Where can I find some references to the algorithms implemented?

[Enigmatisms]

I've been reading the code in this repo, especially the part of BDPT. I find some part of the code difficult to understand, for example Float Path::miWeight which calculates MIS weight for BDPT path, and this part is noticably more complicated than the simplified ones like that in pbrt. So is there any reference that can be referred to to help me understand the idea behind the implementation? Specifically, I'd need some help about the ideas behind this more advanced BDPT in mitsuba, thanks!