feat: add weekday constants and helpers

src/lib/weekday.js

@@ -0,0 +1,17 @@
+// JS getDay() index: 0=Sunday .. 6=Saturday
+export const DOW = ['周日', '周一', '周二', '周三', '周四', '周五', '周六']
+
+// BYDAY codes indexed by JS getDay(): DOW_EN[getDay()] => 'SU'..'SA'
+export const DOW_EN = ['SU', 'MO', 'TU', 'WE', 'TH', 'FR', 'SA']
+
+export const BYDAY_TO_INDEX = { SU: 0, MO: 1, TU: 2, WE: 3, TH: 4, FR: 5, SA: 6 }
+
+/** Return the BYDAY code ('SU'..'SA') for a JS Date. */
+export function bydayFromDate(date) {
+  return DOW_EN[date.getDay()]
+}
+
+/** Return the Chinese weekday label for a JS Date. */
+export function dowLabel(date) {
+  return DOW[date.getDay()]
+}

test/weekday.test.js

@@ -0,0 +1,31 @@
+import { describe, it, expect } from 'vitest'
+import { DOW, DOW_EN, BYDAY_TO_INDEX, bydayFromDate, dowLabel } from '../src/lib/weekday.js'
+
+describe('weekday', () => {
+  it('DOW has 7 entries starting Sunday', () => {
+    expect(DOW).toHaveLength(7)
+    expect(DOW[0]).toBe('周日')
+    expect(DOW[6]).toBe('周六')
+  })
+
+  it('DOW_EN indexed by JS getDay (0=Sunday)', () => {
+    expect(DOW_EN).toEqual(['SU', 'MO', 'TU', 'WE', 'TH', 'FR', 'SA'])
+  })
+
+  it('BYDAY_TO_INDEX maps SU..SA to 0..6', () => {
+    expect(BYDAY_TO_INDEX.SU).toBe(0)
+    expect(BYDAY_TO_INDEX.SA).toBe(6)
+  })
+
+  it('bydayFromDate returns BYDAY code for a Date', () => {
+    // 2026-07-27 is a Monday -> getDay()=1 -> 'MO'
+    expect(bydayFromDate(new Date(2026, 6, 27))).toBe('MO')
+    // 2026-08-01 is a Saturday -> getDay()=6 -> 'SA'
+    expect(bydayFromDate(new Date(2026, 7, 1))).toBe('SA')
+  })
+
+  it('dowLabel returns Chinese label for a Date', () => {
+    expect(dowLabel(new Date(2026, 6, 27))).toBe('周一')
+    expect(dowLabel(new Date(2026, 6, 26))).toBe('周日')
+  })
+})