I want to code a Chinese chess engine, and I want to modify the Stockfish source code to do so, but the Stockfish source code is too hard to understand. There is no document that explains everything. I don't know where to start. For example, today I tried to understand the bitbase.cpp file. This line of code in this file is not well documented:
constexpr unsigned MAX_INDEX = 2*24*64*64; // stm * psq * wksq * bksq = 196608
I really don't know where to ask questions about these kinds of things. It's open source but it seems to be understood by only a few people. Please give me suggestions to understand the source code better.