long readVarInt() {
    byte b;
    long res = 0;
    for (b = readByte(); b & 0x80; b = readByte()) {
      res = (res | (b & 0x7F)) << 8;
    }
    return res | b;
  }
  
  string readString() {
    long len = readVarInt();
    
    if (len < 0 || len > 100000) {
      std::stringstream e;
      e << "Requested reading a string of length " << len;
      throw e.str();
    }
    char buf[len];
    len = fread(buf, 1, len, data->file);
    return string(buf, len);
  }