zhengruifeng commented on pull request #36048:
URL: https://github.com/apache/spark/pull/36048#issuecomment-1086633584
check against the Pandas side:
```
In [65]: s = pd.Series([.2, .0, .6, .2, np.nan, .5, .6])
In [66]: s.autocorr()
Out[66]: -0.14121975762272054
In [67]: s.autocorr(0)
Out[67]: 1.0
In [68]: s.autocorr(-3)
Out[68]: 0.2773500981126146
In [69]: s.autocorr(5)
Out[69]: -0.9999999999999998
In [70]: s.autocorr(6)
/home/zrf/.zrf/anaconda3/lib/python3.9/site-packages/numpy/lib/function_base.py:2683:
RuntimeWarning: Degrees of freedom <= 0 for slice
c = cov(x, y, rowvar, dtype=dtype)
/home/zrf/.zrf/anaconda3/lib/python3.9/site-packages/numpy/lib/function_base.py:2542:
RuntimeWarning: divide by zero encountered in true_divide
c *= np.true_divide(1, fact)
Out[70]: nan
```
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