Sugar++: A SAT-Based MAX-CSP/COP Solver

This paper briefly describes some features of Sugar++, a SAT-based MAXCSP/COP solver entering the Third International CSP Solver Competition. In our approach, a MAX-CSP is translated into a Constraint Optimization Problem (COP), and then it is encoded into a SAT problem by the order encoding method [1]. SAT-encoded COP can be efficiently solved by repeatedly using the MiniSat solver [2]. The order encoding method encodes a comparison x ≤ a by a different boolean variable for each integer variable x and integer value a. Sugar [3] is a SAT-based CSP solver based on order encoding, and its effectiveness has been shown through application to Open-Shop Scheduling in [1]. A solution to COP can be obtained by repeatedly solving a certain number of CSPs. In Sugar, each CSP is solved by a different MiniSat process independently. Therefore the learnt clauses generated in each process disappear and can not be reused. This slows down the execution speed. To solve this problem, we have developed a SAT-based MAX-CSP/COP solver Sugar++ that is an enhancement of Sugar by using the incremental search feature of MiniSat. Sugar++ invokes only one MiniSat process to solve MAXCSP/COP and can reuse the learnt clauses generated during the search.