[jimnotgym]

> functionally correct.

It is often true that someone with very strong domain knowledge may have had this project in their head for a long time, and one day, all-of-a-sudden, has to code it all in a rush. I have had this, where external factors shifted the companies priorities.

In this case you can 'brain dump' a lot of code very quickly, get it functionally correct and make intelligent judgement calls about the threats of edge cases... especially if it is internal company software.

However, coming at the problem from cold, without the benefit of this knowledge requires significant analysis and engineering effort to get equivalent results. So tread very carefully.

But yes, add tests