August 2017
IMPORTANT NOTE regarding version 6.0 or later:
With version 6.0 the compilation process has changed. Instead of 'make', run the script 'compile.sh'. By default, the QBF solver Nenofex will be downloaded, compiled, and linked to DepQBF.
Nenofex source code: https://github.com/lonsing/nenofex
IMPORTANT NOTE: please see the guidelines on preprocessing, incremental solving and API usage below.
This is version 6.03 of the search-based QBF solver DepQBF. This recent version implements conflict-driven clause and solution-driven cube learning (QCDCL) with generalized axioms. To this end, DepQBF applies the SAT solver PicoSAT and the expansion-based QBF solver Nenofex as oracles to check the satisfiability of formulas that arise in the context of axiom applications. The approach is described in the following papers:
Florian Lonsing, Uwe Egly: DepQBF 6.0: A Search-Based QBF Solver Beyond Traditional QCDCL. In Proceedings of CADE 2017, LNCS, Springer, 2017.
Florian Lonsing, Uwe Egly, Martina Seidl: "Q-Resolution with Generalized Axioms". In Proceedings of SAT 2016, LNCS, Springer, 2016.
In version 6.03, the solver Nenofex replaces the QBF preprocessor Bloqqer which was used in the previous versions 6.0 and 6.01. The replacement was necessary since Bloqqer (version 37) is not reentrant, which may cause problems when running multiple instances of DepQBF.
Since version 5.0 DepQBF includes blocked clause elimination (QBCE) as a pre- and inprocessing technique and as a novel dynamic approach (enabled by default) where QBCE is interleaved with the search process. The novel dynamic QBCE approach is described in the following paper (proceedings of LPAR 2015):
F. Lonsing, F. Bacchus, A. Biere, U. Egly, and M. Seidl: "Enhancing Search-Based QBF Solving by Dynamic Blocked Clause Elimination". In Proceedings of LPAR 2015, LNCS, Springer, 2015.
DepQBF provides an API for incremental solving based on clause groups. A clause group is a set of clauses which is incrementally added to and removed from a formula. A description of the API can be found in the following tool paper (proceedings of SAT 2015):
F. Lonsing and U. Egly: "Incrementally Computing Minimal Unsatisfiable Cores of QBFs via a Clause Group Solver API". In Proceedings of SAT 2015, volume 9340 of LNCS, Springer, 2015. Preprint: http://arxiv.org/abs/1502.02484
This release also includes DepQBF4J, a Java interface to DepQBF which allows to call DepQBF as a library from Java programs. Please see the README file in the subdirectory './DepQBF4J-0.2' for further information and usage examples. DepQBF4J is based on the Java Native Interface (JNI) and was implemented by Martin Kronegger and Andreas Pfandler.
PLEASE SEE the header file 'qdpll.h', the examples in the subdirectory 'examples', and the command line documentation (call './depqbf -h') for further information on using DepQBF and its library.
The example './examples/basic-api-example2.c' demonstrates the basic use of the API and, in particular, the 'qdpll_gc' function. The clause group API is illustrated by './examples/basic-clause-groups-api-example.c'
Many thanks to Robert Koenighofer, Adria Gascon, Thomas Krennwallner, Martin Kronegger, Andreas Pfandler, and Simon Cruanes for valuable feedback.
General features of DepQBF:
Since version 6.0. QCDCL with generalized axioms.
Since version 5.0: pre- and inprocessing by blocked clause elimination (QBCE) and dynamic QBCE (enabled by default). See also './depqbf -h' for configuration options.
The solver can be used as a library. The API is declared in file 'qdpll.h' and the examples in the subdirectory 'examples' demonstrate its basic use.
Java API called DepQBF4J, which allows DepQBF to be integrated in Java programs.
Incremental solving: Incremental solving can be beneficial in applications where a sequence of closely related formulae must be solved. This way, the solver does not have to solve each formula from scratch but tries to reuse information learned from previously solved formulae. DepQBF supports incremental solving by a push/pop API and a clause group API. The push/pop API allows to add and remove sets of clauses in a stack-based way. The clause group API is more general and supports addition and deletion of arbitrary sets of clauses.
Extraction of unsatisfiable cores: for unsatisfiable formulas, an unsatisfiable core (i.e. subset of clauses) can be extracted via the clause group API in a convenient way.
Solving under assumptions: assumptions are fixed variable assignments from the leftmost quantifier block of a QBF. Assumptions can be added through the solver API. In forthcoming calls, the solver tries to solve the formula under the assignments given by the added assumptions.
Long-distance resolution for clause and cube learning: traditional Q-resolution explicitly rules out the generation of tautological resolvents. In contrast to that, long-distance resolution admits certain tautological resolvents. It was first implemented in the QBF solver 'quaffle' and is now also available in DepQBF.
Advanced clause and cube learning based on QBF Pseudo Unit Propagation as presented in the following paper: "Florian Lonsing, Uwe Egly, Allen Van Gelder: Efficient Clause Learning for Quantified Boolean Formulas via QBF Pseudo Unit Propagation. In Proc. SAT 2013."
NOTE: by default, this version of DepQBF applies a lazy variant of QPUP-based QCDCL where no resolution steps are carried out. The traditional approach to QCDCL which was implemented in earlier versions of DepQBF is still available by command line option '--traditional-qcdcl'. Please see also the command line documentation by calling './depqbf -h'.
Generation of QDIMACS output (partial certificate): if the outermost (i.e. leftmost) quantifier block of a satisfiable QBF is existentially quantified, then DepQBF can print an assignment to the variables of this block (and dually for unsatisfiable QBFs and universal variables from the outermost block, if that block is universally quantified). To enable QDIMACS output generation, run DepQBF with parameter '--qdo'. Note that the assignment printed by DepQBF can be partial, i.e. not all variables are necessarily assigned. In this case, the variables for which no value was printed can be assigned arbitrarily.
