export const normalizeHostname = (host: string): string =>
  host
    .toLowerCase()
    .replace(/^\[|\]$/g, "")
    .replace(/\.+$/u, "");

const parseIpv4Address = (host: string): readonly number[] | null => {
  const parts = normalizeHostname(host).split(".").map(Number);
  return parts.length === 4 &&
    parts.every((part) => Number.isInteger(part) && part >= 0 && part <= 255)
    ? parts
    : null;
};

const parseIpv4MappedIpv6Address = (host: string): readonly number[] | null => {
  const normalized = normalizeHostname(host);
  if (!normalized.startsWith("::ffff:")) return null;
  const suffix = normalized.slice("::ffff:".length);
  const dotted = parseIpv4Address(suffix);
  if (dotted) return dotted;
  const hextets = suffix.split(":");
  if (hextets.length !== 2 || hextets.some((part) => !/^[\da-f]{1,4}$/u.test(part))) return null;
  const high = Number.parseInt(hextets[0]!, 16);
  const low = Number.parseInt(hextets[1]!, 16);
  return [high >>> 8, high & 0xff, low >>> 8, low & 0xff];
};

const parseIpv6Address = (host: string): readonly number[] | null => {
  const normalized = normalizeHostname(host);
  if (!normalized.includes(":")) return null;
  const halves = normalized.split("::");
  if (halves.length > 2) return null;
  const head = halves[0] ? halves[0].split(":") : [];
  const tail = halves[1] ? halves[1].split(":") : [];
  if ([...head, ...tail].some((part) => !/^[\da-f]{1,4}$/u.test(part))) return null;
  const missing = 8 - head.length - tail.length;
  if ((halves.length === 1 && missing !== 0) || (halves.length === 2 && missing < 1)) return null;
  return [...head, ...Array.from({ length: missing }, () => "0"), ...tail].map((part) =>
    Number.parseInt(part, 16),
  );
};

const ipv6PrefixMatches = (
  address: readonly number[],
  prefix: readonly number[],
  prefixLength: number,
): boolean => {
  const fullHextets = Math.floor(prefixLength / 16);
  if (address.slice(0, fullHextets).some((part, index) => part !== prefix[index])) return false;
  const remainingBits = prefixLength % 16;
  if (remainingBits === 0) return true;
  const mask = (0xffff << (16 - remainingBits)) & 0xffff;
  return (address[fullHextets]! & mask) === (prefix[fullHextets]! & mask);
};

const isPrivateIpv4Address = (parts: readonly number[]): boolean =>
  parts[0] === 0 ||
  parts[0] === 10 ||
  parts[0] === 127 ||
  (parts[0] === 100 && parts[1]! >= 64 && parts[1]! <= 127) ||
  (parts[0] === 172 && parts[1]! >= 16 && parts[1]! <= 31) ||
  (parts[0] === 192 && parts[1] === 168) ||
  (parts[0] === 169 && parts[1] === 254) ||
  (parts[0] === 198 && parts[1]! >= 18 && parts[1]! <= 19);

const isSpecialPurposeIpv4Address = (parts: readonly number[]): boolean =>
  isPrivateIpv4Address(parts) ||
  parts[0]! >= 224 ||
  // Suppress the entire protocol-assignment block, including public .9/.10, to avoid disclosing special-purpose addresses.
  (parts[0] === 192 && parts[1] === 0 && parts[2] === 0) ||
  (parts[0] === 192 && parts[1] === 0 && parts[2] === 2) ||
  (parts[0] === 192 && parts[1] === 88 && parts[2] === 99) ||
  (parts[0] === 198 && parts[1] === 51 && parts[2] === 100) ||
  (parts[0] === 203 && parts[1] === 0 && parts[2] === 113);

export const isLocalLoopbackHost = (host: string): boolean => {
  const normalized = normalizeHostname(host);
  if (normalized === "localhost" || normalized === "::1") return true;
  return parseIpv4Address(normalized)?.[0] === 127;
};

