// rrule@2.8.1 lacks a proper "exports" field, so named ESM imports fail at
// runtime with NodeNext resolution. Use the default import and destructure.
import rrulePkg from "rrule";
import type { RRule as RRuleType } from "rrule";
const { RRule, rrulestr } = rrulePkg as unknown as typeof import("rrule");
import { DateTime } from "luxon";

export const MIN_INTERVAL_MS = 5 * 60 * 1000;

export function parseRRule(rule: string): RRuleType {
  const parsed = rrulestr(rule);
  if (!(parsed instanceof RRule)) {
    throw new Error("Compound RRULE/RRSET not supported");
  }
  return parsed;
}

export function nextOccurrence(rule: string, timezone: string, after: Date): Date | null {
  const parsed = parseRRule(rule);
  const afterInZone = DateTime.fromJSDate(after).setZone(timezone).toJSDate();
  const next = parsed.after(afterInZone, false);
  return next ?? null;
}

export type IntervalCheck = { ok: true } | { ok: false; reason: string };

export function validateMinInterval(rule: string, timezone: string): IntervalCheck {
  const parsed = parseRRule(rule);
  const now = new Date();
  const first = parsed.after(now, false);
  if (!first) return { ok: true };
  const second = parsed.after(first, false);
  if (!second) return { ok: true };
  const gap = second.getTime() - first.getTime();
  if (gap < MIN_INTERVAL_MS) {
    return {
      ok: false,
      reason: `Recurrence fires every ${Math.round(gap / 1000)}s; minimum interval is ${MIN_INTERVAL_MS / 1000}s.`,
    };
  }
  return { ok: true };
}