private double[] autoCorrelation(double[] x) {
    double[] corr = new double[x.length];
    for (int i = 0; i < x.length; i++) {
      corr[i] = pearson(x, x.length - i);
    }
    return corr;
  }
The function is used to compute the period of a numeric time series.
library-udf/src/main/java/org/apache/iotdb/library/dprofile/UDAFPeriod.java