Trace generation (contributed by Aina Niemetz): DepQBF can produce traces in QRP format (ASCII and binary version of the QRP format are supported; see also the command line documentation). If called with the '--trace' option, the solver prints every resolution step during clause and cube learning to
DepQBF consists of a dependency manager (file 'qdpll_dep_man_qdag.c') and a core QDPLL solver (file 'qdpll.c'). During a run the solver queries the dependency manager to find out if there is a dependency between two variables, say 'x' and 'y'. Given the original quantifier prefix of a QBF, there is such dependency if 'x' is quantified to the left of 'y' and 'x' and 'y' are quantified differently. In contrast to that simple approach, DepQBF (in general) is able to extract more sophisticated dependency information from the given QBF. It computes the so-called 'standard dependency scheme' which is represented as a compact graph by the dependency manager.
If you are interested only in the core solver based on QDPLL then it is probably best not to look at the code of the dependency manager in file 'qdpll_dep_man_qdag.c' at all but only consider file 'qdpll.c'.
DepQBF is free software released under GPLv3:
https://www.gnu.org/copyleft/gpl.html
See also the file COPYING.
IMPORTANT NOTE: since version 6.0, DepQBF requires the SAT solver PicoSAT and the QBF solver Nenofex as external libraries.
Nenofex: https://github.com/lonsing/nenofex PicoSAT http://fmv.jku.at/picosat/
Please see the individual licenses that come with PicoSAT and Nenofex.
The latest release is available from http://lonsing.github.io/depqbf/
Unpack the archive and run the script './compile.sh', which will first download and compile PicoSAT and Nenofex and then compile DepQBF.
The build process produces optimized code without assertions (default).
If you want to use the solver as a library in your own applications, then link against 'libqdpll.a'.
Note: set the flag 'FULL_ASSERT' in file 'qdpll_config.h' from 0 to 1 to switch on expensive assertions (recommended only for debugging). The solver will run substantially slower in this case. As usual, using the compiler flag 'DNDEBUG' removes all assertions from the code, regardless from the value of 'FULL_ASSERT'.
Call './depqbf -h' to display usage information. Further, undocumented command line parameters can be found in function 'qdpll_configure(...)' in file 'qdpll.c'. These parameters are mostly experimental.
The solver returns exit code 10 if the given instance was found satisfiable and exit code 20 if the instance was found unsatisfiable. Any other exit code indicates that the instance was not solved.
Parameter '-v' enables basic verbose mode where the solver prints information
on restarts and backtracks to
Trace generation can be enabled by parameter '--trace'. Note that printing the tracing information causes I/O overhead and might slow down the solver. Writing traces in binary QRP format (enabled by parameter '--trace=bqrp') usually produces smaller traces, as far as byte size is concerned.
Calling DepQBF without command line parameters results in default behaviour which was tuned on instances from QBFLIB. For performance comparisons with other solvers it is recommended not to pass any command line parameters to DepQBF.
By default, statistical output is disabled. To enable statistics, set the flag 'COMPUTE_STATS' in file 'qdpll_config.h' from 0 to 1. Similarly, time statistics can be enabled by setting flag 'COMPUTE_STATS'.
Version 5.0 (or later) includes blocked clause elimination (QBCE) as a pre- and inprocessing technique and as a novel dynamic approach (enabled by default) where QBCE is interleaved with the search process.
Depending on your application, preprocessors such as Bloqqer [1] and/or QxBF [2], for example, may improve the performance of DepQBF further.
HOWEVER: depending on the given instance, preprocessing by Bloqqer may be harmful to the performance of DepQBF version 5.0 (or later) in its default configuration (using dynamic QBCE).
[1] http://fmv.jku.at/bloqqer/
Please see the header file 'qdpll.h' for some documentation of the API functions.
When using the API of the solver (versions 3.0 or later), it is HIGHLY RECOMMENDED to first add all the variables to the quantifier prefix and then all the clauses of the formula rather than adding variables and clauses in interleaved fashion. In the latter case, runtime overhead will occur for large formulas. Many thanks to Mathias Preiner for pointing out this problem.
In applications which involve a very large number of incremental calls, the overhead of maintaining the internal data structures in this release of DepQBF might become non-negligible. In this case, please contact Florian Lonsing; your feedback is highly appreciated.
Incremental solving must be enabled by calling the API function 'qdpll_configure' with the parameters '--dep-man=simple' and '--incremental-use', respectively. Please see also the example programs in the subdirectory 'examples'.
The push/pop API allows to add and remove clauses in a stack-based way. Therefore, clauses which are shared between many formulas to be solved should be pushed onto the stack first. Clauses which have to be removed soon should be added last so that they can be popped from the stack easily.
In general, it is beneficial for the performance of the solver to avoid needless push operations. For example, if you know that certain clauses will never be removed from the formula then it is not necessary to call 'qdpll_push' before adding these clauses.
The clause group API generalizes the push/pop API in that it allows sets of clauses to be added and removed arbitrarily. It is recommended to use the push/pop API instead of the clause group API for applications which naturally can be encoded by push and pop operations, that is, where the formulas to be solved incrementally are modified in a rather uniform way.
If assumptions are passed to the solver using 'qdpll_assume' AND clauses are added later to the formula, then the API function 'qdpll_configure' must be called with the parameters '--dep-man=simple' and '--incremental-use' after the solver object has been created by 'qdpll_create'. Otherwise, if no clauses are added, then the aforementioned calls of the API function 'qdpll_configure' can be omitted.
For comments, questions, bug reports etc. related to DepQBF, please do not hesitate to contact Florian Lonsing: