae@users.noreply.github.com>
Date: Tue, 9 Jan 2024 12:26:46 +1000
Subject: [PATCH] Use card.reps - 1 when calculating fuzz (#2933)

https://github.com/open-spaced-repetition/fsrs4anki-helper/issues/343#issuecomment-1879584562

https://forums.ankiweb.net/t/reschedule-is-inconsistent-with-normal-schedule-in-fuzz/39363
---
 proto/anki/scheduler.proto               |  4 ++--
 rslib/src/error/mod.rs                   |  4 ++--
 rslib/src/scheduler/answering/mod.rs     | 19 ++++++++++++++-----
 rslib/src/scheduler/fsrs/memory_state.rs |  2 +-
 rslib/src/scheduler/states/fuzz.rs       |  2 +-
 5 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/proto/anki/scheduler.proto b/proto/anki/scheduler.proto
index 27bb73709..c80026a87 100644
--- a/proto/anki/scheduler.proto
+++ b/proto/anki/scheduler.proto
@@ -53,8 +53,8 @@ service SchedulerService {
       returns (ComputeOptimalRetentionResponse);
   rpc EvaluateWeights(EvaluateWeightsRequest) returns (EvaluateWeightsResponse);
   rpc ComputeMemoryState(cards.CardId) returns (ComputeMemoryStateResponse);
-  // The number of days the calculated interval will be fuzzed by. Utilized by
-  // the FSRS add-on.
+  // The number of days the calculated interval was fuzzed by on the previous
+  // review (if any). Utilized by the FSRS add-on.
   rpc FuzzDelta(FuzzDeltaRequest) returns (FuzzDeltaResponse);
 }
 
diff --git a/rslib/src/error/mod.rs b/rslib/src/error/mod.rs
index 77c6b164e..cde7a4304 100644
--- a/rslib/src/error/mod.rs
+++ b/rslib/src/error/mod.rs
@@ -114,9 +114,9 @@ pub enum AnkiError {
     InvalidMethodIndex,
     InvalidServiceIndex,
     FsrsWeightsInvalid,
-    // Returned by fsrs-rs; may happen even if 1000+ reviews
+    /// Returned by fsrs-rs; may happen even if 1000+ reviews
     FsrsInsufficientData,
-    // Generated by our backend if count < 1000
+    /// Generated by our backend if count < 1000
     FsrsInsufficientReviews {
         count: usize,
     },
diff --git a/rslib/src/scheduler/answering/mod.rs b/rslib/src/scheduler/answering/mod.rs
index 45c805fd8..6624e0a30 100644
--- a/rslib/src/scheduler/answering/mod.rs
+++ b/rslib/src/scheduler/answering/mod.rs
@@ -398,7 +398,7 @@ impl Collection {
         };
         let desired_retention = fsrs_enabled.then_some(config.inner.desired_retention);
         Ok(CardStateUpdater {
-            fuzz_seed: get_fuzz_seed(&card),
+            fuzz_seed: get_fuzz_seed(&card, false),
             card,
             deck,
             config,
@@ -519,14 +519,23 @@ pub mod test_helpers {
 }
 
 impl Card {
-    pub(crate) fn get_fuzz_factor(&self) -> Option<f32> {
-        get_fuzz_factor(get_fuzz_seed(self))
+    /// If for_reschedule is true, we use card.reps - 1 to match the previous
+    /// review.
+    pub(crate) fn get_fuzz_factor(&self, for_reschedule: bool) -> Option<f32> {
+        get_fuzz_factor(get_fuzz_seed(self, for_reschedule))
     }
 }
 
 /// Return a consistent seed for a given card at a given number of reps.
-fn get_fuzz_seed(card: &Card) -> Option<u64> {
-    get_fuzz_seed_for_id_and_reps(card.id, card.reps)
+/// If for_reschedule is true, we use card.reps - 1 to match the previous
+/// review.
+fn get_fuzz_seed(card: &Card, for_reschedule: bool) -> Option<u64> {
+    let reps = if for_reschedule {
+        card.reps.saturating_sub(1)
+    } else {
+        card.reps
+    };
+    get_fuzz_seed_for_id_and_reps(card.id, reps)
 }
 
 /// If in test environment, disable fuzzing.
diff --git a/rslib/src/scheduler/fsrs/memory_state.rs b/rslib/src/scheduler/fsrs/memory_state.rs
index 7cd626502..55565b06e 100644
--- a/rslib/src/scheduler/fsrs/memory_state.rs
+++ b/rslib/src/scheduler/fsrs/memory_state.rs
@@ -95,7 +95,7 @@ impl Collection {
                                         )
                                             as f32;
                                         card.interval = with_review_fuzz(
-                                            card.get_fuzz_factor(),
+                                            card.get_fuzz_factor(true),
                                             interval,
                                             1,
                                             req.max_interval,
diff --git a/rslib/src/scheduler/states/fuzz.rs b/rslib/src/scheduler/states/fuzz.rs
index 1a893aacd..56c2e197d 100644
--- a/rslib/src/scheduler/states/fuzz.rs
+++ b/rslib/src/scheduler/states/fuzz.rs
@@ -49,7 +49,7 @@ impl Collection {
             .or_not_found(card.deck_id)?;
         let config = self.home_deck_config(deck.config_id(), card.original_deck_id)?;
         let fuzzed = with_review_fuzz(
-            card.get_fuzz_factor(),
+            card.get_fuzz_factor(true),
             interval as f32,
             1,
             config.inner.maximum_review_interval,