export type CalendarMode = 'month' | 'week'

/** End times are exclusive; a slot ending at midnight does not occupy the next day. */
export function occursOnDate(startDate: string, endAt: string | undefined, date: string): boolean {
  if (date === startDate) return true
  if (date < startDate || !endAt) return false
  const endDate = endAt.slice(0, 10)
  return date < endDate || (date === endDate && /[1-9]/.test(endAt.slice(11)))
}

/** Calendar dates are warehouse-local dates, never UTC conversions. */
export function dateKey(date: Date): string {
  return `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, '0')}-${String(date.getDate()).padStart(2, '0')}`
}

function parseDate(value: string): Date {
  const date = new Date(`${value}T12:00:00`)
  if (!/^\d{4}-\d{2}-\d{2}$/.test(value) || Number.isNaN(date.getTime()) || dateKey(date) !== value) {
    throw new Error('无效的日历日期')
  }
  return date
}

export function addDays(value: string, days: number): string {
  const date = parseDate(value)
  date.setDate(date.getDate() + days)
  return dateKey(date)
}

export function calendarRange(anchor: string, mode: CalendarMode): { from: string; to: string } {
  const date = parseDate(anchor)
  const nextMonth = new Date(date.getFullYear(), date.getMonth() + 1, 1, 12)
  if (mode === 'month') date.setDate(1)
  date.setDate(date.getDate() - (date.getDay() + 6) % 7)
  const from = dateKey(date)
  // 月视图只补齐月末所在周，不追加整周的下月日期；结束日期仍为开区间。
  return { from, to: mode === 'month' ? addDays(dateKey(nextMonth), (8 - nextMonth.getDay()) % 7) : addDays(from, 7) }
}

export function calendarDates(anchor: string, mode: CalendarMode): string[] {
  const { from, to } = calendarRange(anchor, mode)
  const dates: string[] = []
  for (let date = from; date < to; date = addDays(date, 1)) dates.push(date)
  return dates
}

export function moveCalendar(anchor: string, mode: CalendarMode, offset: number): string {
  if (mode === 'week') return addDays(anchor, offset * 7)
  const date = parseDate(anchor)
  const day = date.getDate()
  date.setDate(1)
  date.setMonth(date.getMonth() + offset)
  const lastDay = new Date(date.getFullYear(), date.getMonth() + 1, 0).getDate()
  date.setDate(Math.min(day, lastDay))
  return dateKey(date)
}