// I-3 時間感知：所有作息判斷一律以 Asia/Taipei 當地時間為準，不受伺服器所在時區影響。
const TAIPEI_PARTS_FORMATTER = new Intl.DateTimeFormat("en-US", {
  timeZone: "Asia/Taipei",
  weekday: "short",
  hour: "2-digit",
  minute: "2-digit",
  hour12: false,
  year: "numeric",
  month: "2-digit",
  day: "2-digit",
});

const WEEKDAY_INDEX: Record<string, number> = {
  Sun: 0,
  Mon: 1,
  Tue: 2,
  Wed: 3,
  Thu: 4,
  Fri: 5,
  Sat: 6,
};

export interface TaipeiTimeParts {
  weekday: number; // 0=週日 ... 6=週六
  hour: number;
  minute: number;
  year: number;
  month: number;
  day: number;
}

export function taipeiParts(date: Date): TaipeiTimeParts {
  const parts = TAIPEI_PARTS_FORMATTER.formatToParts(date);
  const get = (type: string) => parts.find((p) => p.type === type)?.value ?? "";
  return {
    weekday: WEEKDAY_INDEX[get("weekday")] ?? 0,
    hour: Number(get("hour")),
    minute: Number(get("minute")),
    year: Number(get("year")),
    month: Number(get("month")),
    day: Number(get("day")),
  };
}

export function minutesOfDay(date: Date): number {
  const { hour, minute } = taipeiParts(date);
  return hour * 60 + minute;
}

export function isSameTaipeiDate(a: Date, b: Date): boolean {
  const pa = taipeiParts(a);
  const pb = taipeiParts(b);
  return pa.year === pb.year && pa.month === pb.month && pa.day === pb.day;
}

// 回傳「當地日期」的午夜時間點（以 UTC Date 物件表示，僅用於資料庫時間比較的下界）。
export function startOfTaipeiDay(date: Date): Date {
  const { year, month, day } = taipeiParts(date);
  // Asia/Taipei 為 UTC+8，當地 00:00 對應 UTC 前一天 16:00。
  return new Date(Date.UTC(year, month - 1, day, -8, 0, 0, 0));
}