/**
 * The calendar day the beekeeper sees on their phone, as "YYYY-MM-DD".
 *
 * Not `toISOString().slice(0, 10)`: that is the UTC day, which in Germany is
 * still yesterday until 01:00 (winter) or 02:00 (summer) - a form opened
 * shortly after midnight would prefill the wrong date.
 */
export function toLocalIsoDate(date: Date): string {
  const year = String(date.getFullYear()).padStart(4, "0");
  const month = String(date.getMonth() + 1).padStart(2, "0");
  const day = String(date.getDate()).padStart(2, "0");
  return `${year}-${month}-${day}`;
}

/** Today's local calendar day, e.g. the default date of a new record. */
export function todayLocalIsoDate(): string {
  return toLocalIsoDate(new Date());
}

const CALENDAR_DAY = /^\d{4}-\d{2}-\d{2}$/;

/**
 * A day picked in a date field ("YYYY-MM-DD") as the API stores calendar
 * days: midnight UTC of exactly that day, whatever the phone's time zone.
 * Throws on anything that is not a real calendar day rather than sending a
 * shifted or invalid date.
 */
export function toApiDate(calendarDay: string): string {
  if (!CALENDAR_DAY.test(calendarDay)) {
    throw new RangeError(`Kein Kalendertag: "${calendarDay}"`);
  }
  const iso = new Date(`${calendarDay}T00:00:00.000Z`).toISOString();
  if (iso.slice(0, 10) !== calendarDay) {
    throw new RangeError(`Kein Kalendertag: "${calendarDay}"`);
  }
  return iso;
}

const CALENDAR_DAY_LENGTH = 10;

/** The "YYYY-MM-DD" part of an ISO timestamp as written, without any time zone shift. */
export function toCalendarDay(isoTimestamp: string): string {
  return isoTimestamp.slice(0, CALENDAR_DAY_LENGTH);
}