/** MagicDNS or fd7a:115c:a1e0::/48 identifies Tailscale; shared 100.64.0.0/10 addresses alone do not. */
export const isTailnetHost = (host: string): boolean => {
  const normalized = normalizeHostname(host);
  if (normalized.endsWith(".ts.net")) return true;
  const ipv6 = parseIpv6Address(normalized);
  return ipv6 !== null && ipv6PrefixMatches(ipv6, [0xfd7a, 0x115c, 0xa1e0, 0, 0, 0, 0, 0], 48);
};

export const isSharedAddressSpaceHost = (host: string): boolean => {
  const parts = parseIpv4Address(host);
  return parts !== null && parts[0] === 100 && parts[1]! >= 64 && parts[1]! <= 127;
};

export const isPrivateNetworkHost = (host: string): boolean => {
  const normalized = normalizeHostname(host);
  if (
    normalized === "::" ||
    isLocalLoopbackHost(normalized) ||
    normalized.endsWith(".localhost") ||
    normalized.endsWith(".local") ||
    normalized === "home.arpa" ||
    normalized.endsWith(".home.arpa") ||
    (!normalized.includes(".") && !normalized.includes(":"))
  ) {
    return true;
  }
  if (normalized.endsWith(".ts.net")) return true;
  const parts = parseIpv4Address(normalized) ?? parseIpv4MappedIpv6Address(normalized);
  if (parts) return isPrivateIpv4Address(parts);
  const firstIpv6Token = normalized.split(":", 1)[0] ?? "";
  if (!normalized.includes(":") || !/^[\da-f]{1,4}$/u.test(firstIpv6Token)) return false;
  const firstIpv6Hextet = Number.parseInt(firstIpv6Token, 16);
  return (
    Number.isInteger(firstIpv6Hextet) &&
    ((firstIpv6Hextet & 0xfe00) === 0xfc00 || (firstIpv6Hextet & 0xffc0) === 0xfe80)
  );
};

// Includes private TLDs from RFC 6762 Appendix G.
const PRIVATE_FAVICON_TLDS = [
  ".alt",
  ".corp",
  ".example",
  ".home",
  ".internal",
  ".intranet",
  ".invalid",
  ".lan",
  ".onion",
  ".private",
  ".test",
];
const INTERNAL_HOST_LABELS: ReadonlySet<string> = new Set(["corp", "internal", "intranet"]);

export const isPublicFaviconHost = (host: string): boolean => {
  // One trailing DNS dot is valid; repeated dots can conceal legacy numeric addresses.
  if (host.endsWith("..")) return false;
  const normalized = normalizeHostname(host);
  if (isPrivateNetworkHost(normalized)) return false;
  if (
    PRIVATE_FAVICON_TLDS.some(
      (suffix) => normalized === suffix.slice(1) || normalized.endsWith(suffix),
    )
  ) {
    return false;
  }
  // Public domains can contain internal service labels; keep them off third-party requests.
  if (normalized.split(".").some((label) => INTERNAL_HOST_LABELS.has(label))) return false;
  const ipv4 = parseIpv4Address(normalized) ?? parseIpv4MappedIpv6Address(normalized);
  if (ipv4) return !isSpecialPurposeIpv4Address(ipv4);
  if (!normalized.includes(":")) return true;
  const ipv6 = parseIpv6Address(normalized);
  if (!ipv6) return false;
  if (ipv6PrefixMatches(ipv6, [0x0064, 0xff9b, 0, 0, 0, 0, 0, 0], 96)) {
    const embeddedIpv4 = [ipv6[6]! >>> 8, ipv6[6]! & 0xff, ipv6[7]! >>> 8, ipv6[7]! & 0xff];
    return !isSpecialPurposeIpv4Address(embeddedIpv4);
  }
  const first = ipv6[0]!;
  if ((first & 0xe000) !== 0x2000) return false;
  if (ipv6PrefixMatches(ipv6, [0x2001, 0, 0, 0, 0, 0, 0, 0], 23)) {
    const publicProtocolAssignment =
      (ipv6[1] === 1 &&
        ipv6.slice(2, 7).every((part) => part === 0) &&
        [1, 2, 3].includes(ipv6[7]!)) ||
      ipv6PrefixMatches(ipv6, [0x2001, 3, 0, 0, 0, 0, 0, 0], 32) ||
      ipv6PrefixMatches(ipv6, [0x2001, 4, 0x0112, 0, 0, 0, 0, 0], 48) ||
      ipv6PrefixMatches(ipv6, [0x2001, 0x20, 0, 0, 0, 0, 0, 0], 28) ||
      ipv6PrefixMatches(ipv6, [0x2001, 0x30, 0, 0, 0, 0, 0, 0], 28);
    return publicProtocolAssignment;
  }
  if (ipv6PrefixMatches(ipv6, [0x2001, 0x0db8, 0, 0, 0, 0, 0, 0], 32)) return false;
  if (ipv6PrefixMatches(ipv6, [0x2002, 0, 0, 0, 0, 0, 0, 0], 16)) return false;
  if (first === 0x3fff && (ipv6[1]! & 0xf000) === 0) return false;
  return true;
};