homeio

1/**
2 * Minimal cron next-run calculator for 5-field cron expressions.
3 * Supports: * (wildcard), single values, comma-separated lists.
4 * Fields: minute hour day-of-month month day-of-week (0–6, Sunday=0)
5 */
6
7function parseCronField(field: string, min: number, max: number): Set<number> {
8  const result = new Set<number>();
9  if (field === "*") {
10    for (let i = min; i <= max; i++) result.add(i);
11    return result;
12  }
13  for (const part of field.split(",")) {
14    const n = parseInt(part, 10);
15    if (!isNaN(n) && n >= min && n <= max) result.add(n);
16  }
17  return result;
18}
19
20export function nextCronRun(expression: string, after: Date): Date | null {
21  const parts = expression.trim().split(/\s+/);
22  if (parts.length !== 5) return null;
23
24  const [minuteField, hourField, domField, monthField, dowField] = parts;
25
26  const minutes = parseCronField(minuteField, 0, 59);
27  const hours = parseCronField(hourField, 0, 23);
28  const doms = parseCronField(domField, 1, 31);
29  const months = parseCronField(monthField, 1, 12);
30  const dows = parseCronField(dowField, 0, 6);
31
32  const candidate = new Date(after);
33  candidate.setSeconds(0);
34  candidate.setMilliseconds(0);
35  // Start searching from next minute
36  candidate.setMinutes(candidate.getMinutes() + 1);
37
38  // Search up to 1 year (525,600 minutes)
39  const limit = 525_600;
40  for (let i = 0; i < limit; i++) {
41    const m = candidate.getMonth() + 1; // 1–12
42    const dom = candidate.getDate();
43    const dow = candidate.getDay(); // 0=Sun
44    const h = candidate.getHours();
45    const min = candidate.getMinutes();
46
47    if (
48      months.has(m) &&
49      doms.has(dom) &&
50      dows.has(dow) &&
51      hours.has(h) &&
52      minutes.has(min)
53    ) {
54      return new Date(candidate);
55    }
56
57    candidate.setMinutes(candidate.getMinutes() + 1);
58  }
59
60  return null;
61}
62