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

export function recordedDayKey(iso: string): string {
  return toDayKey(new Date(iso));
}

export function dayKeyToMonthStart(dayKey: string): Date | null {
  const match = /^(\d{4})-(\d{2})-(\d{2})$/.exec(dayKey);
  if (!match) return null;
  return new Date(Number(match[1]), Number(match[2]) - 1, 1);
}

export function formatDayKeyLabel(dayKey: string): string {
  const match = /^(\d{4})-(\d{2})-(\d{2})$/.exec(dayKey);
  if (!match) return dayKey;
  const date = new Date(Number(match[1]), Number(match[2]) - 1, Number(match[3]));
  return date.toLocaleDateString(undefined, { month: "long", day: "numeric", year: "numeric" });
}

export function buildEntryDaySet(entries: Array<{ recordedAt: string }>): Set<string> {
  return new Set(entries.map((entry) => recordedDayKey(entry.recordedAt)));
}