// Server-only — calls the European Commission's public VIES REST API to
// confirm an EU VAT ID is actually registered, not just correctly
// formatted (see lib/vatId.ts's own comment: format alone is never
// enough to zero-rate an invoice). Confirmed live and working against
// the real endpoint 2026-07-23 (POST {countryCode, vatNumber} →
// {valid: boolean, ...}) — this is the Commission's own documented REST
// API, not a guess.
const VIES_URL = "https://ec.europa.eu/taxation_customs/vies/rest-api/check-vat-number";

export type ViesCheckResult =
  | { ok: true; valid: boolean; name: string | null; address: string | null }
  | { ok: false; reason: string };

// `vatNumber` must NOT include the country prefix (VIES wants it split
// out) — callers pass the full "DE123456789"-shaped id and this function
// does the splitting, since every call site already has the normalized
// full id (see lib/vatId.ts's normalizeVatId()) rather than the two parts
// separately.
export async function checkVatIdViaVies(vatId: string): Promise<ViesCheckResult> {
  const countryCode = vatId.slice(0, 2);
  const vatNumber = vatId.slice(2);
  if (!countryCode || !vatNumber) return { ok: false, reason: "Ungültiges USt-IdNr.-Format." };

  try {
    // 8s timeout — VIES is a shared EU-wide government service with no
    // uptime SLA to this shop; a slow/unreachable response must not hang
    // checkout indefinitely. Callers treat `ok: false` as "couldn't
    // confirm" and fail closed (no exemption), never as "confirmed invalid".
    const res = await fetch(VIES_URL, {
      method: "POST",
      headers: { "Content-Type": "application/json" },
      body: JSON.stringify({ countryCode, vatNumber }),
      signal: AbortSignal.timeout(8000),
    });
    if (!res.ok) return { ok: false, reason: `VIES antwortete mit ${res.status}` };
    const data: { actionSucceed?: boolean; valid?: boolean; name?: string; address?: string; errorWrappers?: { error?: string }[] } = await res.json();
    // VIES answers 200 even when it couldn't actually perform the check —
    // `actionSucceed: false` (e.g. `MS_UNAVAILABLE`, the member state's own
    // national gateway being temporarily down — Germany's in particular is
    // known to do this) means "couldn't confirm", not "confirmed invalid".
    // Without this check a `MS_UNAVAILABLE` response fell through to
    // `Boolean(data.valid)` on a body that has no `valid` field at all,
    // silently reading as `valid: false` — a real, currently-registered VAT
    // ID would then look rejected instead of "VIES unavailable, try again".
    if (data.actionSucceed === false) {
      const reason = data.errorWrappers?.[0]?.error ?? "VIES konnte die Anfrage nicht bearbeiten.";
      return { ok: false, reason: `VIES: ${reason}` };
    }
    return {
      ok: true,
      valid: Boolean(data.valid),
      name: data.name && data.name !== "---" ? data.name : null,
      address: data.address && data.address !== "---" ? data.address : null,
    };
  } catch (err) {
    return { ok: false, reason: err instanceof Error ? err.message : "VIES ist gerade nicht erreichbar." };
  }
}