People want greenfield projects, but maintaining and improving legacy code is where the real, fundamental learnings occur ;). Probably the cruftiest code was a

People want greenfield projects, but maintaining and improving legacy code is where the real, fundamental learnings occur ;). Probably the cruftiest code was a ship autopilots UI code that was 20k lines of C in one file with no automated testing. There was a very manual process for loading code onto a test display. The debugging experience was terrible. There was a way to manually inspect memory addresses, but that was about it. Oh and there were threads, managed by the homegrown OS (basically just a scheduler that ran on a 10 ms ticks or on an interrupt). These threads had priorities so they could a sometimes get into deadlocks waiting for each other. As insane as it was, it was the best learning experience I could have had. I’m glad I stuck with it. I learned a lot about low level systems, prioritizing the highest value refactoring, and introducing automated testing to a legacy code base.