feat: add shared recurrence helpers

tests/recurrence.test.ts

@@ -0,0 +1,83 @@
+import { describe, expect, test } from "bun:test";
+import {
+	formatRecurrenceText,
+	getRecurrencePreview,
+	parseRecurrenceRule,
+	serializeRecurrenceRule,
+	validateRecurrence,
+} from "@/lib/recurrence";
+
+describe("recurrence helpers", () => {
+	test("serializes a weekly weekday recurrence into an RFC5545 RRULE string", () => {
+		const rule = serializeRecurrenceRule({
+			freq: "WEEKLY",
+			interval: 2,
+			byDay: ["TU", "TH"],
+			until: "2026-06-30",
+		});
+
+		expect(rule).toBe("FREQ=WEEKLY;INTERVAL=2;UNTIL=20260630T235959Z;BYDAY=TU,TH");
+	});
+
+	test("rehydrates an existing RRULE string into picker-friendly values", () => {
+		const parsed = parseRecurrenceRule(
+			"FREQ=WEEKLY;INTERVAL=1;BYDAY=MO,WE,FR;COUNT=6",
+		);
+
+		expect(parsed).toEqual({
+			freq: "WEEKLY",
+			interval: 1,
+			byDay: ["MO", "WE", "FR"],
+			count: 6,
+			until: undefined,
+		});
+	});
+
+	test("returns a preview of upcoming recurrence instances from the public helper", () => {
+		const preview = getRecurrencePreview(
+			{
+				freq: "MONTHLY",
+				interval: 1,
+				byDay: [],
+				count: 3,
+			},
+			"2026-04-09T10:00:00.000Z",
+		);
+
+		expect(preview).toHaveLength(3);
+		expect(preview[0]).toContain("2026-04-09T10:00:00.000Z");
+		expect(preview[1]).toContain("2026-05-09T10:00:00.000Z");
+		expect(preview[2]).toContain("2026-06-09T10:00:00.000Z");
+	});
+
+	test("rejects invalid recurrence endings that mix count and until", () => {
+		const result = validateRecurrence({
+			freq: "DAILY",
+			interval: 1,
+			byDay: [],
+			count: 5,
+			until: "2026-05-01",
+		});
+
+		expect(result.isValid).toBe(false);
+		expect(result.errors.rule).toContain("either count or until");
+	});
+
+	test("rejects malformed count inputs", () => {
+		const result = validateRecurrence({
+			freq: "MONTHLY",
+			interval: 1,
+			byDay: [],
+			count: 0,
+		});
+
+		expect(result.isValid).toBe(false);
+		expect(result.errors.count).toContain("greater than 0");
+	});
+
+	test("formats supported recurrence strings into human text", () => {
+		expect(formatRecurrenceText("FREQ=WEEKLY;INTERVAL=1;BYDAY=MO,WE")).toContain(
+			"every week",
+		);
+	});
+});