/** Normalize to local midnight; invalid input falls back to today. */
export function startOfLocalDay(d: Date): Date {
  if (Number.isNaN(d.getTime())) {
    const t = new Date();
    return new Date(t.getFullYear(), t.getMonth(), t.getDate(), 0, 0, 0, 0);
  }
  return new Date(d.getFullYear(), d.getMonth(), d.getDate(), 0, 0, 0, 0);
}

/** Local calendar bounds for a date (browser timezone). */
export function getLocalDayIsoRange(day: Date): { from: string; to: string } {
  const start = startOfLocalDay(day);
  const end = new Date(start.getFullYear(), start.getMonth(), start.getDate() + 1, 0, 0, 0, 0);
  return { from: start.toISOString(), to: end.toISOString() };
}

export function toDateInputValue(d: Date): string {
  const y = d.getFullYear();
  const m = String(d.getMonth() + 1).padStart(2, '0');
  const day = String(d.getDate()).padStart(2, '0');
  return `${y}-${m}-${day}`;
}

export function parseDateInput(value: string): Date {
  const [y, m, d] = value.split('-').map(Number);
  return new Date(y, m - 1, d, 0, 0, 0, 0);
}

export function combineLocalDateAndTime(day: Date, timeHHmm: string): Date {
  const [h, min] = timeHHmm.split(':').map(Number);
  return new Date(day.getFullYear(), day.getMonth(), day.getDate(), h, min, 0, 0);
}

export function formatTimeForInput(d: Date): string {
  const h = String(d.getHours()).padStart(2, '0');
  const m = String(d.getMinutes()).padStart(2, '0');
  return `${h}:${m}`;
}

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

export function formatHourLabel(hour: number): string {
  const d = new Date(2000, 0, 1, hour, 0, 0, 0);
  return d.toLocaleTimeString(undefined, { hour: 'numeric', hour12: true });
}

/** Local midnight + delta calendar days. */
export function addCalendarDays(day: Date, delta: number): Date {
  return new Date(day.getFullYear(), day.getMonth(), day.getDate() + delta, 0, 0, 0, 0);
}

/** Compare two calendar days at local midnight (ordering by date only). */
export function compareLocalDayStart(a: Date, b: Date): number {
  const ta = new Date(a.getFullYear(), a.getMonth(), a.getDate()).getTime();
  const tb = new Date(b.getFullYear(), b.getMonth(), b.getDate()).getTime();
  return ta - tb;
}