export const RESERVED_SUBDOMAINS = new Set([
  'app',
  'api',
  'www',
  'admin',
  'auth',
  'mail',
  'static',
]);

const SUBDOMAIN_RE = /^[a-z0-9-]{3,32}$/;

export function validateSubdomain(input: unknown):
  | { ok: true; value: string }
  | { ok: false; error: string } {
  if (typeof input !== 'string') {
    return { ok: false, error: 'subdomain must be a string' };
  }
  const value = input.trim().toLowerCase();
  if (!SUBDOMAIN_RE.test(value)) {
    return {
      ok: false,
      error: 'subdomain must be 3–32 chars, lowercase a–z, 0–9, hyphen',
    };
  }
  if (RESERVED_SUBDOMAINS.has(value)) {
    return { ok: false, error: `'${value}' is reserved` };
  }
  return { ok: true, value };
}