I'm the person that you're replying to responded to. This is exactly what I went through during that four month period. The existing tests weren't very useful

I'm the person that you're replying to responded to. This is exactly what I went through during that four month period. The existing tests weren't very useful and I was completely changing the structure of how the program was architected, but the input v output could be measured. I, like you, also discovered critical bugs in the existing implementation because I needed to cover the entire combinatorial set or possible inputs whilst the original implementers presumably went through them as they came up.