const MINUTE = 60_000;
const HOUR = 60 * MINUTE;
const DAY = 24 * HOUR;

/**
 * Approximate period of a 5-field cron expression (minute hour day-of-month month day-of-week),
 * used for staleness checks ("no success within 2× the schedule interval"). Exact evaluation is
 * not needed: monthly → 31 d, weekly → 7 d, daily → 1 d (÷ number of listed hours), hourly → 1 h
 * (or N minutes for step expressions). Returns null for malformed expressions.
 */
export function scheduleIntervalMs(cron: string | undefined | null): number | null {
  if (!cron) return null;
  const f = cron.trim().split(/\s+/);
  if (f.length !== 5) return null;
  const [minute, hour, dom, month, dow] = f as [string, string, string, string, string];
  if (month !== '*' || dom !== '*') return 31 * DAY;
  if (dow !== '*') return (7 * DAY) / Math.max(1, listCount(dow));
  if (hour !== '*') {
    const step = stepOf(hour);
    if (step) return step * HOUR;
    return DAY / Math.max(1, listCount(hour));
  }
  if (minute !== '*') {
    const step = stepOf(minute);
    if (step) return step * MINUTE;
    return HOUR / Math.max(1, listCount(minute));
  }
  return MINUTE;
}

function listCount(field: string): number {
  return field.split(',').reduce((n, part) => {
    const m = /^(\d+)-(\d+)$/.exec(part);
    return n + (m ? Math.max(1, Number(m[2]) - Number(m[1]) + 1) : 1);
  }, 0);
}

function stepOf(field: string): number | null {
  const m = /^\*\/(\d+)$/.exec(field);
  return m ? Math.max(1, Number(m[1])) : null;
}

/** Stale = no success for more than `factor` × the schedule interval (default 2×). */
export function isStale(lastSuccessAt: Date | string | null | undefined, cron: string | undefined | null, now = Date.now(), factor = 2): { stale: boolean; ageMs: number | null; limitMs: number | null } {
  const limitMs = scheduleIntervalMs(cron);
  const ageMs = lastSuccessAt ? now - new Date(lastSuccessAt).getTime() : null;
  if (limitMs === null) return { stale: false, ageMs, limitMs: null };
  if (ageMs === null) return { stale: true, ageMs: null, limitMs: limitMs * factor };
  return { stale: ageMs > limitMs * factor, ageMs, limitMs: limitMs * factor };
}

/** Human duration ("3 d 4 h", "12 min"). */
export function humanDuration(ms: number | null | undefined): string {
  if (ms === null || ms === undefined || !Number.isFinite(ms)) return '-';
  const s = Math.round(ms / 1000);
  if (s < 60) return `${s} s`;
  const m = Math.floor(s / 60);
  if (m < 60) return `${m} min`;
  const h = Math.floor(m / 60);
  if (h < 48) return `${h} h ${m % 60} min`;
  const d = Math.floor(h / 24);
  return `${d} d ${h % 24} h`;
}