A little confused about the class `FiniteBernoulliBanditLaplace` in `finite_arm.py`

[zhiyiZeng]

Hello~ I'm new to Thompson sampling. In the class FiniteBernoulliBanditLaplace in finite_arm.py, I'm a little confused about laplace_sample. How does the equation come out? And It doesn't look like the example in section 5.2. Can someone give a little help? I would appreciate any help.