findSidx.js

/**
 * Translate groups of 4 big-endian bytes to Integer.
 * @param {Uint8Array} bytes
 * @param {Number} offset - The offset (from the start of the given array)
 * @returns {Number}
 */
function be4toi(bytes, offset) {
  return (
    (bytes[offset + 0] * 0x1000000) +
    (bytes[offset + 1] * 0x0010000) +
    (bytes[offset + 2] * 0x0000100) +
    (bytes[offset + 3]));
}

/**
 * Find the right box in an isobmff.
 * @param {Uint8Array} buf - the isobmff
 * @param {Number} wantedName
 * @returns {Number} - Offset where the box begins. -1 if not found.
 */
function findBox(buf, wantedName) {
  const len = buf.length;
  let i = 0;
  while (i + 8 < len) {
    const size = be4toi(buf, i);
    if (size <= 0) {
      console.error("size out of range");
      return - 1;
    }

    const name = be4toi(buf, i + 4);
    if (name === wantedName) {
      if (i + size <= len) {
        return i;
      }
      console.error("box out of range");
      return -1;
    }

    i += size;
  }
  return -1;
}

/**
 * @param {Uint8Array} segment - the isobmff
 * @returns {Object}
 */
function findSidx(segment) {
  const offset = findBox(segment, 0x73696478 /* "sidx" */);
  if (offset === -1) {
    console.log("sidx not found");
    return null;
  }

  // do things with the sidx:
  // offset       -> (offset + 4)       = length
  // (offset + 4) -> (offset + 8)       = 0x73696478 ("sidx")
  // (offset + 8) -> (offset + length)  = box content
}