test: add isoWeekToMonday boundary tests and fix render test mock

- Extract isoWeekToMonday to deployer/week.ts for reuse and testability
- Add boundary value tests: year boundaries (W1 2025/2026), W53 (2020),
  Jan 1 Monday (2024 W1), last week of year (W52 2026)
- Fix render.test.ts mock to pass through real isoWeekToMonday
- Verify card dateRange E2E: weekId -> isoWeekToMonday -> formatted output

Author: unhappychoice

src/cli/commands/render.test.ts

@@ -47,10 +47,14 @@ vi.mock("../../renderer/rss.js", () => ({
   buildRSSFeed: (...args: unknown[]) => mockBuildRSSFeed(...args),
 }));
 
-// Mock week
-vi.mock("../../deployer/week.js", () => ({
-  getWeekId: () => ({ year: 2026, week: 14, path: "2026/W14" }),
-}));
+// Mock week (keep real isoWeekToMonday)
+vi.mock("../../deployer/week.js", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("../../deployer/week.js")>();
+  return {
+    ...actual,
+    getWeekId: () => ({ year: 2026, week: 14, path: "2026/W14" }),
+  };
+});
 
 const GITHUB_DATA_YAML = `
 username: testuser

src/cli/commands/render.ts

@@ -6,7 +6,7 @@ import { join } from "node:path";
 import { parse as parseYaml } from "yaml";
 import { renderReport } from "../../renderer/index.js";
 import { renderIndexPage, buildReportEntry, type ReportEntry } from "../../deployer/index-page.js";
-import { getWeekId } from "../../deployer/week.js";
+import { getWeekId, isoWeekToMonday } from "../../deployer/week.js";
 import { parseLocalDate } from "../../collector/date-range.js";
 import { generateOGImage, generateIndexOGImage } from "../../renderer/og-image.js";
 import { generateCard, generateDarkCard } from "../../renderer/card.js";
@@ -175,12 +175,6 @@ const run = async (options: RenderOptions): Promise<void> => {
 
   // Generate animated SVG summary cards (light + dark)
   // Compute Mon-Sun date range from ISO week number (independent of data file)
-  const isoWeekToMonday = (year: number, week: number): Date => {
-    const jan4 = new Date(Date.UTC(year, 0, 4));
-    const dow = jan4.getUTCDay() || 7;
-    const w1Mon = new Date(jan4.getTime() - (dow - 1) * 86_400_000);
-    return new Date(w1Mon.getTime() + (week - 1) * 7 * 86_400_000);
-  };
   const fmtShort = (d: Date): string =>
     d.toLocaleDateString("en-US", { month: "short", day: "numeric", timeZone: "UTC" });
   const monday = isoWeekToMonday(weekId.year, weekId.week);

src/deployer/week.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from "vitest";
-import { getWeekId, getCurrentWeekId } from "./week.js";
+import { getWeekId, getCurrentWeekId, isoWeekToMonday } from "./week.js";
 
 describe("getWeekId", () => {
   // -------------------------------------------------------------------
@@ -220,3 +220,63 @@ describe("getCurrentWeekId", () => {
     expect(result.path).toBe("2026/W01");
   });
 });
+
+describe("isoWeekToMonday", () => {
+  const dayOfWeek = (d: Date): string =>
+    d.toLocaleDateString("en-US", { weekday: "long", timeZone: "UTC" });
+  const fmt = (d: Date): string =>
+    d.toISOString().slice(0, 10);
+
+  it("returns Monday for 2026 W14 (Mar 30)", () => {
+    const mon = isoWeekToMonday(2026, 14);
+    expect(fmt(mon)).toBe("2026-03-30");
+    expect(dayOfWeek(mon)).toBe("Monday");
+  });
+
+  it("returns Monday for 2026 W1 (Dec 29, 2025)", () => {
+    // ISO 2026 W1 starts on Mon Dec 29 2025
+    const mon = isoWeekToMonday(2026, 1);
+    expect(fmt(mon)).toBe("2025-12-29");
+    expect(dayOfWeek(mon)).toBe("Monday");
+  });
+
+  it("returns Monday for 2025 W1 (Dec 30, 2024)", () => {
+    const mon = isoWeekToMonday(2025, 1);
+    expect(fmt(mon)).toBe("2024-12-30");
+    expect(dayOfWeek(mon)).toBe("Monday");
+  });
+
+  it("handles 2024 W1 (Jan 1, 2024 is Monday)", () => {
+    const mon = isoWeekToMonday(2024, 1);
+    expect(fmt(mon)).toBe("2024-01-01");
+    expect(dayOfWeek(mon)).toBe("Monday");
+  });
+
+  it("handles last week of year: 2026 W52", () => {
+    const mon = isoWeekToMonday(2026, 52);
+    expect(fmt(mon)).toBe("2026-12-21");
+    expect(dayOfWeek(mon)).toBe("Monday");
+  });
+
+  it("handles W53 in a long year (2020 has 53 weeks)", () => {
+    const mon = isoWeekToMonday(2020, 53);
+    expect(fmt(mon)).toBe("2020-12-28");
+    expect(dayOfWeek(mon)).toBe("Monday");
+  });
+
+  it("Sunday is always 6 days after Monday", () => {
+    const cases = [
+      { year: 2026, week: 1 },
+      { year: 2026, week: 14 },
+      { year: 2026, week: 52 },
+      { year: 2020, week: 53 },
+      { year: 2024, week: 1 },
+    ];
+    cases.forEach(({ year, week }) => {
+      const mon = isoWeekToMonday(year, week);
+      const sun = new Date(mon.getTime() + 6 * 86_400_000);
+      expect(dayOfWeek(mon)).toBe("Monday");
+      expect(dayOfWeek(sun)).toBe("Sunday");
+    });
+  });
+});

src/deployer/week.ts

@@ -34,6 +34,14 @@ export const getWeekId = (
 
 // Current ISO week ID. Used by daily-fetch to store events for the
 // week that is still in progress.
+// Monday (00:00 UTC) of the given ISO week. Week 1 contains January 4.
+export const isoWeekToMonday = (year: number, week: number): Date => {
+  const jan4 = new Date(Date.UTC(year, 0, 4));
+  const dow = jan4.getUTCDay() || 7;
+  const w1Mon = new Date(jan4.getTime() - (dow - 1) * 86_400_000);
+  return new Date(w1Mon.getTime() + (week - 1) * 7 * 86_400_000);
+};
+
 export const getCurrentWeekId = (
   date: Date = new Date(),
   timezone: string = "UTC",