@Override
  public void beforeStart(UDFParameters parameters, UDTFConfigurations configurations)
      throws Exception {
    configurations.setAccessStrategy(new RowByRowAccessStrategy()).setOutputDataType(Type.DOUBLE);
    this.interval = Util.parseTime(parameters.getStringOrDefault("interval", "1s"), parameters);
    SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    this.start = 0;
    if (parameters.hasAttribute(START_PARAM)) {
      this.start = format.parse(parameters.getString(START_PARAM)).getTime();
    }
  }
This function does Inverse Fast Fourier Transform for input series.
library-udf/src/main/java/org/apache/iotdb/library/frequency/UDTFIFFT.java