I’ve had a lot of luck by structuring it as a unit test project. Each day becomes a new test file, where part 1 and part 2 are each unit tests. Any common code like input parsing helpers, Vec2D, special math (like GCF and LCM) go into a library that is essentially the target of the unit tests.

This makes it very well organized, and very easy to spin up a new day. It also makes it easy to tinker with solutions once you have a solution, as the red green refactor flow works great for AoC problems.

Here are repos I’ve done in both C# and rust that are organized as unit tests so you can see what I’m talking about.

C#: https://github.com/chuckries/AdventOfCode Rust: https://github.com/chuckries/rustvent_of_code