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A fully-automated domain-independent planner for PDDL2.2 domains
(bib entry, bib entry)

Awarded at the 4th International Planning Competition (IPC4)
Top performer at IPC4 in domains involving "Timed Initial Literals"

Top performer at IPC4 in plan quality (satisficing planning track)

Awarded best fully-automated planner at the 3rd International Planning Competition (IPC3)
new: ICAPS 2019 Influential Paper Award for the Paper "LPG: A Planner Based on Local Search for Planning Graphs with Action Costs".

The LPG-td version (June 2004)

LPG-td is a new version of LPG that extends and improves version 1.2 (see below), and that took part in the 4th International Planning Competition, 2004. LPG-td is an extension of LPG to handle the new features of the standard planning domain description languages PDDL2.2: "Timed initial literals" and "derived predicates". Timed initial literals are a simply way to express facts that become true or false at certain known time points, and they correspond to deterministic unconditional exogenous events, which are practically important for modeling planning domains. Derived predicates are predicates that are not affected by any of the actions available to the planner. Their truth values are determined by a set of implicative domain rules.  More details on the new features of PDDL and links to related material are available from  the official web site of IPC4.

At IPC4 we submitted three versions of LPG-td, but only the first two were evaluated due to time constraints in producing and evaluating all data:
An abstract describing the most important new features of LPG-td can be found here, while some papers describing the LPG approach to planning can be found below. Some experimental results from the IPC4 and a comparison with the previous version of the planner using the IPC3 test domains:
Download of LPG-td (executable code)

The 1.2 version of LPG (September 2003)

Download of LPG 1.2
(executable and source code)

Short description of LPG 1.2 (see also available papers below)

LPG (Local search for Planning Graphs) is a planner based on local search and planning graphs that handles PDDL2.1 domains involving numerical quantities and durations. The system can solve both plan generation and plan adaptation problems.  The basic search scheme of LPG was inspired by Walksat, an efficient procedure to solve SAT-problems. The search space of LPG consists of "action graphs", particular subgraphs of the planning graph representing partial plans. The search steps are certain graph modifications transforming an action graph into another one.  LPG exploits a compact representation of the planning graph to define the search neighborhood and to evaluate its elements using a parametrized function, where the parameters weight different types of inconsistencies in the current partial plan, and are dynamically evaluated during search using discrete Lagrange multipliers.
The evaluation function uses some  heuristics to estimate the "search cost" and the "execution cost" of achieving a (possibly numeric) precondition.  Action durations and numerical quantities (e.g., fuel consumption) are represented in the actions graphs, and are modeled in the evaluation function. In temporal domains, actions are ordered using a "precedence graph" that is maintained during search, and that takes into account the mutex relations of the planning graph.
The system can produce good quality plans in terms of one or more criteria. This is achieved by an anytime process producing a sequence of plans, each of which is an improvement of the previous ones in terms of its quality. LPG is integrated with a best-first algorithm similar to the one used by FF. The system can automatically switch to best-first search after a certain number of search steps and "restarts" have been performed. Finally, LPG can be used as a preprocessor to produce a quasi-solution that is then repaired by ADJ, a plan-analysis technique for fast plan-adaptation.

Experimental Results (3rd IPC test problems)

Color Plots and data showing the performance of LPG for all test problems of the planning competition. Notice that these are new results, improving the official results of the competition. Overall, the number of problems attempted in the new tests by our planner was 468 (over a total of 508 problems), and the success ratio was 94.4%  (the problems attempted by LPG  in the competition were 428 and the  success ratio 87%), which by far the highest success ratio among the fully-automated planners of the competition

Performance of LPG in Satellite HardNumeric: Speed, Quality
This domain was not addressed in the 3rd IPC because the current version of the system did not handle maximization of plan metrics expressions. LPG is compared with FF and MIPS. (Note that we consider valid plans only those that are not empty). LPG solves more problems than the other planners and produces plans which have a much better quality.

Performance of  LPG's RelaxedPlan heuristics with and without counting "threats" in the relaxed plans. Details on these heuristics and this experiment are given in the JAIR paper: Color Plots.  

Main papers related to LPG (from 1999)

LPG Team

Current members: Alfonso E. Gerevini , Alessandro Saetti, Ivan Serina, and Mauro Vallati 
Planning group coordinator: Alfonso E. Gerevini
Undergraduate students (now graduated) previously involved: Marco Lazzaroni, Stefano Orlandi, Valerio Lorini, Fabrizio Morbini, Sergio Spinoni, Alberto Bettini, Paolo Toninelli, Fabrizio Bonfadini.
Programmer previouly involved: Maurizio Vitale (for InLPG)

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