(function exposeCalendar(root, factory) {
  const api = factory();
  if (typeof module === "object" && module.exports) module.exports = api;
  if (root) root.ValleyCalendar = api;
})(typeof window === "undefined" ? globalThis : window, () => {
  const seasons = ["spring", "summer", "fall", "winter"];
  // Days since Spring 1, Year 1 (which is day 1).
  const dayNumber = ({ year, season, day }) => (year - 1) * 112 + seasons.indexOf(season) * 28 + day;
  const fromDayNumber = number => {
    const offset = number - 1;
    return { year: Math.floor(offset / 112) + 1, season: seasons[Math.floor(offset % 112 / 28)], day: offset % 28 + 1 };
  };

  return {
    birthdayOn(birthdays, season, day, year) {
      const entry = birthdays?.[season]?.[day];
      if (!entry || (entry[1] && entry[1] !== year)) return null;
      return entry[0];
    },

    // Crops are harvestable `growDays` after planting when watered daily.
    harvestDate(planted, growDays) {
      return fromDayNumber(dayNumber(planted) + growDays);
    },

    daysBetween(from, to) {
      return dayNumber(to) - dayNumber(from);
    }
  };
});