/** 财务账单日 / 账期一律按中国北京时间（Asia/Shanghai，无夏令时） */

export const SHANGHAI_TIME_ZONE = 'Asia/Shanghai';

function pad2(n: number): string {
  return String(n).padStart(2, '0');
}

export function shanghaiYmd(d: Date): string {
  return d.toLocaleDateString('sv-SE', { timeZone: SHANGHAI_TIME_ZONE });
}

export function parseShanghaiYmd(ymd: string): Date {
  return new Date(`${ymd}T00:00:00+08:00`);
}

export function startOfShanghaiDay(d: Date = new Date()): Date {
  return parseShanghaiYmd(shanghaiYmd(d));
}

export function addShanghaiDays(d: Date, days: number): Date {
  const [y, m, day] = shanghaiYmd(startOfShanghaiDay(d)).split('-').map(Number);
  const utc = new Date(Date.UTC(y, m - 1, day + days));
  const ymd = `${utc.getUTCFullYear()}-${pad2(utc.getUTCMonth() + 1)}-${pad2(utc.getUTCDate())}`;
  return new Date(`${ymd}T00:00:00+08:00`);
}

/** 门店 T+1：核销窗口 [昨日 00:00, 今日 00:00)；出账日 = 今天 */
export function shanghaiT1DayWindow(anchor = new Date()) {
  const billDate = startOfShanghaiDay(anchor);
  const start = addShanghaiDays(billDate, -1);
  return { start, end: billDate, billDate };
}

/**
 * 酒厂日滞后窗口（旧：每天出账只收 lagDays 前那一天）。
 * 现网酒厂已改为「每 periodDays 天出一期」，见 shanghaiWineryPeriodWindow。
 */
export function shanghaiLaggedIssueWindow(anchor = new Date(), lagDays: number) {
  const billDate = startOfShanghaiDay(anchor);
  const start = addShanghaiDays(billDate, -lagDays);
  const end = addShanghaiDays(start, 1);
  return { start, end, billDate };
}

/** 两个北京自然日之间相差天数（b − a；可为负） */
export function shanghaiCalendarDaysBetween(a: Date, b: Date): number {
  const sa = startOfShanghaiDay(a).getTime();
  const sb = startOfShanghaiDay(b).getTime();
  return Math.round((sb - sa) / (24 * 60 * 60 * 1000));
}

/** 是否酒厂出账日：相对 epoch，每 periodDays 天一次（首张 = epoch + periodDays） */
export function isWineryIssueDay(anchor: Date, periodDays: number, epochYmd: string): boolean {
  const billDate = startOfShanghaiDay(anchor);
  const epoch = parseShanghaiYmd(epochYmd);
  const diff = shanghaiCalendarDaysBetween(epoch, billDate);
  return diff >= periodDays && diff % periodDays === 0;
}

/**
 * 酒厂 T+N 期账单窗口：出账日 = anchor 当天；纳入 [出账日 − periodDays, 出账日) 内完成的订单。
 * 调用方应先用 isWineryIssueDay 判断是否出账日。
 */
export function shanghaiWineryPeriodWindow(anchor = new Date(), periodDays: number) {
  const billDate = startOfShanghaiDay(anchor);
  const start = addShanghaiDays(billDate, -periodDays);
  return { start, end: billDate, billDate };
}

/** 订单完成日 → 所属酒厂出账日（落入下一期出账日） */
export function wineryIssueDateFromCompletedAt(
  completedAt: Date,
  periodDays: number,
  epochYmd: string,
) {
  const c = startOfShanghaiDay(completedAt);
  const epoch = parseShanghaiYmd(epochYmd);
  const daysFromEpoch = shanghaiCalendarDaysBetween(epoch, c);
  if (daysFromEpoch < 0) {
    return addShanghaiDays(epoch, periodDays);
  }
  const k = Math.floor(daysFromEpoch / periodDays) + 1;
  return addShanghaiDays(epoch, k * periodDays);
}

/** 枚举 [from, to] 内（含）的全部酒厂出账日 */
export function listWineryIssueDays(from: Date, to: Date, periodDays: number, epochYmd: string): Date[] {
  const start = startOfShanghaiDay(from);
  const end = startOfShanghaiDay(to);
  const out: Date[] = [];
  for (let d = start; d.getTime() <= end.getTime(); d = addShanghaiDays(d, 1)) {
    if (isWineryIssueDay(d, periodDays, epochYmd)) out.push(d);
  }
  return out;
}

export function shanghaiMonthRange(year: number, month: number): { start: Date; endExclusive: Date } {
  const start = new Date(`${year}-${pad2(month)}-01T00:00:00+08:00`);
  const endMonth = month === 12 ? 1 : month + 1;
  const endYear = month === 12 ? year + 1 : year;
  const endExclusive = new Date(`${endYear}-${pad2(endMonth)}-01T00:00:00+08:00`);
  return { start, endExclusive };
}

export function shanghaiMonthLastInstant(year: number, month: number): Date {
  return new Date(shanghaiMonthRange(year, month).endExclusive.getTime() - 1);
}

export function previousShanghaiMonth(anchor = new Date()): { year: number; month: number } {
  const [y, m] = shanghaiYmd(anchor).split('-').map(Number);
  if (m === 1) return { year: y - 1, month: 12 };
  return { year: y, month: m - 1 };
}

export function shanghaiYearMonth(anchor = new Date()): { year: number; month: number } {
  const [y, m] = shanghaiYmd(anchor).split('-').map(Number);
  return { year: y, month: m };
}

/** 账期展示：以 periodStart 所在北京月的首末日，避免 UTC 23:59:59 滚到下月 */
export function shanghaiPeriodYmds(periodStart: Date): { periodStart: string; periodEnd: string } {
  const [y, m] = shanghaiYmd(periodStart).split('-').map(Number);
  const { start, endExclusive } = shanghaiMonthRange(y, m);
  return {
    periodStart: shanghaiYmd(start),
    periodEnd: shanghaiYmd(new Date(endExclusive.getTime() - 1)),
  };
}