export function startOfMonth(d: Date) {
  return new Date(d.getFullYear(), d.getMonth(), 1);
}

export function endOfMonth(d: Date) {
  return new Date(d.getFullYear(), d.getMonth() + 1, 0, 23, 59, 59, 999);
}

export function addMonths(d: Date, n: number) {
  return new Date(d.getFullYear(), d.getMonth() + n, 1);
}

export function sameDay(a: Date, b: Date) {
  return (
    a.getFullYear() === b.getFullYear() &&
    a.getMonth() === b.getMonth() &&
    a.getDate() === b.getDate()
  );
}

export function toDateKey(d: Date) {
  return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, "0")}-${String(d.getDate()).padStart(2, "0")}`;
}

export function parseDateKey(key: string) {
  const [y, m, day] = key.split("-").map(Number);
  return new Date(y, m - 1, day);
}

/** Monday-first calendar grid cells (includes leading/trailing days). */
export function buildMonthGrid(month: Date): Date[] {
  const start = startOfMonth(month);
  const end = endOfMonth(month);
  const startPad = (start.getDay() + 6) % 7;
  const cells: Date[] = [];
  const gridStart = new Date(start);
  gridStart.setDate(start.getDate() - startPad);

  for (let i = 0; i < 42; i++) {
    const cell = new Date(gridStart);
    cell.setDate(gridStart.getDate() + i);
    cells.push(cell);
    if (i >= 27 && cell >= end && cell.getDay() === 0) break;
  }
  return cells;
}

export function formatMonthYear(d: Date) {
  return d.toLocaleDateString("en-US", { month: "long", year: "numeric" });
}

export function matchDisplayDate(iso: string | null) {
  if (!iso) return "TBD";
  return new Date(iso).toLocaleString(undefined, {
    weekday: "short",
    month: "short",
    day: "numeric",
    hour: "2-digit",
    minute: "2-digit",
  });
}