@Override
  public void beforeStart(UDFParameters parameters, UDTFConfigurations configurations)
      throws Exception {
    value.clear();
    timestamp.clear();
    configurations.setAccessStrategy(new RowByRowAccessStrategy()).setOutputDataType(Type.DOUBLE);
    lag = parameters.getIntOrDefault("lag", -1);
  }
This function solves Yule-Walker equation to calculate partial auto-correlation factor.
library-udf/src/main/java/org/apache/iotdb/library/dprofile/UDTFPACF.java