I am a huge fan of Simon Tatham's puzzles. His developer documentation [0] is absolutely amazing. All the puzzles are written in very high quality C with, in my opinion, a perfect ratio of comments to code [1].

I was hooked on his Tents [2] puzzle for a few weeks, then I started modifying the code to automatically fill in squares based on the tricks I found for adding constraints. Before I knew it, I had written a complete solver! Now I don't have to waste my time on it and I can get back to work :)

[0]: https://www.chiark.greenend.org.uk/~sgtatham/puzzles/devel/

[1]: https://git.tartarus.org/?p=simon/puzzles.git;a=blob;f=tents...

[2]: https://www.chiark.greenend.org.uk/~sgtatham/puzzles/js/tent...