/**
 * 重复网格展开 - 移植自 ical_editor.py 的 occurrence 生成逻辑。
 * occurrence 按需从 exdates 计算, 不存储在模型上。
 */
import { toISODate, toDate, addDays } from './date.js'

/** 重复事件的间隔天数。 */
export function intervalDays(ev) {
  if (!ev.rrule) return 0
  const { freq, interval } = ev.rrule
  const step = interval || 1
  return freq === 'WEEKLY' ? 7 * step : step
}

const MAX_OCCURRENCES = 500

/**
 * 将重复事件展开为 occurrence 网格。
 * 返回 [{ date: 'YYYY-MM-DD', skipped: boolean }]。
 */
export function expandOccurrences(ev) {
  // 非重复事件: 单个 occurrence
  if (!ev.rrule) {
    return [{ date: ev.dtstartDate, skipped: false }]
  }

  const step = intervalDays(ev)
  if (step <= 0) {
    return [{ date: ev.dtstartDate, skipped: false }]
  }

  const until = ev.rrule.untilDate
    ? toDate(ev.rrule.untilDate)
    : new Date(toDate(ev.dtstartDate).getTime() + 365 * 24 * 3600 * 1000)

  const exdateSet = new Set(ev.exdates || [])
  const occs = []
  let cur = ev.dtstartDate
  let count = 0

  while (cur <= toISODate(until) && count < MAX_OCCURRENCES) {
    occs.push({ date: cur, skipped: exdateSet.has(cur) })
    cur = addDays(cur, step)
    count++
  }

  return occs
}