@Override
  public int lengthFor(JavaDataModel model) {
    // 5 is the head, 1<<p means the number of bytes for register
    return (5 + (1 << p));
  }
Converts sparse to dense hll register.
@param sparseRegister
         - sparse register to be converted
@return converted dense register
standalone-metastore/metastore-server/src/main/java/org/apache/hadoop/hive/common/ndv/hll/HyperLogLog.java