Fix/fuzz=false wrong calc next_ivl (#74)

* Fix/enable_fuzz=false wrong calc next_ivl

* 3.5.1

* Test/next_ivl

__tests__/algorithm.test.ts

@@ -0,0 +1,39 @@
+import { fsrs, FSRS, generatorParameters, get_fuzz_range } from "../src/fsrs";
+
+describe("next_interval", () => {
+  const DECAY: number = -0.5;
+  const FACTOR: number = Math.pow(0.9, 1 / DECAY) - 1;
+
+  it("next_ivl", () => {
+    const desired_retentions: number[] = Array.from(
+      { length: 10 },
+      (_, i) => (i + 1) / 10,
+    );
+    const intervals: number[] = desired_retentions.map((r) =>
+      fsrs({ request_retention: r }).next_interval(1.0, 0, false),
+    );
+    expect(intervals).toEqual([422, 102, 43, 22, 13, 8, 4, 2, 1, 1]);
+  });
+
+  // https://github.com/open-spaced-repetition/ts-fsrs/pull/74
+  it("next_ivl[max_limit]", () => {
+    const params = generatorParameters({ maximum_interval: 365 });
+    const intervalModifier =
+      (Math.pow(params.request_retention, 1 / DECAY) - 1) / FACTOR;
+    const f: FSRS = fsrs(params);
+
+    const s = 737.47;
+    const next_ivl = f.next_interval(s, 0, false);
+    expect(next_ivl).toEqual(params.maximum_interval);
+
+    const t_fuzz = 98;
+    const next_ivl_fuzz = f.next_interval(s, t_fuzz, true);
+    const { min_ivl, max_ivl } = get_fuzz_range(
+      s * intervalModifier,
+      t_fuzz,
+      params.maximum_interval,
+    );
+    expect(next_ivl_fuzz).toBeGreaterThanOrEqual(min_ivl);
+    expect(next_ivl_fuzz).toBeLessThanOrEqual(params.maximum_interval);
+  });
+});

package.json

@@ -1,6 +1,6 @@
 {
   "name": "ts-fsrs",
-  "version": "3.5.0",
+  "version": "3.5.1",
   "description": "ts-fsrs is a ES modules package based on TypeScript, used to implement the Free Spaced Repetition Scheduler (FSRS) algorithm. It helps developers apply FSRS to their flashcard applications, there by improving the user learning experience.",
   "main": "dist/index.cjs",
   "module": "dist/index.mjs",

src/fsrs/algorithm.ts

@@ -132,9 +132,9 @@ export class FSRSAlgorithm {
     elapsed_days: number,
     enable_fuzz: boolean = this.param.enable_fuzz,
   ): int {
-    const newInterval = Math.max(
-      1,
-      Math.round(s * this.intervalModifier),
+    const newInterval = Math.min(
+      Math.max(1, Math.round(s * this.intervalModifier)),
+      this.param.maximum_interval,
     ) as int;
     return this.apply_fuzz(newInterval, elapsed_days, enable_fuzz);
   }

src/fsrs/default.ts

@@ -9,7 +9,7 @@ export const default_w = [
 ];
 export const default_enable_fuzz = false;
 
-export const FSRSVersion: string = "3.5.0";
+export const FSRSVersion: string = "3.5.1";
 
 export const generatorParameters = (
   props?: Partial<FSRSParameters>,