refactor(notes): extract card generation into shared module

Pull pure card-generation logic (template rendering and the initial FSRS
state for new cards) out of the server note repository into
src/shared/card-generator.ts so both server and client can call it. The
server's note.create / createMany now delegate the rendering work to the
shared function and only insert the resulting payload, preserving the
existing behavior. Groundwork for offline-first note CRUD on the client.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

1 files changed, 146 insertions, 0 deletions

diff --git a/src/shared/card-generator.test.ts b/src/shared/card-generator.test.ts
new file mode 100644
index 0000000..8c78ca5
--- /dev/null
+++ b/src/shared/card-generator.test.ts
@@ -0,0 +1,146 @@
+import { describe, expect, it } from "vitest";
+import { generateCardsForNote, renderCardTemplate } from "./card-generator";
+
+describe("renderCardTemplate", () => {
+	it("substitutes a single placeholder", () => {
+		const fields = new Map([["Front", "Hello"]]);
+		expect(renderCardTemplate("{{Front}}", fields)).toBe("Hello");
+	});
+
+	it("substitutes multiple placeholders", () => {
+		const fields = new Map([
+			["Front", "Q"],
+			["Back", "A"],
+		]);
+		expect(renderCardTemplate("{{Front}} -> {{Back}}", fields)).toBe("Q -> A");
+	});
+
+	it("substitutes the same placeholder repeated in a template", () => {
+		const fields = new Map([["Word", "x"]]);
+		expect(renderCardTemplate("{{Word}}{{Word}}", fields)).toBe("xx");
+	});
+
+	it("leaves unknown placeholders unchanged", () => {
+		const fields = new Map([["Front", "Q"]]);
+		expect(renderCardTemplate("{{Front}} {{Missing}}", fields)).toBe(
+			"Q {{Missing}}",
+		);
+	});
+
+	it("replaces placeholders with empty strings when value is empty", () => {
+		const fields = new Map([["Front", ""]]);
+		expect(renderCardTemplate("[{{Front}}]", fields)).toBe("[]");
+	});
+});
+
+const noteTypeBase = {
+	frontTemplate: "{{Front}}",
+	backTemplate: "{{Back}}",
+	isReversible: false,
+};
+
+const fieldTypes = [
+	{ id: "ft-front", name: "Front" },
+	{ id: "ft-back", name: "Back" },
+];
+
+const fieldValues = [
+	{ noteFieldTypeId: "ft-front", value: "Question" },
+	{ noteFieldTypeId: "ft-back", value: "Answer" },
+];
+
+describe("generateCardsForNote", () => {
+	it("returns one card when isReversible is false", () => {
+		const cards = generateCardsForNote({
+			noteType: noteTypeBase,
+			fieldTypes,
+			fieldValues,
+		});
+
+		expect(cards).toHaveLength(1);
+		expect(cards[0]).toMatchObject({
+			isReversed: false,
+			front: "Question",
+			back: "Answer",
+		});
+	});
+
+	it("returns two cards when isReversible is true and swaps templates", () => {
+		const cards = generateCardsForNote({
+			noteType: { ...noteTypeBase, isReversible: true },
+			fieldTypes,
+			fieldValues,
+		});
+
+		expect(cards).toHaveLength(2);
+		expect(cards[0]).toMatchObject({
+			isReversed: false,
+			front: "Question",
+			back: "Answer",
+		});
+		expect(cards[1]).toMatchObject({
+			isReversed: true,
+			front: "Answer",
+			back: "Question",
+		});
+	});
+
+	it("initialises FSRS state to a fresh new-card payload", () => {
+		const now = new Date("2026-05-02T10:00:00Z");
+		const [card] = generateCardsForNote({
+			noteType: noteTypeBase,
+			fieldTypes,
+			fieldValues,
+			now,
+		});
+
+		expect(card).toMatchObject({
+			state: 0,
+			stability: 0,
+			difficulty: 0,
+			elapsedDays: 0,
+			scheduledDays: 0,
+			reps: 0,
+			lapses: 0,
+		});
+		expect(card?.due.getTime()).toBe(now.getTime());
+	});
+
+	it("treats missing field values as empty strings", () => {
+		const [card] = generateCardsForNote({
+			noteType: noteTypeBase,
+			fieldTypes,
+			fieldValues: [{ noteFieldTypeId: "ft-front", value: "Q only" }],
+		});
+
+		expect(card?.front).toBe("Q only");
+		expect(card?.back).toBe("{{Back}}");
+	});
+
+	it("ignores field values with unknown field type ids", () => {
+		const [card] = generateCardsForNote({
+			noteType: noteTypeBase,
+			fieldTypes,
+			fieldValues: [
+				...fieldValues,
+				{ noteFieldTypeId: "ft-unknown", value: "ghost" },
+			],
+		});
+
+		expect(card?.front).toBe("Question");
+		expect(card?.back).toBe("Answer");
+	});
+
+	it("returns independent due Date instances per card", () => {
+		const now = new Date("2026-05-02T10:00:00Z");
+		const cards = generateCardsForNote({
+			noteType: { ...noteTypeBase, isReversible: true },
+			fieldTypes,
+			fieldValues,
+			now,
+		});
+
+		expect(cards[0]?.due).not.toBe(cards[1]?.due);
+		expect(cards[0]?.due).not.toBe(now);
+	});
+});