// 前端日期/周期工具: 与 server/checkin/period.js 保持同一套口径 (Asia/Shanghai)
export const TZ = 'Asia/Shanghai'

const FMT = new Intl.DateTimeFormat('en-CA', {
  timeZone: TZ, year: 'numeric', month: '2-digit', day: '2-digit',
})

/** 任意时间 -> 本地日历日 "YYYY-MM-DD" */
export function toDateStr(d = new Date()) {
  return FMT.format(d instanceof Date ? d : new Date(d))
}

function toUTC(s) {
  const [y, m, d] = String(s).split('-').map(Number)
  return new Date(Date.UTC(y, (m || 1) - 1, d || 1))
}

function fromUTC(dt) {
  return dt.toISOString().slice(0, 10)
}

export function addDays(s, n) {
  const d = toUTC(s)
  d.setUTCDate(d.getUTCDate() + n)
  return fromUTC(d)
}

export function addMonths(s, n) {
  const [y, m] = String(s).split('-').map(Number)
  const first = new Date(Date.UTC(y, (m || 1) - 1 + n, 1))
  const day = Number(String(s).slice(8, 10)) || 1
  const lastDay = new Date(Date.UTC(first.getUTCFullYear(), first.getUTCMonth() + 1, 0)).getUTCDate()
  first.setUTCDate(Math.min(day, lastDay))
  return fromUTC(first)
}

/** 周一为一周第一天 */
export function startOfWeek(s) {
  const dow = (toUTC(s).getUTCDay() + 6) % 7
  return addDays(s, -dow)
}

export const PERIOD_LABEL = { day: '每天', week: '每周', month: '每月', year: '每年' }

export function periodRange(period, s) {
  switch (period) {
    case 'week': {
      const start = startOfWeek(s)
      return { start, end: addDays(start, 6) }
    }
    case 'month': {
      const start = String(s).slice(0, 8) + '01'
      const [y, m] = String(s).split('-').map(Number)
      return { start, end: fromUTC(new Date(Date.UTC(y, m || 1, 0))) }
    }
    case 'year': {
      const y = String(s).slice(0, 4)
      return { start: y + '-01-01', end: y + '-12-31' }
    }
    default: return { start: s, end: s }
  }
}

/** 月历网格: 返回 6*7 = 42 天 (含前后月的补位), 周一起始 */
export function monthGrid(monthStr) {
  const first = monthStr + '-01'
  const gridStart = startOfWeek(first)
  const days = []
  for (let i = 0; i < 42; i++) days.push(addDays(gridStart, i))
  return days
}

export function weekdayLabels() {
  return ['一', '二', '三', '四', '五', '六', '日']
}

/** 相对今天的文案: 昨天 / 今天 / 明天 / MM-DD */
export function dayLabel(dateStr, today = toDateStr()) {
  if (dateStr === today) return '今天'
  if (dateStr === addDays(today, -1)) return '昨天'
  if (dateStr === addDays(today, 1)) return '明天'
  return dateStr.slice(5).replace('-', '/')
}

export function fmtMonth(monthStr) {
  const [y, m] = monthStr.split('-')
  return y + '年' + Number(m) + '月'
}