const MONTH_DAY_PATTERN = /^(\d{2})-(\d{2})$/;
// Days per month of a non-leap year: 02-29 is not a valid reference day, so
// the reference day exists in every year.
const DAYS_IN_MONTH = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

/** MM-TT with a real month and a day that exists in a non-leap year. */
export function isValidMonthDay(value: string): boolean {
  const match = MONTH_DAY_PATTERN.exec(value);
  if (!match) {
    return false;
  }
  const month = Number(match[1]);
  const day = Number(match[2]);
  return (
    month >= 1 && month <= 12 && day >= 1 && day <= DAYS_IN_MONTH[month - 1]
  );
}

/** The reference day (YYYY-MM-DD) of a year for a stored MM-TT. */
export function tskReferenceDayOf(year: number, monthDay: string): string {
  if (!isValidMonthDay(monthDay)) {
    throw new Error(`Invalid month-day: ${monthDay}`);
  }
  return `${String(year).padStart(4, '0')}-${monthDay}`;
}