/**
 * Day-tab helpers (MSEP-3): users didn't discover the prev/next arrows, so the
 * calendar shows one labeled tab per fair day. Pure functions, kept
 * dependency-free so they are unit-testable without FullCalendar.
 */

/** Local-timezone ISO date (YYYY-MM-DD) — avoids toISOString() UTC shifts. */
export function isoDateOf(date: Date): string {
  const month = String(date.getMonth() + 1).padStart(2, '0');
  const day = String(date.getDate()).padStart(2, '0');
  return `${date.getFullYear()}-${month}-${day}`;
}

/**
 * All fair days as ISO dates, inclusive of the end date.
 * Accepts the API's datetime strings ("2026-06-17T23:59:59") or bare dates.
 * Capped at 60 days as a runaway guard for malformed ranges.
 */
export function listFairDays(startIso: string, endIso: string): string[] {
  const startDate = new Date(String(startIso).slice(0, 10) + 'T12:00:00');
  const endDate = new Date(String(endIso).slice(0, 10) + 'T12:00:00');

  if (isNaN(startDate.getTime()) || isNaN(endDate.getTime())) {
    return [];
  }

  const days: string[] = [];
  for (
    let current = new Date(startDate);
    current <= endDate && days.length < 60;
    current.setDate(current.getDate() + 1)
  ) {
    days.push(isoDateOf(current));
  }

  return days;
}

/** Localized tab label: weekday + short date (e.g. de: "Mo., 15.06."). */
export function formatDayLabel(isoDate: string, locale?: string): string {
  const date = new Date(isoDate + 'T12:00:00');
  if (isNaN(date.getTime())) {
    return isoDate;
  }

  return date.toLocaleDateString(locale || undefined, {
    weekday: 'short',
    day: '2-digit',
    month: '2-digit',
  });
}