FS is a classical planner that works with the Functional STRIPS planning language [Geffner, 2000], a modeling language based on the quantifier-free fragment of first-order logic that includes constant, function and predicate symbols, but no variable symbols. The increased expressiveness of the Functional STRIPS language with respect to propositional languages such as standard STRIPS (which is indeed subsumed by Functional STRIPS) often results in problem encodings which are more compact, more readable, have fewer ground actions and preserve the structural properties of the problem in a manner which allows the derivation of more effective heuristics.

Along with the core of the Functional STRIPS language, the FS planner supports certain extensions which are useful both from the expressive and the computational point of view. These include existential quantification, state constraints, a fairly large library of global constraints, and the possibility of using externally-defined symbols and built-in arithmetic symbols.

This documentation covers a number of practical issues related to the use of the FS planner. The planner, however, has been used and described in a number of academic publications that can be found here, the most recent of which are [Francès and Geffner, 2015] and [Francès and Geffner, 2016a] and [Francès and Geffner, 2016b].

• Installation
• Usage
• Credits
• References

In order to install and run the FS planner, you need the following software components:

1. The LAPKT Planning Toolkit, which provides the base search algorithms used with our heuristics. You should use the branch v2_work, as explained here.

2. A custom version of the Fast Downward PDDL 3.0 parser (written in Python), modified with the purpose of fully supporting the functional capabilities of the language and allowing for constraints and external procedures to be used on the specification of the domain.

3. The Gecode CSP Solver (tested with version 4.4.0 only). The recommended way to install it is on ~/local, i.e. by running ./configure --prefix=~/local before the actual compilation.

Once you have installed these projects locally, your system needs to be configured with the following environment variables, e.g. by setting them up in your ~/.bashrc configuration file:

export LAPKT_PATH="${HOME}/projects/code/lapkt"
export LAPKT2_PATH="${LAPKT_PATH}/aptk2"
export FD_AIG_PATH="${HOME}/projects/code/downward/downward-aig"
export FS_PATH="${HOME}/projects/code/fs"

# Local C++ library installations
export LD_LIBRARY_PATH=$LIBRARY_PATH:/usr/local/lib
if [[ -d ${HOME}/local/lib ]]; then
	export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:${HOME}/local/lib
fi

# LAPKT and FS libraries
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:${FS_PATH}/lib
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:${LAPKT2_PATH}/lib

# AIG Fast Downward PDDL Parser
if [[ -d ${FD_AIG_PATH}/src/translate ]]; then
	export PYTHONPATH="${FD_AIG_PATH}/src/translate:$PYTHONPATH"
fi

Once all this is set up, you can build the project library by doing

cd $FS_PATH
scons

You can run scons debug=1 to build the debug version of the library, or scons edebug=1 to build an extremely-verbose debug version.

FS is invoked through a high-level Python script which parses any FSTRIPS problem specification and generates certain data which is necessary for the main planner module to run. The main Python preprocessing script is $FS_PATH/preprocessor/runner.py, and can be invoked like (replace $BENCHMARKS by an appropriate directory):

python3 runner.py --tag foo --instance $BENCHMARKS/fn-simple-sokoban/instance_6.pddl --run --driver=smart

Where instance_6.pddl is a Functional STRIPS (or standard STRIPS) instance file, and foo is an arbitrary name that will be used to determine the output directory where the executable solver, related data, and results will be left, which in this case will be $FS_PATH/generated/test/fn-simple-sokoban/instance_6. Read below for further details on the semantics of the --run and --driver options. If externally-defined symbols are used, the parsing process involves the automatic generation of a bunch of C++ classes that is then compiled and linked against the binary solver. If --run is not used, only the parsing stage is run, in which case all files necessary to run the solver are left on the abovementioned directory, and nothing else is done. If that is the case, we can run the automatically generated solver.bin executable from that directory (add the -h flag for further options :

cd $FS_PATH/generated/test/fn-simple-sokoban/instance_6
./solver.bin

Note that only the non-debug executable is built by default, but you can invoke the generator.py script with flags --debug and --edebug to control the debug level of the resulting executable.

Once the planner for a particular problem instance has been compiled, and we are about to run it on the particular planner directory, a number of options can be specified on the command line, the most prominent of them being the search driver.

The planner is pre-configured with a number of "search drivers" that specify the global search strategy (search algorithm plus heuristic, if necessary) that will be followed in the search for a plan. The --driver command-line option is thus mandatory; for instance, to use the planner with the smart driver we would invoke ./solver.bin --driver=smart. The following are the main available drivers:

• lite: The lite driver is a greedy best-first search which works with the non-constrained RPG heuristics, i.e. computes the standard h_FF and h_MAX heuristics, the only difference being that it can work with the more compact Functional STRIPS encodings, thus not being restricted to predicative STRIPS.

