I'll open by saying I've only ever had bad experiences with complete re-writes and these experiences have impacted my aversion to them.
"[Working Effectively with Legacy Code]" by Michael Feathers really helped me get through a situation like this.
My recommendation is not to try to understand the code per se, but understand the business that the code was being used in/by.
From there, over time, just start writing really high level end-to-end tests to represent what the business expects the codebase to do (i.e. starting at the top of the [test pyramid]). This ends up acting as your safety net (your "test harness").
Then it's less a matter of trying to understand what the code does, and becomes a question of what the code should do. You can iterate level by level into the test pyramid, documenting the code with tests and refactoring/improving the code as you go.
It's a long process (I'm about 4.5 years into it and still going strong), but it allowed us to move fast while developing new features with a by-product of continually improving the code base as we went.
[test pyramid]: https://martinfowler.com/bliki/TestPyramid.html
[Working Effectively with Legacy Code]: https://www.amazon.com/FEATHERS-WORK-EFFECT-LEG-CODE/dp/0131...
"[Working Effectively with Legacy Code]" by Michael Feathers really helped me get through a situation like this.
My recommendation is not to try to understand the code per se, but understand the business that the code was being used in/by.
From there, over time, just start writing really high level end-to-end tests to represent what the business expects the codebase to do (i.e. starting at the top of the [test pyramid]). This ends up acting as your safety net (your "test harness").
Then it's less a matter of trying to understand what the code does, and becomes a question of what the code should do. You can iterate level by level into the test pyramid, documenting the code with tests and refactoring/improving the code as you go.
It's a long process (I'm about 4.5 years into it and still going strong), but it allowed us to move fast while developing new features with a by-product of continually improving the code base as we went.
[test pyramid]: https://martinfowler.com/bliki/TestPyramid.html [Working Effectively with Legacy Code]: https://www.amazon.com/FEATHERS-WORK-EFFECT-LEG-CODE/dp/0131...