/**
 * How long to let the panel sleep.
 *
 * In BYOS there is no way to push: the device sleeps, wakes, asks, and sleeps
 * again. The only lever is how long it sleeps for, so this is the entire
 * freshness-versus-battery trade-off of the product, in one function.
 *
 * Two rules:
 *   - sleep briefly while the shop is trading, and at length overnight;
 *   - never sleep through a change of state. If the door opens in twenty
 *     minutes, wake in twenty-one, whatever the base interval says.
 */

import { civilDaysBetween, minutesOfTime, toCivilDate, toCivilTime } from '@/lib/schedule/civil';
import type { ShopStatus } from '@/lib/schedule/types';

/** Below this the panel would spend its life awake. */
export const MIN_REFRESH_SEC = 60;

/** A panel that has not checked in for six hours is indistinguishable from a dead one. */
export const MAX_REFRESH_SEC = 21_600;

/** Wake just after the change, not exactly on it. */
export const CHANGE_MARGIN_SEC = 60;

export type RefreshInput = {
  now: Date;
  status: ShopStatus;
  timezone: string;
  openSec: number;
  closedSec: number;
};

export function computeRefreshRate({
  now,
  status,
  timezone,
  openSec,
  closedSec,
}: RefreshInput): number {
  // "Opening soon" gets the short interval too: that is the moment the screen
  // is about to be wrong, and the moment someone is standing at the window.
  const busy =
    status.status === 'OPEN' || status.status === 'CLOSING_SOON' || status.status === 'OPENING_SOON';

  let seconds = busy ? openSec : closedSec;

  if (status.nextChangeAt) {
    const untilChange = secondsUntil(now, status.nextChangeAt, timezone);
    if (untilChange > 0) {
      seconds = Math.min(seconds, untilChange + CHANGE_MARGIN_SEC);
    }
  }

  return clamp(Math.round(seconds));
}

function clamp(seconds: number): number {
  return Math.min(MAX_REFRESH_SEC, Math.max(MIN_REFRESH_SEC, seconds));
}

/**
 * Seconds from now until a wall-clock moment.
 *
 * Measured on the wall clock, which is off by an hour on the two nights a year
 * the clocks change. That only shifts one wake-up, and the alternative — a
 * real timezone conversion back to an instant — buys nothing the panel can
 * perceive.
 */
function secondsUntil(now: Date, target: { date: string; time: string }, timezone: string): number {
  const today = toCivilDate(now, timezone);
  const nowMinutes = minutesOfTime(toCivilTime(now, timezone));
  const days = civilDaysBetween(today, target.date);
  const minutes = days * 1440 + minutesOfTime(target.time) - nowMinutes;
  return minutes * 60 - now.getUTCSeconds();
}