/** The RTC's day counter wraps every 20 weeks (FixDays keeps wCurDay below
 * 140), and day differences wrap with it (_CalcDaysSince). */
export const RTC_DAYS = 20 * 7;

/** A day countdown (InitNDaysCountdown's two bytes): the days remaining
 * and the wCurDay it was last checked on. */
export interface DayTimer {
  remaining: number;
  day: number;
}

/** wCurDay for a native day number. */
export const rtcDay = (day: number) => ((day % RTC_DAYS) + RTC_DAYS) % RTC_DAYS;

/** InitNDaysCountdown. */
export function initCountdown(days: number, today: number): DayTimer {
  return { remaining: days, day: rtcDay(today) };
}

/** CheckDayDependentEventHL: CalcDaysSince (which also moves the stored
 * day to today) and UpdateTimeRemaining. True (carry) once the countdown
 * has run out; a timer never started (`undefined`, all zeros in the
 * original) is already out. */
export function checkCountdown(timer: DayTimer | undefined, today: number) {
  const current = rtcDay(today);
  const start = timer ?? { remaining: 0, day: 0 };
  const elapsed = (current - start.day + RTC_DAYS) % RTC_DAYS;
  const remaining = Math.max(0, start.remaining - elapsed);
  return { done: remaining === 0, timer: { remaining, day: current } };
}

/** The days until next Friday (RestartLuckyNumberCountdown's
 * .GetDaysUntilNextFriday): 7 on Fridays and 6 on Saturdays. */
export function daysUntilNextFriday(weekday: number) {
  const FRIDAY = 5;
  const days = FRIDAY - weekday;
  return days > 0 ? days : days + 7;
}

export function validDayTimer(value: unknown): value is DayTimer {
  const timer = value as DayTimer;
  return (
    Boolean(timer) &&
    typeof timer === "object" &&
    Number.isInteger(timer.remaining) &&
    timer.remaining >= 0 &&
    timer.remaining <= 255 &&
    Number.isInteger(timer.day) &&
    timer.day >= 0 &&
    timer.day < RTC_DAYS
  );
}