From 0eedbfd5b2331bccdf7687f14ee1904fdc78ce20 Mon Sep 17 00:00:00 2001
From: Rekty <rekty@users.noreply.github.com>
Date: Tue, 25 Aug 2026 14:08:39 +0700
Subject: [PATCH] test(common): pin the Beijing weekend shift with edge
 instants
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Closes #1090

The two weekend tests sat at 02:00Z, where the +8h shift and a plain
getUTCDay() agree — deleting the shift left the suite green. Export
isBeijingWeekend and pin its four edges (Fri 15:00Z/16:00Z, Sun
15:00Z/16:00Z), the only instants where the calendars disagree, so the
predicate can no longer silently regress to UTC days.
---
 .../src/__tests__/freebuff-peak-hours.test.ts  | 18 ++++++++++++++++++
 common/src/constants/freebuff-peak-hours.ts    |  9 ++++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/common/src/__tests__/freebuff-peak-hours.test.ts b/common/src/__tests__/freebuff-peak-hours.test.ts
index 72abc8f97d..527c227e63 100644
--- a/common/src/__tests__/freebuff-peak-hours.test.ts
+++ b/common/src/__tests__/freebuff-peak-hours.test.ts
@@ -4,6 +4,7 @@ import {
   DEEPSEEK_EXPENSIVE_WINDOW_UTC,
   deepSeekExpensiveWindowEndsAt,
   deepseekPricingWindow,
+  isBeijingWeekend,
   isDeepSeekExpensiveWindow,
 } from '../constants/freebuff-peak-hours'
 
@@ -72,3 +73,20 @@ test.each(['2026-08-29T02:00:00Z', '2026-08-30T02:00:00Z'])(
     expect(isDeepSeekExpensiveWindow(date)).toBe(false)
   },
 )
+
+describe('the Beijing weekend shift', () => {
+  // The Beijing weekend runs Fri 16:00Z -> Sun 16:00Z, a band where every UTC
+  // hour is off-peak anyway — so the window functions cannot see the +8h shift
+  // and neither can the weekend tests above (they sit at 02:00Z, inside the
+  // agreement band of both calendars). These instants sit exactly on the four
+  // edges, where the shift and a plain getUTCDay() disagree; only the
+  // predicate itself can tell them apart.
+  test.each([
+    ['2026-08-28T15:00:00Z', false], // Fri 23:00 Beijing — last weekday hour
+    ['2026-08-28T16:00:00Z', true], // Sat 00:00 Beijing — weekend opens
+    ['2026-08-30T15:00:00Z', true], // Sun 23:00 Beijing — still weekend
+    ['2026-08-30T16:00:00Z', false], // Mon 00:00 Beijing — weekend closed
+  ])('%s is weekend=%p in Beijing', (instant, weekend) => {
+    expect(isBeijingWeekend(new Date(instant))).toBe(weekend as boolean)
+  })
+})
diff --git a/common/src/constants/freebuff-peak-hours.ts b/common/src/constants/freebuff-peak-hours.ts
index 99c92a6ec5..6b657b07f0 100644
--- a/common/src/constants/freebuff-peak-hours.ts
+++ b/common/src/constants/freebuff-peak-hours.ts
@@ -29,7 +29,14 @@ export const DEEPSEEK_PEAK_HOUR_RANGES_UTC: ReadonlyArray<
 
 export type DeepSeekPricingWindow = 'peak' | 'off-peak'
 
-function isBeijingWeekend(at: Date): boolean {
+/**
+ * Whether `at` falls on a Saturday or Sunday in Beijing time (UTC+8).
+ *
+ * Exported for tests: the Beijing weekend runs Fri 16:00Z -> Sun 16:00Z, a
+ * band the peak-hour windows never touch, so only this predicate can pin the
+ * shift directly.
+ */
+export function isBeijingWeekend(at: Date): boolean {
   const beijingDay = new Date(at.getTime() + 8 * 60 * 60 * 1000).getUTCDay()
   return beijingDay === 0 || beijingDay === 6
 }