  SAPA is a domain-independent heuristic forward chaining planner that can
handle durative actions, metric resource constraints, and deadline goals. It is
designed to be capable of handling the multi-objective nature of metric
temporal planning. Our technical contributions include (i) planning-graph based
methods for deriving heuristics that are sensitive to both cost and makespan
(ii) techniques for adjusting the heuristic estimates to take action
interactions and metric resource limitations into account and (iii) a linear
time greedy post-processing technique to improve execution flexibility of the
solution plans. An implementation of SAPA using many of the techniques
presented in this paper was one of the best domain independent planners for
domains with metric and temporal constraints in the third International
Planning Competition, held at AIPS-02. We describe the technical details of
extracting the heuristics and present an empirical evaluation of the current
implementation of SAPA.
