The software I developed, CryptoMiniSat 2.5 won the SAT Race of 2010. It is a great honour to win such a difficult race, as there were 19 other solvers submitted from 9 different countries.
Thinking back and trying to answer the question why CryptoMiniSat won, I think there are no clear answers. Naturally, I put immense effort into it, but that is only one part of the picture. Other important factors include those who supported me: my fiancée, the PLANETE project of INRIA, and in particular Claude Castelluccia and my colleagues here in Paris, the LIP6 team and my professor here, Jean-Charles Faugere. As with all randomised algorithms, luck played a good part of winning, too.
In all of this, it is interesting to note that I never had a colleague who worked in or even near the field of SAT: both the PLANETE and the LIP6 team use SAT solvers only as a means to an end, not as an end in itself. In other words, I had to get this far mostly alone. To be honest, I think this is one of the reasons I had won, too: there probably is too much misunderstanding in SAT that everybody takes for granted. Since I never really talked with anyone about their understanding of the subject (I only read papers, which state facts), I had a white paper in front of me, which I could fill with my own intuitions, without being lead by those of others. Naturally, this means that I have probably stepped into a large number things that are widely understood to be wrong, but at the end, it seems that I must have also made some good steps, too.
As for the bad steps I have made (and of which I am aware of), I had copied the subsumption algorithm from SatElite, which the authors of MiniSat told me was a bad move, since its self-subsuming resolution is slow. I had in fact observed this, but I thought it was a fact of life. Alas, it isn’t. Other errors I have made include a non-portable pointer-shortening which shortens pointers to 32 bits when using 64-bit architectures (Armin Biere’s lingeling does this right), and a wrong clause-prefetching code that Norbert Manthey put me right about. I have also conceptually misunderstood the possible impact of variable elimination on the difficulty of resolving an instance, a grave mistake that Niklas Sörensson put me right about.
As for the good steps, I think I have made a couple of them. Firstly, I have put the 2009 SAT paper we wrote with my colleagues to its natural conclusion: including XOR clauses and partial XOR reasoning into the innermost parts of the solver. Secondly, I have used the community to its fullest potential: CryptoMiniSat was used by the constraint solver STP far before the SAT Race began, and it was distributed to a number of highly skilled individuals who made use of it and gave feedback. Sometimes I got very negative feedbacks (these were the most valuable), which highlighted where and why the solver was failing. Thirdly, I have tried to take advantage of the academic environment I work in: I have tried to read the relevant papers, used the clusters provided by the French universities to carry out experiments, and generally took my time (rarely an option in industry) to get things right instead of rushing out a solver.