• native: The native driver implements a greedy best-first search coupled with a constrained RPG heuristic that can be either the h_FF (heuristic=hff) or the h_MAX heuristic (heuristic=hmax). The particularity of this driver is that the CSPs into which the computation of the heuristic is mapped are not solved by Gecode, but rather by a native, hand-coded simplified approach which might yield some performance gain, since it avoids the overhead of interacting with Gecode. The downside of this approach is that only a certain subset of FSTRIPS is accepted (namely, that which results in very simple CSPs), and only approximated CSP resolution is used.

• standard: A greedy best-first search with one of the two constrained RPG heuristics, which is computed with a 1-CSP-per-ground-action model.

• smart: The smart driver implements a greedy best-first search coupled with a constrained RPG heuristic that can be either the h_FF (heuristic=hff) or the h_MAX heuristic (heuristic=hmax). When it comes to computing the heuristic, though, this driver does not need to work on a "1-CSP-per-ground-action" basis --- the limitation of this being that the number of ground actions, and thus of CSPs, is exponential in the number of parameters of the action schemas. Instead, this driver works on a "1-CSP-per-action-effect" basis, but the grounding is "smart", i.e. it only grounds as much as necessary so that the effect head is a state variable. Among other things, this means that the number of CSPs is now exponential only in the number of action parameters in the head of the effect, which will tipically be smaller. The smart driver accepts conditional effects.

• lifted: The lifted driver implements a fully-lifted greedy-best first search, meaning that actions are never grounded, but instead the constraint-based nature of the planner are used to model the task of deciding which actions are applicable in a given state as a particular CSP which is then solved whenever we need to expand a node during the search. This can yield a benefit in problems with a huge number of ground actions, which usually will not work well with traditional planners that ground all the action schemas, as they will never go beyond the grounding phase.

• smart_lifted: This driver conducts a fully-lifted search as the lifted driver above, This is thus equivalent to the smart driver above, but using lifted search as in the lifted driver.

• unreached_atom: An experimental driver which performs a greedy best-first search on a version of the constrained RPG heuristics which is computed in a somewhat different manner that iterates through atoms that have not yet been reached in the RPG, trying to achieve them one by one. Seems to perform better than other options in some domains, but not in general.

• iw: Iterated Width search.

• novelty_best_first: A Greedy best-first search with a novelty-based heuristic. Namely, the search favors states with (1) lower novelty, (2) higher number of satisfied goal atoms, if novelty is equal, and (3) lower accumulated cost, the two first factors being equal.

• bfs: A blind, standard breadth-first search.

There are some other important options that can also be specified on the command line. Some of them are particular to each driver, some of them are general to more than one (perhaps to all) drivers. These options are not mandatory and, if not provided, the defaut value for each of them is the one read from the defaults.json configuration file which is located in the planner directory. If you want to override the value of a particular option through the command line, you can do so with the --options parameter, which accepts a list of comma-separated name=value pairs, as in e.g. ./solver.bin --driver=smart --options="heuristic=hff,novelty=false".

The most common configuration options are:

• heuristic: Usually either hff or hmax.
• goal_resolution: Either full or approximate: whether to fully or only approximately solve the action goal CSPs.
• precondition_resolution: Same than goal_resolution but for action precondition formulas.
• novelty: Either true or false. Some CSP models for the computation of support some kind of extra constraint to enforce that the solutions of the CSP do indeed map into atoms which are novel in the RPG. This variable controls the usage of these constraints.

Besides, there are some other obscure / experimental options, mostly for internal usage and testing:

• plan_extraction: Either propositional or extended. The type of plan extraction procedure.
• goal_value_selection: Either min_val and min_hmax. The type of CSP value selection to use in goal CSPs.
• action_value_selection: Same than goal_value_selection, but for action CSPs.
• support_priority: Either first or min_hmaxsum. Which support sets should be given priority.

The FS planner is partially built upon the Lightweight Automated Planning Toolkit and the PDDL parser from the Fast Downward distribution.

• Francès, G., and Geffner, H. (2015), Modeling and Computation in Planning: Better Heuristics from More Expressive Languages, ICAPS 2015.

• Francès, G., and Geffner, H. (2016a), E-STRIPS: Existential Quantification in Planning and Constraint Satisfaction, IJCAI 2016.

• Francès, G., and Geffner, H. (2016b), Effective Planning with More Expressive Languages, IJCAI 2016.

• Geffner, H. (2000), Functional STRIPS: A more flexible lan- guage for planning and problem solving. In Minker, J., ed., Logic-Based Artificial Intelligence. Kluwer. 187–205.