const dateFormatter = new Intl.DateTimeFormat("de-AT", {
  day: "2-digit",
  month: "2-digit",
  year: "numeric",
});

export function fmtDate(date: string | Date | null | undefined): string {
  if (!date) return "–";
  const d = typeof date === "string" ? new Date(date) : date;
  if (Number.isNaN(d.getTime())) return "–";
  return dateFormatter.format(d);
}

export function initials(firstName: string, lastName: string): string {
  const a = firstName.trim().charAt(0).toUpperCase();
  const b = lastName.trim().charAt(0).toUpperCase();
  return `${a}${b}`;
}

export function fmtAge(birthDate: string | Date): number {
  const d = typeof birthDate === "string" ? new Date(birthDate) : birthDate;
  const today = new Date();
  let age = today.getFullYear() - d.getFullYear();
  const hasHadBirthdayThisYear =
    today.getMonth() > d.getMonth() ||
    (today.getMonth() === d.getMonth() && today.getDate() >= d.getDate());
  if (!hasHadBirthdayThisYear) age -= 1;
  return age;
}

export function tenure(entryDate: string | Date, endDate?: string | Date | null): string {
  const start = typeof entryDate === "string" ? new Date(entryDate) : entryDate;
  const end = endDate ? (typeof endDate === "string" ? new Date(endDate) : endDate) : new Date();

  let years = end.getFullYear() - start.getFullYear();
  let months = end.getMonth() - start.getMonth();
  if (end.getDate() < start.getDate()) months -= 1;
  if (months < 0) {
    years -= 1;
    months += 12;
  }
  if (years < 0) return "0 Monate";

  const yearPart = years > 0 ? `${years} ${years === 1 ? "Jahr" : "Jahre"}` : "";
  const monthPart = months > 0 ? `${months} ${months === 1 ? "Monat" : "Monate"}` : "";
  if (yearPart && monthPart) return `${yearPart}, ${monthPart}`;
  return yearPart || monthPart || "unter 1 Monat";
}

export function daysBetween(a: string | Date, b: string | Date = new Date()): number {
  const start = typeof a === "string" ? new Date(a) : a;
  const end = typeof b === "string" ? new Date(b) : b;
  return Math.round((end.getTime() - start.getTime()) / (1000 * 60 * 60 * 24));
}