From 8f1a08fefee3a8e928baec741c830a88a4cd7200 Mon Sep 17 00:00:00 2001
From: nsfisis <nsfisis@gmail.com>
Date: Sat, 2 May 2026 10:41:12 +0900
Subject: feat(study): submit reviews offline via IndexedDB

Move FSRS scheduling to a shared module so the client can compute next
card state without contacting the server. StudyPage now writes the
updated card and review log straight to IndexedDB and lets the existing
sync engine push them on reconnect, instead of POSTing to
/api/decks/:deckId/study/:cardId. Online sessions still trigger a sync
immediately so server-side aggregates stay fresh; offline sessions
accumulate in pendingCountAtom until the next online tick.

The legacy study POST route is preserved for backwards compatibility.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
---
 src/shared/fsrs.ts | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 74 insertions(+)
 create mode 100644 src/shared/fsrs.ts

(limited to 'src/shared/fsrs.ts')

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,
+	};
+}
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