// This app stores dates as date-only strings ("YYYY-MM-DD") and timestamps as
// timestamptz, and is used from a single timezone. Every conversion here is
// pinned to Europe/Vienna rather than the runtime's zone: the server renders
// in UTC (Docker/Vercel) while the browser renders in Vienna, so an unpinned
// formatter produces a different day on each side — wrong dates for the user
// near midnight, and a React hydration mismatch.
const TIMEZONE = "Europe/Vienna";

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

// en-CA formats as "YYYY-MM-DD", which is the shape the rest of the app and
// the database speak.
const isoFormatter = new Intl.DateTimeFormat("en-CA", {
  day: "2-digit",
  month: "2-digit",
  year: "numeric",
  timeZone: TIMEZONE,
});

const DATE_ONLY = /^\d{4}-\d{2}-\d{2}$/;

// Today in Vienna. Deliberately not new Date().toISOString().slice(0, 10):
// that is the *UTC* date, which for part of every day is already tomorrow
// relative to Austria.
export function todayIso(): string {
  return isoFormatter.format(new Date());
}

// Normalizes either input to a "YYYY-MM-DD" string. A date-only string is
// returned as-is — parsing it into a Date first would anchor it to UTC
// midnight and shift it in any negative-offset zone.
export function toIsoDate(value: string | Date): string {
  if (typeof value === "string") return DATE_ONLY.test(value) ? value : isoFormatter.format(new Date(value));
  return isoFormatter.format(value);
}

export function fmtDate(date: string | Date | null | undefined): string {
  if (!date) return "–";
  if (typeof date === "string" && DATE_ONLY.test(date)) {
    const [year, month, day] = date.split("-");
    return `${day}.${month}.${year}`;
  }
  const d = typeof date === "string" ? new Date(date) : date;
  if (Number.isNaN(d.getTime())) return "–";
  return dateTimeFormatter.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}`;
}

// "Dr. Max Mustermann, MSc MBA" — prefix titles precede the name, suffix
// titles follow after a comma, both space-joined in the stored order.
export function fmtFullName(
  firstName: string,
  lastName: string,
  titlePrefix: string[] | null | undefined,
  titleSuffix: string[] | null | undefined
): string {
  const prefix = titlePrefix && titlePrefix.length > 0 ? `${titlePrefix.join(" ")} ` : "";
  const suffix = titleSuffix && titleSuffix.length > 0 ? `, ${titleSuffix.join(" ")}` : "";
  return `${prefix}${firstName} ${lastName}${suffix}`;
}

// Whole years between two ISO dates. Compares the "MM-DD" tails as strings,
// which is exact and needs no Date arithmetic at all.
export function yearsBetweenIso(from: string, to: string): number {
  const years = Number(to.slice(0, 4)) - Number(from.slice(0, 4));
  return to.slice(5) < from.slice(5) ? years - 1 : years;
}

export function fmtAge(birthDate: string | Date): number {
  return yearsBetweenIso(toIsoDate(birthDate), todayIso());
}

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

  let years = Number(end.slice(0, 4)) - Number(start.slice(0, 4));
  let months = Number(end.slice(5, 7)) - Number(start.slice(5, 7));
  if (Number(end.slice(8, 10)) < Number(start.slice(8, 10))) 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";
}

// Anchored at UTC midnight on both sides so the difference is a whole number
// of calendar days regardless of DST transitions in between.
export function daysBetweenIso(from: string, to: string = todayIso()): number {
  const start = Date.parse(`${from}T00:00:00Z`);
  const end = Date.parse(`${to}T00:00:00Z`);
  return Math.round((end - start) / 86_400_000);
}

export function addDaysIso(iso: string, days: number): string {
  const d = new Date(`${iso}T00:00:00Z`);
  d.setUTCDate(d.getUTCDate() + days);
  return d.toISOString().slice(0, 10);
}