// Österreichische Sozialversicherungsnummer (SVNR).
//
// Ten digits: three-digit serial, one check digit, then the date of birth as
// TTMMJJ — e.g. "1237 010180". The check digit is the weighted sum of the
// other nine digits modulo 11; a serial whose sum yields 11 leaves no usable
// digit and is simply never issued, which is why 10 has to be rejected
// rather than wrapped.
//
// Only employees at an Austrian location have one. Everyone else keeps the
// field free-form, since the German/Czech/Slovenian equivalents have their
// own formats and are not what this validates.

const WEIGHTS = [3, 7, 9, 0, 5, 8, 4, 2, 1, 6] as const;
const CHECK_INDEX = 3;

export type SvnrError = "length" | "serial" | "checksum" | "date" | "birthDateMismatch";

const MESSAGES: Record<SvnrError, string> = {
  length: "Die SV-Nummer muss aus 10 Ziffern bestehen (4 Ziffern, dann TTMMJJ).",
  serial: "Die laufende Nummer darf nicht 000 sein.",
  checksum: "Die Prüfziffer stimmt nicht. Bitte die Eingabe kontrollieren.",
  date: "Die letzten 6 Stellen ergeben kein gültiges Geburtsdatum (TTMMJJ).",
  birthDateMismatch: "Die SV-Nummer enthält ein anderes Geburtsdatum als im Stammdatensatz.",
};

export function svnrErrorMessage(error: SvnrError): string {
  return MESSAGES[error];
}

/** Strips spaces and separators; keeps everything else so bad input still fails loudly. */
export function normalizeSvnr(input: string): string {
  return input.replace(/[\s./-]/g, "");
}

/** "1237010180" → "1237 010180"; leaves anything non-canonical untouched. */
export function formatSvnr(input: string): string {
  const n = normalizeSvnr(input);
  return /^\d{10}$/.test(n) ? `${n.slice(0, 4)} ${n.slice(4)}` : input;
}

export function svnrCheckDigit(digits: string): number | null {
  const n = normalizeSvnr(digits);
  if (!/^\d{10}$/.test(n)) return null;
  let sum = 0;
  for (let i = 0; i < 10; i++) {
    if (i === CHECK_INDEX) continue;
    sum += Number(n[i]) * WEIGHTS[i];
  }
  const check = sum % 11;
  return check === 10 ? null : check;
}

/**
 * `birthDate` (ISO yyyy-mm-dd) is optional; when given, the TTMMJJ tail is
 * cross-checked against it — the single most common data-entry slip is a
 * transposed birth date, which the checksum alone will not catch.
 */
export function validateSvnr(input: string, birthDate?: string | null): SvnrError | null {
  const n = normalizeSvnr(input);
  if (!/^\d{10}$/.test(n)) return "length";
  if (n.slice(0, 3) === "000") return "serial";

  const day = Number(n.slice(4, 6));
  const month = Number(n.slice(6, 8));
  if (month < 1 || month > 12 || day < 1 || day > 31) return "date";
  // Without a century the tail cannot be resolved to a real date, so the
  // day-of-month bound is the generous one; a supplied birthDate settles it.
  if (day > [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month - 1]) return "date";

  const expected = svnrCheckDigit(n);
  if (expected === null || expected !== Number(n[CHECK_INDEX])) return "checksum";

  if (birthDate) {
    const [y, m, d] = birthDate.split("-");
    if (d !== n.slice(4, 6) || m !== n.slice(6, 8) || y?.slice(-2) !== n.slice(8, 10)) return "birthDateMismatch";
  }

  return null;
}

export function isValidSvnr(input: string, birthDate?: string | null): boolean {
  return validateSvnr(input, birthDate) === null;
}

/** Countries whose employees this validation applies to. */
export function requiresAustrianSvnr(locationCountry: string | null | undefined): boolean {
  return locationCountry === "Österreich";
}