import { DateTime } from "luxon";

/**
 * Resolve and validate the scheduledAt for `updateReminderAction`.
 *
 * Why this exists: the action is invoked from four edit forms
 * (account / message / groups / when). Three of those don't change
 * the time — they pass the original `scheduledAt` straight through.
 * For a paused/ended one-off whose original time is now in the past
 * (the operator paused it after a fire and is editing the message),
 * a strict "must be future" check rejects the edit even though the
 * operator only wanted to fix a typo. So:
 *
 *   - Reject malformed ISO outright.
 *   - Allow past timestamps when:
 *       * the reminder's existing status is `paused` or `ended` —
 *         it won't fire in those states regardless, AND/OR
 *       * the submitted timestamp matches the existing one (within
 *         a second of rounding), i.e. the form is passing it through
 *         unchanged.
 *   - Otherwise reject past timestamps so an active reminder can't
 *     be scheduled into a moment that's already gone.
 *
 * Pure function — takes `now` as input so tests can pin the clock.
 */
export function validateUpdateScheduledAt(args: {
  iso: string;
  timezone: string;
  existingStatus: string;
  existingScheduledAt: Date | null;
  now: Date;
}): { ok: true; scheduledAt: Date } | { ok: false; error: string } {
  const dt = DateTime.fromISO(args.iso, { zone: args.timezone }).toJSDate();
  if (Number.isNaN(dt.getTime())) {
    return { ok: false, error: "Invalid date" };
  }
  const isPaused = args.existingStatus === "paused" || args.existingStatus === "inactive";
  const sameAsExisting =
    args.existingScheduledAt !== null &&
    Math.abs(args.existingScheduledAt.getTime() - dt.getTime()) < 1000;
  if (dt.getTime() <= args.now.getTime() && !isPaused && !sameAsExisting) {
    return { ok: false, error: "Time is in the past" };
  }
  return { ok: true, scheduledAt: dt };
}