/**
 * Format an elapsed millisecond count as a compact human string:
 *   < 60s   -> "12s"
 *   < 1h    -> "5m 23s"
 *   >= 1h   -> "2h 05m"
 * Negatives clamp to 0s (clock skew tolerance).
 */
export function formatDuration(ms: number): string {
    if (!Number.isFinite(ms) || ms < 0) ms = 0;
    const totalSeconds = Math.floor(ms / 1000);
    if (totalSeconds < 60) return `${totalSeconds}s`;
    const minutes = Math.floor(totalSeconds / 60);
    const seconds = totalSeconds % 60;
    if (minutes < 60) return `${minutes}m ${String(seconds).padStart(2, '0')}s`;
    const hours = Math.floor(minutes / 60);
    const mins = minutes % 60;
    return `${hours}h ${String(mins).padStart(2, '0')}m`;
}

/**
 * Convenience: elapsed since an ISO timestamp, formatted. Returns null when
 * the anchor is null so callers can `{#if}` on it.
 */
export function elapsedSince(iso: string | null, now: number): string | null {
    if (!iso) return null;
    return formatDuration(now - new Date(iso).getTime());
}