CHANGES.md

Release notes

Fast Downward has been in development since 2003, but the current timed release model was not adopted until 2019. This file documents the changes since the first timed release, Fast Downward 19.06.

For more details, check the repository history (https://github.com/aibasel/downward) and the issue tracker (http://issues.fast-downward.org). Repository branches are named after the corresponding tracker issues.

Changes since the last release

• Add debugging methods to LP solver interface. http://issues.fast-downward.org/issue960 You can now assign names to LP variables and constraints for easier debugging. Since this incurs a slight runtime penalty, we recommend against using this feature when running experiments.

• Support integer variables in linear programs. http://issues.fast-downward.org/issue891 You can now use the LP solver interface to solve mixed integer programs. In particular, the operator-counting heuristics now have an option use_integer_operator_counts that improves the heuristic value by forcing operator counts to take integer values. Solving a MIP is NP-hard and usually takes much longer than solving the corresponding LP.

• For developers: move functionality used during search away from LandmarkGraph, making it constant after creation. http://issues.fast-downward.org/issue988 http://issues.fast-downward.org/issue1000

• For developers: new state class http://issues.fast-downward.org/issue348 We unified the classes GlobalState and State into a new class also called State. This removed a lot of code duplication and hacks from the code. A description of the new class can be found in the wiki: http://www.fast-downward.org/ForDevelopers/Blog/A%20Deeper%20Look%20at%20States

• For developers: introduce class for delete-relaxation based landmark factories and move usage of exploration object to subclasses of (abstract) landmark factory class. http://issues.fast-downward.org/issue990

• For developers: add support for Github actions http://issues.fast-downward.org/issue940

• For developers: We cleaned up the code of LandmarkGraph. Some of the public methods were renamed. This class will undergo further changes in the future. http://issues.fast-downward.org/issue989

• Debug builds with LP solvers vs. the _GLIBCXX_DEBUG flag http://issues.fast-downward.org/issue982 Previously, we used the flag _GLIBCXX_DEBUG in debug builds for additional checks. This makes the binary incompatible with external libraries such as LP solvers. The flag is now disabled by default. If no LP solvers are present or LP solvers are disabled, it can be enabled by setting the CMake option USE_GLIBCXX_DEBUG. The build configurations debugnolp and releasenolp have been removed, and the build configuration glibcxx_debug has been added.

• For developers: decide on rules regarding software support and improve Github actions accordingly http://issues.fast-downward.org/issue1003

• For developers: add CPLEX support to our GitHub Actions for Windows http://issues.fast-downward.org/issue1005

• Fix a bug in the computation of RHW landmarks http://issues.fast-downward.org/issue1004

Fast Downward 20.06

Released on July 26, 2020.

Highlights:

• The Singularity and Docker distributions of the planner now include LP support using the SoPlex solver out of the box. Thank you to ZIB for their solver and for giving permission to include it in the release.

• The Vagrant distribution of the planner now includes LP support using the SoPlex and/or CPLEX solvers out of the box if they are made available when the virtual machine is first provisioned. See http://www.fast-downward.org/QuickStart for more information.

• A long-standing bug in the computation of derived predicates has been fixed. Thanks to everyone who provided bug reports for their help and for their patience!

• A new and much faster method for computing stubborn sets has been added to the planner.

• The deprecated merge strategy aliases merge_linear and merge_dfp have been removed.

Details:

• Fix crash of --show-aliases option of fast-downward.py.

• Fix incorrect computation of derived predicates. http://issues.fast-downward.org/issue453 Derived predicates that were only needed in negated form and cyclically depended on other derived predicates could be computed incorrectly.

• Integrate new pruning method atom_centric_stubborn_sets. http://issues.fast-downward.org/issue781 We merged the code for the SoCS 2020 paper "An Atom-Centric Perspective on Stubborn Sets" (https://ai.dmi.unibas.ch/papers/roeger-et-al-socs2020.pdf). See http://www.fast-downward.org/Doc/PruningMethod.

• Remove deprecated merge strategy aliases merge_linear and merge_dfp. The deprecated merge strategy aliases merge_linear for linear merge strategies and merge_dfp for the DFP merge strategy are no longer available. See http://www.fast-downward.org/Doc/MergeStrategy for equivalent command line options to use these merge strategies.

• For developers: use global logging mechanism for all output. http://issues.fast-downward.org/issue963 All output of the planner is now handled by a global logging mechanism, which prefaces printed lines with time and memory information. For developers, this means that output should no longer be passed to cout but to utils::g_log. Further changes to logging are in the works.

• For developers: store enum options as enums (not ints) in Options objects. http://issues.fast-downward.org/issue962

• For developers: allow creating Timers in stopped state. http://issues.fast-downward.org/issue965

Fast Downward 19.12

Released on December 20, 2019.

Highlights:

• Fast Downward no longer supports Python 2.7, which reaches its end of support on January 1, 2020. The minimum supported Python version is now 3.6.

• Fast Downward now supports the SoPlex LP solver.

Details:

• general: raise minimum supported Python version to 3.6 http://issues.fast-downward.org/issue939 Fast Downward now requires Python 3.6 or newer; support for Python 2.7 and Python 3.2-3.5 has been dropped. The main reason for this change is Python 2 reaching its end of support on January 1, 2020. See https://python3statement.org/ for more background.

• LP solver: add support for the solver SoPlex http://issues.fast-downward.org/issue752 The relative performance of CPLEX and SoPlex depends on the domain and configuration with each solver outperforming the other in some cases. See the issue for a more detailed discussion of performance.

• LP solver: add support for version 12.9 of CPLEX http://issues.fast-downward.org/issue925 Older versions are still supported but we recommend using 12.9. In our experiments, we saw performance differences between version 12.8 and 12.9, as well as between using static and dynamic linking. However, on average there was no significant advantage for any configuration. See the issue for details.

• LP solver: update build instructions of the open solver interface http://issues.fast-downward.org/issue752 http://issues.fast-downward.org/issue925 The way we recommend building OSI now links dynamically against the solvers and uses zlib. If your existing OSI installation stops working, try installing zlib (sudo apt install zlib1g-dev) or re-install OSI (http://www.fast-downward.org/LPBuildInstructions).

• merge-and-shrink: remove trivial factors http://issues.fast-downward.org/issue914 When the merge-and-shrink computation terminates with several factors (due to using a time limit), only keep those factors that are non-trivial, i.e., which have a non-goal state or which represent a non-total function.

• tests: use pytest for running most tests http://issues.fast-downward.org/issue935 http://issues.fast-downward.org/issue936

• tests: test Python code with all supported Python versions using tox http://issues.fast-downward.org/issue930

• tests: adjust style of Python code as suggested by flake8 and add this style check to the continuous integration test suite http://issues.fast-downward.org/issue929 http://issues.fast-downward.org/issue931 http://issues.fast-downward.org/issue934

• scripts: move Stone Soup generator scripts to separate repository at https://github.com/aibasel/stonesoup. http://issues.fast-downward.org/issue932

Fast Downward 19.06

Released on June 11, 2019. First time-based release.