Fix/clip post lapse stability (#142)

package.json

@@ -1,6 +1,6 @@
 {
   "name": "ts-fsrs",
-  "version": "4.5.0",
+  "version": "4.5.1",
   "description": "ts-fsrs is a versatile package based on TypeScript that supports ES modules, CommonJS, and UMD. It implements the Free Spaced Repetition Scheduler (FSRS) algorithm, enabling developers to integrate FSRS into their flashcard applications to enhance the user learning experience.",
   "main": "dist/index.cjs",
   "umd": "dist/index.umd.js",

src/fsrs/algorithm.ts

@@ -233,6 +233,8 @@ export class FSRSAlgorithm {
   /**
    * The formula used is :
    * $$S^\prime_f(D,S,R) = w_{11}\cdot D^{-w_{12}}\cdot ((S+1)^{w_{13}}-1) \cdot e^{w_{14}\cdot(1-R)}$$
+   * enable_short_term = true : $$S^\prime_f \in \min \lbrace \max \lbrace S^\prime_f,0.01\rbrace, \frac{S}{e^{w_{17} \cdot w_{18}}} \rbrace$$
+   * enable_short_term = false : $$S^\prime_f \in \min \lbrace \max \lbrace S^\prime_f,0.01\rbrace, S \rbrace$$
    * @param {number} d Difficulty D \in [1,10]
    * @param {number} s Stability (interval when R=90%)
    * @param {number} r Retrievability (probability of recall)

src/fsrs/default.ts

@@ -11,7 +11,7 @@ export const default_w = [
 export const default_enable_fuzz = false
 export const default_enable_short_term = true
 
-export const FSRSVersion: string = 'v4.5.0 using FSRS-5.0'
+export const FSRSVersion: string = 'v4.5.1 using FSRS-5.0'
 
 export const generatorParameters = (
   props?: Partial<FSRSParameters>

src/fsrs/impl/basic_scheduler.ts

@@ -186,11 +186,20 @@ export default class BasicScheduler extends AbstractScheduler {
       difficulty,
       Rating.Again
     )
-    next_again.stability = this.algorithm.next_forget_stability(
-      difficulty,
-      stability,
-      retrievability
+    const nextSMin =
+      stability /
+      Math.exp(
+        this.algorithm.parameters.w[17] * this.algorithm.parameters.w[18]
+      )
+    next_again.stability = Math.min(
+      +nextSMin.toFixed(8),
+      this.algorithm.next_forget_stability(
+        difficulty,
+        stability,
+        retrievability
+      )
     )
+
     next_hard.difficulty = this.algorithm.next_difficulty(
       difficulty,
       Rating.Hard

src/fsrs/impl/long_term_scheduler.ts

@@ -112,11 +112,15 @@ export default class LongTermScheduler extends AbstractScheduler {
       difficulty,
       Rating.Again
     )
-    next_again.stability = this.algorithm.next_forget_stability(
-      difficulty,
+    next_again.stability = Math.min(
       stability,
-      retrievability
+      this.algorithm.next_forget_stability(
+        difficulty,
+        stability,
+        retrievability
+      )
     )
+
     next_hard.difficulty = this.algorithm.next_difficulty(
       difficulty,
       Rating.Hard