Two Approaches to Semi-Dynamic Disjunctive Temporal Problems

This paper considers two approaches to improving efficiency and stability in semi-dynamic Disjunctive Temporal Problems (DTPs). A semi-dynamic DTP is a sequence of DTPs (Stergiou and Koubarakis 1998) in which each problem is a restriction of the problem before. We consider three basic types of DTP restrictions—tightening the bound of an existing constraint, adding a simple constraint, and adding a disjunctive constraint. The techniques we use for solving semi-dynamic DTPs come from the literature on dynamic Constraint Satisfaction Problems (CSPs) and include nogood recording (Schiex and Verfaillie 1993) and oracles (van Hentenryck and Provost 1991). Experimental results show that nogood recording improves efficiency but hurts stability, whereas oracles improve efficiency even more while also improving stability. The performance of nogood recording and oracles used in combination is not significantly different than the performance of oracles alone.