Factor sm2 retention into memory state calculation (#101)

ankitects/anki#2702

src/inference.rs

@@ -89,14 +89,20 @@ impl<B: Backend> FSRS<B> {
     /// If a card has incomplete learning history, memory state can be approximated from
     /// current sm2 values.
     /// Weights must have been provided when calling FSRS::new().
-    pub fn memory_state_from_sm2(&self, ease_factor: f32, interval: f32) -> MemoryState {
-        let stability = interval.max(0.1);
+    pub fn memory_state_from_sm2(
+        &self,
+        ease_factor: f32,
+        interval: f32,
+        sm2_retention: f32,
+    ) -> MemoryState {
+        let stability = interval.max(0.1) * 0.9_f32.ln() / sm2_retention.ln();
         let w = &self.model().w;
         let w8: f32 = w.get(8).into_scalar().elem();
         let w9: f32 = w.get(9).into_scalar().elem();
         let w10: f32 = w.get(10).into_scalar().elem();
         let difficulty = 11.0
-            - (ease_factor - 1.0) / (w8.exp() * stability.powf(-w9) * ((0.1 * w10).exp() - 1.0));
+            - (ease_factor - 1.0)
+                / (w8.exp() * stability.powf(-w9) * (((1.0 - sm2_retention) * w10).exp() - 1.0));
         MemoryState {
             stability,
             difficulty: difficulty.clamp(1.0, 10.0),
@@ -471,14 +477,14 @@ mod tests {
     fn memory_from_sm2() -> Result<()> {
         let fsrs = FSRS::new(Some(&[]))?;
         assert_eq!(
-            fsrs.memory_state_from_sm2(2.5, 10.0),
+            fsrs.memory_state_from_sm2(2.5, 10.0, 0.9),
             MemoryState {
                 stability: 10.0,
                 difficulty: 6.265295
             }
         );
         assert_eq!(
-            fsrs.memory_state_from_sm2(1.3, 20.0),
+            fsrs.memory_state_from_sm2(1.3, 20.0, 0.9),
             MemoryState {
                 stability: 20.0,
                 difficulty: 9.956561
@@ -488,7 +494,7 @@ mod tests {
         let ease_factor = 2.0;
         let fsrs_factor = fsrs
             .next_states(
-                Some(fsrs.memory_state_from_sm2(ease_factor, interval as f32)),
+                Some(fsrs.memory_state_from_sm2(ease_factor, interval as f32, 0.9)),
                 0.9,
                 interval,
             )