Using constraint programming for solving RCPSP/max-cal

Resource-constrained project scheduling with the objective of minimizing project duration (RCPSP) is one of the most studied scheduling problems. In this paper we consider the RCPSP with general temporal constraints and calendar constraints. Calendar constraints make some resources unavailable on certain days in the scheduling period and force activity execution to be delayed while resources are unavailable. They arise in practice from, e.g., unavailabilities of staff during public holidays and weekends. The resulting problems are challenging optimization problems. We develop not only six different constraint programming (CP) models to tackle the problem, but also a specialized propagator for the cumulative resource constraints taking the calendar constraints into account. This propagator includes the ability to explain its inferences so it can be used in a lazy clause generation solver. We compare these models, and different search strategies on a challenging set of benchmarks using the lazy clause generation solver chuffed and IBM CPLEX CP Optimizer, respectively. We close all but 8 of the open problems of the benchmark set, extend the benchmark set by instances with up to 500 activities, and show that CP solutions are highly competitive with existing Mip models of the problem.