Reading an EBML variable-length integer

readVariableInt.js

/**
 * Read an EBML tag header or length and return their respective values into
 * single numbers.
 *
 * @example
 * var tagHeader = new Uint8Array([0x1A, 0x45, 0xDF, 0xA3, 0x01, 0x00, 0x00,
 *                                 0x00, 0x00, 0x00, 0x00, 0x1F]);
 * var tagId = readVariableInt(tagHeader, 4, 0);
 * console.log('The current tag ID is: ', tagId.value, ', skipping ', tagId.size, ' bytes.');
 * // The current tag ID is: 172351395, skipping 4 bytes.
 *
 * var tagLength = readVariableInt(tagHeader, 8, tagId.size);
 * console.log('The tag\'s length is: ', tagLength.value, ', skipping ', tagLength.size, ' bytes.');
 * // The tag's length is 31, skipping 8 bytes.
 *
 * @param {Uint8Array} bytes
 * @param {number} maxSize
 * @param {number} offset
 * @returns {{size: number, value: number}}
 */
function readVariableInt(bytes, maxSize, offset) {
  var index = (offset && offset > 0) ? offset : 0;
  var count = 1;
  var length = bytes[index];
  var bytesRead = 1;
  var lengthMask = 0x80;

  if (!length) {
    return null;
  }

  while (bytesRead <= maxSize && !(length & lengthMask)) {
    bytesRead++;
    lengthMask >>= 1;
  }

  if (bytesRead > maxSize) {
    return null;
  }

  length &= ~lengthMask;
  while (count++ < bytesRead) {
    length = (length << 8) | bytes[++index];
  }

  return {
    size: bytesRead,
    value: length
  };
}