feat: 完善视频任务与 KOC 资源库

lib/video-file.ts

@@ -0,0 +1,45 @@
+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;
+}