Fix/calculate current retrievability with FSRS-4.5 (#192)

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "fsrs"
-version = "0.6.3"
+version = "0.6.4"
 authors = ["Open Spaced Repetition"]
 categories = ["algorithms", "science"]
 edition = "2021"

src/inference.rs

@@ -267,7 +267,7 @@ impl<B: Backend> FSRS<B> {
     /// How well the user is likely to remember the item after `days_elapsed` since the previous
     /// review.
     pub fn current_retrievability(&self, state: MemoryState, days_elapsed: u32) -> f32 {
-        (days_elapsed as f32 / (state.stability * 9.0) + 1.0).powi(-1)
+        (days_elapsed as f64 / state.stability as f64 * FACTOR + 1.0).powf(DECAY) as f32
     }
 
     /// Returns the universal metrics for the existing and provided parameters. If the first value
@@ -571,6 +571,19 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn current_retrievability() {
+        let fsrs = FSRS::new(None).unwrap();
+        let state = MemoryState {
+            stability: 1.0,
+            difficulty: 5.0,
+        };
+        assert_eq!(fsrs.current_retrievability(state, 0), 1.0);
+        assert_eq!(fsrs.current_retrievability(state, 1), 0.9);
+        assert_eq!(fsrs.current_retrievability(state, 2), 0.82502866);
+        assert_eq!(fsrs.current_retrievability(state, 3), 0.76613088);
+    }
+
     #[test]
     fn memory_from_sm2() -> Result<()> {
         let fsrs = FSRS::new(Some(&[]))?;