Past exam of the computer science course of the University of Oxford / 2020 / Artificial intelligence / 2 / f / Solution

The translation gives a sound and complete route through an optimized SAT solver; it trades access to general-purpose SAT machinery against encoding cost and the possible loss of constraint-specific structure. Modern SAT solvers use efficient unit propagation, adaptive branching and clause learning. Learned Boolean clauses can summarize conflicts involving many original variables, allowing later branches to avoid repeating the same failed combination. The reduction also offers a standard, reusable interface: improvements to a SAT solver can benefit many finite-domain constraint satisfaction problems.

The Boolean formula may be much larger than the original constraint satisfaction problem, especially for a succinct high-arity relation. One original variable becomes many indicators, and general branching may lose the interpretation that the minimum remaining values and degree heuristic exploit directly. Native constraint propagation can exploit a relation without enumerating all its forbidden tuples. Even in the binary case, stronger propagation does not follow just from switching to SAT.

For example, take $D_X=\{1,2\}$, $D_Y=\{a,b,c\}$, and let the only allowed pairs be $(1,c),(2,a),(2,b)$. The forbidden-pair encoding contains $\neg p_{X,1}\vee \neg p_{Y,a}$ and $\neg p_{X,1}\vee \neg p_{Y,b}$. If $p_{Y,c}$ becomes false, arc consistency deletes $X=1$ because it has no support. Yet the direct encoding has no unit Boolean clause forcing $\neg p_{X,1}$: both $a,b$ remain possible individually, so plain unit propagation does not make that deletion. A SAT solver can eventually derive it through additional reasoning or search. Conversely, clause learning can combine information beyond the local consistency enforced by AC-3. Neither approach uniformly dominates: performance depends on representation, propagation strength and the instance's structure; both still face exponential worst-case search on general inputs.