locate-moov.ts

function toAscii(i: number) {
  return [i >> 24, (i & 0xff0000) >> 16, (i & 0xff00) >> 8, i & 0xff]
    .map((c) => String.fromCharCode(c))
    .join("");
}

type Optional<T> = { r: T; e: null } | { r: null; e: Error };

export async function deflate<T>(p: Promise<T>): Promise<Optional<T>> {
  return p
    .then((res) => Promise.resolve<{ r: T; e: null }>({ r: res, e: null }))
    .catch((err) =>
      Promise.resolve<{ r: null; e: Error }>({ r: null, e: err })
    );
}

const rRange = /(\d+)-(\d+)\/(\d+)/;

interface FetchRangeParams {
  from: number;
  to: number;
  url: string;
}

interface FetchRangeResult {
  begin: number;
  end: number;
  total: number;
  bytes: ArrayBuffer;
}

async function fetchRange({
  from,
  to,
  url,
}: FetchRangeParams): Promise<Optional<FetchRangeResult>> {
  const resp = await deflate(
    fetch(url, {
      headers: {
        Range: `bytes=${from}-${to}`,
      },
    })
  );
  if (resp.e) return { r: null, e: resp.e };

  const mathResults = resp.r.headers.get("Content-Range")?.match(rRange);
  if (!mathResults) return { r: null, e: new Error("deformed response") };

  const [, begin, end, total] = mathResults;
  const bytes = await resp.r.arrayBuffer();

  return {
    r: {
      begin: parseInt(begin, 10),
      end: parseInt(end, 10),
      total: parseInt(total, 10),
      bytes,
    },
    e: null,
  };
}

interface FindMoovResult {
  size: number;
  from: number;
  times: number;
}
async function locateMoov(url: string): Promise<Optional<FindMoovResult>> {
  const times = 10;

  let i = times;
  let from = 0;
  let to = 8;

  while (times) {
    const resp = await fetchRange({
      from,
      to,
      url,
    });
    if (resp.e) return { r: null, e: resp.e };

    const dv = new DataView(resp.r.bytes, 0, resp.r.bytes.byteLength);
    const size = dv.getUint32(0);
    const boxtype = toAscii(dv.getUint32(4));

    if (boxtype === "moov") {
      return { r: { size, from, times: times - i }, e: null };
    }

    console.log(`box is [${boxtype}], continue seeking...`);

    from = from + size;
    to = from + 8;

    i--;
  }

  return { r: null, e: new Error("cannot find moov") };
}

(async () => {
  const url = "https://www.dropbox.com/s/fl02xpyhz8693b0/mv.mp?dl=1";
  const resp = await locateMoov(url);
  if (resp.e) {
    console.error(resp.e);
    return;
  }
  console.log(
    `found moov at ${resp.r.from}, size is ${resp.r.size} after ${resp.r.times} times seeking`
  );
})();