/**
 * Get ISO week number for a given date
 * ISO 8601 week numbering: week starts on Monday, first week contains Jan 4
 */
export function getISOWeek(date: Date): number {
  const d = new Date(Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()));
  const dayNum = d.getUTCDay() || 7;
  d.setUTCDate(d.getUTCDate() + 4 - dayNum);
  const yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1));
  return Math.ceil(((d.getTime() - yearStart.getTime()) / 86400000 + 1) / 7);
}

/**
 * Get week number and year for a given date
 * Returns format: "S06-2026"
 */
export function getWeekYearLabel(date: Date = new Date()): string {
  const week = getISOWeek(date);
  const year = date.getFullYear();
  return `S${week.toString().padStart(2, '0')}-${year}`;
}