/** 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 T+N：出账日 = 今天；纳入窗口为「今天 − lagDays」当天完成的订单
 */
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 };
}

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)),
  };
}