include decay in ComputeMemoryStateResponse (#4102)

* include decay in ComputeMemoryStateResponse

* Add decay attribute to ComputedMemoryState and update Collection methods

* Refactor decay calculation into a helper function for improved readability and maintainability in memory state management

* format & clippy

proto/anki/scheduler.proto

@@ -450,6 +450,7 @@ message EvaluateParamsResponse {
 message ComputeMemoryStateResponse {
   optional cards.FsrsMemoryState state = 1;
   float desired_retention = 2;
+  float decay = 3;
 }
 
 message FuzzDeltaRequest {

pylib/anki/collection.py

@@ -122,6 +122,7 @@ class ComputedMemoryState:
     desired_retention: float
     stability: float | None = None
     difficulty: float | None = None
+    decay: float | None = None
 
 
 @dataclass
@@ -1189,9 +1190,13 @@ class Collection(DeprecatedNamesMixin):
                 desired_retention=resp.desired_retention,
                 stability=resp.state.stability,
                 difficulty=resp.state.difficulty,
+                decay=resp.decay,
             )
         else:
-            return ComputedMemoryState(desired_retention=resp.desired_retention)
+            return ComputedMemoryState(
+                desired_retention=resp.desired_retention,
+                decay=resp.decay,
+            )
 
     def fuzz_delta(self, card_id: CardId, interval: int) -> int:
         "The delta days of fuzz applied if reviewing the card in v3."

rslib/src/scheduler/fsrs/memory_state.rs

@@ -30,6 +30,18 @@ pub struct ComputeMemoryProgress {
     pub total_cards: u32,
 }
 
+/// Helper function to determine the appropriate decay value based on FSRS
+/// parameters
+fn get_decay_from_params(params: &[f32]) -> f32 {
+    if params.is_empty() {
+        FSRS6_DEFAULT_DECAY // default decay for FSRS-6
+    } else if params.len() < 21 {
+        FSRS5_DEFAULT_DECAY // default decay for FSRS-4.5 and FSRS-5
+    } else {
+        params[20]
+    }
+}
+
 #[derive(Debug)]
 pub(crate) struct UpdateMemoryStateRequest {
     pub params: Params,
@@ -77,15 +89,7 @@ impl Collection {
                 .then(|| Rescheduler::new(self))
                 .transpose()?;
             let fsrs = FSRS::new(req.as_ref().map(|w| &w.params[..]).or(Some([].as_slice())))?;
-            let decay = req.as_ref().map(|w| {
+            let decay = req.as_ref().map(|w| get_decay_from_params(&w.params));
-                if w.params.is_empty() {
-                    FSRS6_DEFAULT_DECAY // default decay for FSRS-6
-                } else if w.params.len() < 21 {
-                    FSRS5_DEFAULT_DECAY // default decay for FSRS-4.5 and FSRS-5
-                } else {
-                    w.params[20]
-                }
-            });
             let historical_retention = req.as_ref().map(|w| w.historical_retention);
             let items = fsrs_items_for_memory_states(
                 &fsrs,
@@ -190,7 +194,9 @@ impl Collection {
             .or_not_found(conf_id)?;
         let desired_retention = config.inner.desired_retention;
         let historical_retention = config.inner.historical_retention;
-        let fsrs = FSRS::new(Some(config.fsrs_params()))?;
+        let params = config.fsrs_params();
+        let decay = get_decay_from_params(params);
+        let fsrs = FSRS::new(Some(params))?;
         let revlog = self.revlog_for_srs(SearchNode::CardIds(card.id.to_string()))?;
         let item = fsrs_item_for_memory_state(
             &fsrs,
@@ -204,6 +210,7 @@ impl Collection {
             Ok(ComputeMemoryStateResponse {
                 state: card.memory_state.map(Into::into),
                 desired_retention,
+                decay,
             })
         } else {
             card.memory_state = None;
@@ -211,6 +218,7 @@ impl Collection {
             Ok(ComputeMemoryStateResponse {
                 state: None,
                 desired_retention,
+                decay,
             })
         }
     }