feat: add weekday constants and helpers

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')
+  })
+})