1 files changed, 90 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);
+	});
+});