function indexGB18030CodePointFor(pointer) {
    if ((pointer > 39419 && pointer < 189000) || (pointer > 1237575)) {
      return null;
    }
    var /** @type {number} */ offset = 0,
        /** @type {number} */ code_point_offset = 0,
        /** @type {Array.<Array.<number>>} */ idx = index('gb18030');
    var i;
    for (i = 0; i < idx.length; ++i) {
      var entry = idx[i];
      if (entry[0] <= pointer) {
        offset = entry[0];
        code_point_offset = entry[1];
      } else {
        break;
      }
    }
    return code_point_offset + pointer - offset;
  }
@param {number} pointer The |pointer| to search for in the gb18030 index.
@return {?number} The code point corresponding to |pointer| in |index|,
    or null if |code point| is not in the gb18030 index.