/** 中国星期名称 */
const WEEKDAY_CN = ["周日", "周一", "周二", "周三", "周四", "周五", "周六"] as const;

/** 中国时区 */
const TZ = "Asia/Shanghai";

/**
 * 获取中国时区下的日期数字（月、日、星期）
 */
function getChinaDateParts(): { year: number; month: number; day: number; weekday: number } {
  const now = new Date();
  const formatter = new Intl.DateTimeFormat("en-CA", {
    timeZone: TZ,
    year: "numeric",
    month: "numeric",
    day: "numeric",
    weekday: "short",
  });
  const parts = formatter.formatToParts(now);
  const get = (type: string) => parseInt(parts.find((p) => p.type === type)?.value ?? "0", 10);
  const weekdayStr = parts.find((p) => p.type === "weekday")?.value ?? "Sun";
  const weekdayMap: Record<string, number> = { Sun: 0, Mon: 1, Tue: 2, Wed: 3, Thu: 4, Fri: 5, Sat: 6 };
  return {
    year: get("year"),
    month: get("month"),
    day: get("day"),
    weekday: weekdayMap[weekdayStr] ?? 0,
  };
}

/**
 * 格式化为中文日期：M月D日 周X
 */
function formatDateCN(month: number, day: number, weekday: number): string {
  return `${month}月${day}日 ${WEEKDAY_CN[weekday]}`;
}

/**
 * 获取下一个周六、周日的日期（中国日历）
 * session-1 对应周六，session-2 对应周日
 */
export function getNextWeekendDates(): { saturday: string; sunday: string } {
  const { year, month, day, weekday } = getChinaDateParts();

  let satM = month;
  let satD = day;
  let satW = 6;
  let sunM = month;
  let sunD = day;
  let sunW = 0;

  if (weekday === 0) {
    // 今天周日：下周六 +6 天，下周日 +7 天
    const d = new Date(year, month - 1, day);
    d.setDate(d.getDate() + 6);
    satM = d.getMonth() + 1;
    satD = d.getDate();
    d.setDate(d.getDate() + 1);
    sunM = d.getMonth() + 1;
    sunD = d.getDate();
  } else if (weekday === 6) {
    // 今天周六：明天周日
    const d = new Date(year, month - 1, day);
    d.setDate(d.getDate() + 1);
    sunM = d.getMonth() + 1;
    sunD = d.getDate();
  } else {
    // 周一至周五
    const daysToSat = 6 - weekday;
    const d = new Date(year, month - 1, day);
    d.setDate(d.getDate() + daysToSat);
    satM = d.getMonth() + 1;
    satD = d.getDate();
    d.setDate(d.getDate() + 1);
    sunM = d.getMonth() + 1;
    sunD = d.getDate();
  }

  return {
    saturday: formatDateCN(satM, satD, 6),
    sunday: formatDateCN(sunM, sunD, 0),
  };
}