I haven't completed one before and I have a question for those who have. Can I treat the entire month as one code based which evolves over time, with an evolving set of unit and integration tests? Or do some days require starting from scratch? Lots of people here are talking about completing it in exotic languages but I'm more interested in using it to sharpen enterprise coding techniques.
Every day is mostly independent of the others. I generally don't reuse code because most of the common code is easy enough to rewrite, sometimes I copy/paste earlier code in to a new solution.
The main exception is 2019's Intcode VM/interpreter which was used over about a dozen days (I mentioned it elsewhere, he hasn't done anything like that since). Occasionally, beyond basic algorithms there is some continuity between 2-3 days but that's not terribly common, I don't think there were any pairs of puzzles like that last year or the year before.
If you want to make one code base, you'll still have 25 or so separate solutions. You might have some continuity with common algorithms (like A*) that get used a fair bit in search problems or abstracting out the input handling when you identify common structures for parsing.
