Solving NxN Sudokus with CSPs

I've built a site that lets you play around with solving NxN sudokus, up to 36x36, backed by a Constraint Satisfaction Problem (CSP) solver. The site exposes a bunch of parameters that influence how this problem is solved, including how the cells are selected, what values are attempted first, how the problem is modeled, and how quickly the solver attempts to eliminate impossible states after each run. Each of these comes with a set of trade-offs, like a more thorough elimination algorithm takes longer to run between each step, so each step takes longer, and it can be interesting to try out different combinations to see which result in fasted solutions, fewest backtracks, and so on.

This is definitely not the fastest sudoku solver out there, but the visualizations tend to be fun to watch and should give you an idea of how search incrementally builds up the solution, eliminates uninteresting parts, backtracks on failure, and how the various parameters affect its behavior. You can expand the "?" to see the FAQ about what the colors represent and a bit more details on the parameters.

I also have a bunch of the technical details about how this all comes together in my blog (e.g., this post gives an overview: https://blog.dawnofthe.dad/posts/solver-basics/), and if you want to see another application of this same solver, check out the crossword builder on the same site. The model for crosswords is of course very different, but the underlying engine is the same. The only thing that crosswords share with sudoku is one all-different constraint: while sudokus are basically just one giant glob of all-different constraints, the crossword only has the one, to ensure no word is used twice. This all-different constraint is the "Sparse" one (in the UX), and being tailored for the crossword use-case, unsurprisingly tends to perform poorly for sudokus (because while it's very fast, it's also very simple).

In any event, it's been a fun project to nerd out on, and I hope you enjoy poking at it. I'll be happy to take any questions about it.