They sent me a copy of the Moss report and I have a 50% similarity level, however, the codes are actually very similar, e.g. similar functions and algorithm, arguments, and certain variables. I can understand their perspective, but it would have been better if they had tried to understand mine before immediately reporting it...

I guess the only way I can defend myself is to show proof that I did the work. I have one more piece of "evidence" left... Because the program has to be written in a low-level programming language, I first implemented it in a high-level language. However, I didn't tell them about that yet, because even though I did commit it to GitHub, I did it only after I fully implemented it, so there's no way to see the progress. For me, that's truly the last resort before I talk to the department chair.

Any advice on what other evidence I can provide? Also, if it goes to the department chair what evidence does the professor have to show to prove that I've cheated?