const formattersCache = {} as Record<string, Intl.DateTimeFormat>;

export type Locale = string | null | undefined;

export const getFormatterForLocale = (
  locale: Locale,
  options?: Intl.DateTimeFormatOptions,
): Intl.DateTimeFormat => {
  const l = locale ?? navigator.language;
  return (
    formattersCache[l + JSON.stringify(options)] ??
    new Intl.DateTimeFormat(l, options)
  );
};

/**
 * the day on which a week should start
 */
export type StartOfWeek = "monday" | "sunday";

export const getWeeDayNames = (
  locale: Locale,
  startOfWeek: StartOfWeek,
  weekday: Intl.DateTimeFormatOptions["weekday"],
) =>
  Array.from(
    { length: 7 },
    (_, i) =>
      getFormatterForLocale(locale, { weekday })
        .formatToParts(
          new Date(2024, 0, i + (startOfWeek === "sunday" ? 0 : 1)),
        )
        .find((part) => part.type === "weekday")!.value,
  );

/**
 * get the localized month name according to a given format
 */
export const getMonthName = (
  value: Date,
  locale: Locale,
  month?: Intl.DateTimeFormatOptions["month"],
) =>
  getFormatterForLocale(locale, { month })
    .formatToParts(value)
    .find(({ type }) => type === "month")!.value;