I had access to a repo (from a closed startup) with 800K lines of python & C code, written from the 90s to today. They had some very interesting approach to a specific chemistry problem. 20-30 years of work of several persons.

But God, I could not understand the code, and I could not easily make it work with modern technologies (GPU etc).

So I used Claude and Gemini to reverse engineer the codebase, extract the core ideas, and rewrite it from scratch with modern frameworks (with guidance from the original authors)

It took me only 10 days to have a functioning equivalent, in 10K lines of code (using many libraries that did not exist in the 90s and 00s), which I find much easier to understand, even though I wrote none of it myself.

10 days to rewrite 20-30 year of a few persons. That was quite scary.