2 files changed, 164 insertions, 0 deletions
diff --git a/src/shared/fsrs.test.ts b/src/shared/fsrs.test.ts
new file mode 100644
index 0000000..83f2309
--- /dev/null
+++ b/src/shared/fsrs.test.ts
@@ -0,0 +1,90 @@
+import { describe, expect, it } from "vitest";
+import { computeNextSchedule, type ScheduleInput } from "./fsrs";
+
+const baseCard: ScheduleInput = {
+	state: 0,
+	due: new Date("2026-05-01T00:00:00Z"),
+	stability: 0,
+	difficulty: 0,
+	elapsedDays: 0,
+	scheduledDays: 0,
+	reps: 0,
+	lapses: 0,
+	lastReview: null,
+};
+
+describe("computeNextSchedule", () => {
+	it("schedules a new card forward when rated Good", () => {
+		const now = new Date("2026-05-02T10:00:00Z");
+		const result = computeNextSchedule(baseCard, 3, now);
+
+		expect(result.reps).toBe(1);
+		expect(result.lastReview.getTime()).toBe(now.getTime());
+		expect(result.due.getTime()).toBeGreaterThan(now.getTime());
+		expect(result.stability).toBeGreaterThan(0);
+		expect(result.difficulty).toBeGreaterThan(0);
+	});
+
+	it("counts a lapse when a Review-state card is rated Again", () => {
+		const card: ScheduleInput = {
+			state: 2,
+			due: new Date("2026-05-01T00:00:00Z"),
+			stability: 10,
+			difficulty: 5,
+			elapsedDays: 5,
+			scheduledDays: 5,
+			reps: 3,
+			lapses: 0,
+			lastReview: new Date("2026-04-26T00:00:00Z"),
+		};
+		const now = new Date("2026-05-02T10:00:00Z");
+		const result = computeNextSchedule(card, 1, now);
+
+		expect(result.lapses).toBe(1);
+	});
+
+	it("computes reviewElapsedDays from previous lastReview", () => {
+		const lastReview = new Date("2026-04-29T00:00:00Z");
+		const card: ScheduleInput = {
+			...baseCard,
+			state: 2,
+			stability: 5,
+			difficulty: 5,
+			lastReview,
+			reps: 1,
+		};
+		const now = new Date("2026-05-02T00:00:00Z");
+		const result = computeNextSchedule(card, 3, now);
+
+		expect(result.reviewElapsedDays).toBe(3);
+	});
+
+	it("uses 0 reviewElapsedDays for a card without lastReview", () => {
+		const now = new Date("2026-05-02T00:00:00Z");
+		const result = computeNextSchedule(baseCard, 3, now);
+
+		expect(result.reviewElapsedDays).toBe(0);
+	});
+
+	it("higher ratings yield longer scheduled intervals than lower ratings", () => {
+		const card: ScheduleInput = {
+			state: 2,
+			due: new Date("2026-05-01T00:00:00Z"),
+			stability: 10,
+			difficulty: 5,
+			elapsedDays: 10,
+			scheduledDays: 10,
+			reps: 5,
+			lapses: 0,
+			lastReview: new Date("2026-04-21T00:00:00Z"),
+		};
+		const now = new Date("2026-05-02T00:00:00Z");
+
+		const hard = computeNextSchedule(card, 2, now);
+		const good = computeNextSchedule(card, 3, now);
+		const easy = computeNextSchedule(card, 4, now);
+
+		expect(easy.scheduledDays).toBeGreaterThanOrEqual(good.scheduledDays);
+		expect(good.scheduledDays).toBeGreaterThanOrEqual(hard.scheduledDays);
+	});
+});
diff --git a/src/shared/fsrs.ts b/src/shared/fsrs.ts
new file mode 100644
index 0000000..1cf60ba
--- /dev/null
+++ b/src/shared/fsrs.ts
@@ -0,0 +1,74 @@
+import {
+	type Card as FSRSCard,
+	type State as FSRSState,
+	fsrs,
+	type Grade,
+} from "ts-fsrs";
+
+const f = fsrs({ enable_fuzz: true });
+
+export interface ScheduleInput {
+	state: number;
+	due: Date;
+	stability: number;
+	difficulty: number;
+	elapsedDays: number;
+	scheduledDays: number;
+	reps: number;
+	lapses: number;
+	lastReview: Date | null;
+}
+
+export interface ScheduleResult {
+	state: number;
+	due: Date;
+	stability: number;
+	difficulty: number;
+	elapsedDays: number;
+	scheduledDays: number;
+	reps: number;
+	lapses: number;
+	lastReview: Date;
+	/** Days elapsed since the previous review (for ReviewLog). */
+	reviewElapsedDays: number;
+}
+
+export function computeNextSchedule(
+	card: ScheduleInput,
+	rating: 1 | 2 | 3 | 4,
+	now: Date,
+): ScheduleResult {
+	const fsrsCard: FSRSCard = {
+		due: card.due,
+		stability: card.stability,
+		difficulty: card.difficulty,
+		elapsed_days: card.elapsedDays,
+		scheduled_days: card.scheduledDays,
+		reps: card.reps,
+		lapses: card.lapses,
+		state: card.state as FSRSState,
+		last_review: card.lastReview ?? undefined,
+		learning_steps: 0,
+	};
+
+	const result = f.next(fsrsCard, now, rating as Grade);
+
+	const reviewElapsedDays = card.lastReview
+		? Math.round(
+				(now.getTime() - card.lastReview.getTime()) / (1000 * 60 * 60 * 24),
+			)
+		: 0;
+
+	return {
+		state: result.card.state,
+		due: result.card.due,
+		stability: result.card.stability,
+		difficulty: result.card.difficulty,
+		elapsedDays: result.card.elapsed_days,
+		scheduledDays: result.card.scheduled_days,
+		reps: result.card.reps,
+		lapses: result.card.lapses,
+		lastReview: now,
+		reviewElapsedDays,
+	};
+}