const MP4_BRANDS = new Set([
  "avc1",
  "dash",
  "isom",
  "M4A ",
  "M4B ",
  "M4P ",
  "M4V ",
  "mp41",
  "mp42",
  "MSNV",
]);

function fourCharacters(bytes: Uint8Array, offset: number) {
  return String.fromCharCode(...bytes.slice(offset, offset + 4));
}

function isMp4Brand(brand: string) {
  return (
    MP4_BRANDS.has(brand) ||
    /^iso[2-9]$/.test(brand) ||
    /^3g[2p]$/.test(brand.slice(0, 3))
  );
}

/** Rejects HTML/JSON/error payloads and non-MP4 containers before download. */
export function hasMp4FileSignature(input: ArrayBuffer | Uint8Array) {
  const bytes = input instanceof Uint8Array ? input : new Uint8Array(input);
  if (bytes.byteLength < 12 || fourCharacters(bytes, 4) !== "ftyp") {
    return false;
  }
  const declaredSize = new DataView(
    bytes.buffer,
    bytes.byteOffset,
    bytes.byteLength,
  ).getUint32(0);
  const boxEnd = Math.min(
    bytes.byteLength,
    declaredSize >= 12 ? declaredSize : bytes.byteLength,
  );
  for (let offset = 8; offset + 4 <= boxEnd; offset += 4) {
    if (isMp4Brand(fourCharacters(bytes, offset))) return true;
  }
  return false;
}