metroboy VS which.gb

He wrote a game engine, it renders transparent items using the painter's algorithm, which requires the items to be sorted in Z. He has a routine that iterates over all items to render them, and bubble-sorts the items in-place while iterating over the items (all in the same thread). The "compare and swap" in this case is the innermost functionality of bubblesort.

I assume the author is https://github.com/aappleby but I don't see any obvious candidate for this code. Maybe this? https://github.com/aappleby/metroboy but I can't see why a gate-level emulator woudl need to render transparent items using painter's algorithm.

If you want to go deeper, you can look at the reverse engineered schematic of most of the stuff except the CPU core in the DMG-CPU-Inside project. You could also consult the MetroBoy project, which builds on the previous project and aims to be accurate pretty much down to the logic gate level.

Well then, here's the ground truth - https://github.com/aappleby/MetroBoy/blob/master/src/GateBoyLib/GateBoyPixPipe.cpp

First I would try to deduce the timing from this gate-level simulation of the game boy, but the master branch was broken at the time, so I didn't manage to compile it. So, I tried deducing it from its underling reverse engineered schematics, but it would take too much time, if it even was possible.

You might find my hobby project interesting - https://github.com/aappleby/MetroBoy

Did you know the original Game Boy has non-deterministic behavior? Due to some strangeness in the integrated circuit that provides the CPU and some related parts (the SoC (system on a chip)), the same ROM may behave differently at different times when you'd think it should not. GateBoy is an attempt to make a perfect gate-level emulator for the Game Boy. Its README says:

Reminds me of GateBoy, which was posted a while back: https://news.ycombinator.com/item?id=28396927, https://github.com/aappleby/MetroBoy

GateBoy runs surprisingly fast for a gate-level simulation.