I know what you're describing well, I've dabbled quite a bit in chess engine dev myself, and I'm planning to get back into it soon; I've got some interesting new ideas recently I wanna try out(once they're fleshed out enough to actually be implemented, right now they're just fanciful ideas I'm kicking around my head).

Testing is a bitch though, for sure. I know that stockfish is constantly being playtasted against itself, with a new instance spawned for every pull request etc, and then given an elo rating. That way they can tell if a potential change makes it weaker or stronger.

Debugging isn't easy either. Forget about stepping over code in the debugger. You have no idea whether the bug is only triggered after billions of nodes. That's a lot of stack frames to step through. And forget about debug prints too, for the most part, because putting an unconditional debug print in your search() , qsearch() or eval() will quickly lead to gigabytes and gigabytes of output...

Only helpful thing I found was to use asserts. Find invariants, and in your debug version check them every node, die if they don't hold and barf out your stack frame or a core dump. If you're lucky the bug is somewhere near where the assert failed in the call tree. Even that isn't guaranteed though.