## Refactoring Varisat 5: Incremental Solving and Proofs

This is the fifth and final post in my series about refactoring varisat. In the last post varisat gained the heuristics needed to solve some non-trivial instances. In this post we’ll add incremental solving and proof generation. This brings varisat to feature parity with the old version.

Incremental solving enables new use cases that just aren’t feasible with batch solving. Building SMT solvers on top of a SAT solver or techniques like symbolic execution often generate a vast amount of related SAT solver queries. Starting a new solving process for each query would result in a lot of repeated work. Incremental solving avoids some of this.

Proof generation makes it possible to independently check the work performed by a SAT solver. State of the art SAT solvers have a large codebase where it’s easy for a bug to hide. When the SAT solver finds a solution it is easy to tell whether it satisfies the input formula. For the unsatisfiable case, without proof generation, we have to trust the solver to be free of bugs. As solvers tend to have bugs from time to time, the yearly SAT competition started to require proof generation a few years ago.

Being able to generate proofs isn’t only helpful for uses who want to have better assurance of the solver’s correctness, it’s also great for finding and analyzing bugs.