const DEFAULT_TIMEZONE = "Asia/Shanghai";
let appTimezone = DEFAULT_TIMEZONE;

export function getAppTimezone(): string {
  return appTimezone;
}

export function setAppTimezone(timezone: string): void {
  try {
    new Intl.DateTimeFormat("en", { timeZone: timezone }).format();
    appTimezone = timezone;
  } catch {
    appTimezone = DEFAULT_TIMEZONE;
  }
}

export function formatDateTime(value: Date | number | string, timezone = appTimezone): string {
  return new Intl.DateTimeFormat("zh-CN", {
    dateStyle: "medium",
    timeStyle: "short",
    timeZone: timezone,
  }).format(value instanceof Date || typeof value === "number" ? new Date(value) : new Date(value));
}

export const formatDate = formatDateTime;
export const dateText = formatDateTime;

export function toDateInputValue(value: Date | number | string, timezone = appTimezone): string {
  const date = value instanceof Date || typeof value === "number" ? new Date(value) : new Date(value);
  const parts = new Intl.DateTimeFormat("en-CA", {
    timeZone: timezone,
    year: "numeric",
    month: "2-digit",
    day: "2-digit",
    hour: "2-digit",
    minute: "2-digit",
    hourCycle: "h23",
  }).formatToParts(date);
  const values = Object.fromEntries(parts.map((part) => [part.type, part.value]));
  return `${values.year}-${values.month}-${values.day}T${values.hour}:${values.minute}`;
}

export const formatDateTimeInput = toDateInputValue;

export function fromDateInput(value: string, timezone = appTimezone): string {
  const match = /^(\d{4})-(\d{2})-(\d{2})[ T](\d{2}):(\d{2})$/.exec(value.trim());
  if (!match) throw new Error("请选择有效的支付日期和时间");
  const year = Number(match[1]);
  const month = Number(match[2]);
  const day = Number(match[3]);
  const hour = Number(match[4]);
  const minute = Number(match[5]);
  const calendarProbe = new Date(0);
  calendarProbe.setUTCFullYear(year, month - 1, day);
  calendarProbe.setUTCHours(0, 0, 0, 0);
  if (month < 1 || month > 12 || day < 1 || day > 31 || calendarProbe.getUTCFullYear() !== year || calendarProbe.getUTCMonth() !== month - 1 || calendarProbe.getUTCDate() !== day || hour < 0 || hour > 23 || minute < 0 || minute > 59) {
    throw new Error("支付时间无效");
  }

  // Interpret the form value as a wall-clock time in the configured zone.
  const wallUtc = Date.UTC(year, month - 1, day, hour, minute);
  const observedParts = new Intl.DateTimeFormat("en-CA", {
    timeZone: timezone,
    year: "numeric",
    month: "2-digit",
    day: "2-digit",
    hour: "2-digit",
    minute: "2-digit",
    hourCycle: "h23",
  }).formatToParts(new Date(wallUtc));
  const observed = Object.fromEntries(observedParts.map((part) => [part.type, part.value]));
  const observedUtc = Date.UTC(Number(observed.year), Number(observed.month) - 1, Number(observed.day), Number(observed.hour), Number(observed.minute));
  return new Date(wallUtc + wallUtc - observedUtc).toISOString();
}

export const dateFromInput = fromDateInput;

export function currentMonth(timezone = appTimezone): string {
  const parts = Object.fromEntries(new Intl.DateTimeFormat("en-CA", {
    timeZone: timezone,
    year: "numeric",
    month: "2-digit",
  }).formatToParts(new Date()).map((part) => [part.type, part.value]));
  return `${parts.year}-${parts.month}`;
}

export const monthNow = currentMonth;