## Editor: Hélène Verhaeghe

Authors, Title and Abstract | Paper | Talk |
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ABSTRACT. Constraint Programming is a powerful method to solve combinatorial problems, but due to a large search space, solving can be very time consuming. Diagnosis, planning, product configuration are example use cases. These systems are often used in an online format answering queries. Compilation methods were developed to deal with the complexity of solving the problems offline and create a representation that is able to answer queries in polynomial time. Symmetry breaking is the addition of constraints to eliminate symmetries, thus in general speeding up search and reducing the number of solutions. Knowledge compilation looks at finding succinct representations that are also tractable, that is, they support queries and transformations in polytime. Finding the smallest representation is often the bottleneck of compilation methods. Compiled representations are Directed Acyclic Graphs representing the set of all solutions. In this paper we investigate if breaking symmetries, that is representing less solutions, always leads to a smaller compiled representation? We considered four compilers and three highly symmetrical problems. A reduction is observed in all the problems for the compilation size when we break symmetries, with top-down compilers obtaining more reduction. | Aug 01 15:00 | |

ABSTRACT. To automate the discovery of conjectures on combinatorial objects, we introduce the concept of a map of sharp bounds on characteristics of combinatorial objects, that provides a set of interrelated sharp bounds for these combinatorial objects. We then describe a Bound Seeker, a CP-based system, that gradually acquires maps of conjectures. The system was tested for searching conjectures on bounds on characteristics of digraphs: it constructs sixteen maps involving 431 conjectures on sharp lower and upper-bounds on eight digraph characteristics. | Aug 01 17:26 | |

ABSTRACT. Branching decisions have a strong impact on performance in Constraint Programming (CP). Therefore robust generic variable ordering heuristics are an important area of research in the CP community. By allowing us to compute marginal probability distributions over the domain of each variable in the model, CP-based Belief Propagation also allows us to compute the entropy of each variable. We present two new branching heuristics exploiting entropy: one chooses the variable with the smallest entropy; the other is inspired by impact-based search and chooses the variable whose fixing is believed to have the strongest impact on the entropy of the model. We also propose a dynamic stopping criterion for iterated belief propagation based on variations in model entropy. We test our heuristics on various constraint satisfaction problems from XCSP and minizinc benchmarks. | Aug 01 14:20 | |

ABSTRACT. This paper presents two exact optimization models for the Constrained Single-Row Facility Layout Problem. It is a linear arrangement problem considering departments in a facility with given lengths and traffic intensities. The first approach is an extension of the state-of-the-art mixed-integer programming model for the unconstrained problem with the additional constraints. The second one is a decision diagram-based branch-and-bound that takes advantage of the recursive nature of the problem through a dynamic programming model. The computational experiments show that both approaches significantly outperform the only mixed-integer programming model in the literature. | Aug 01 12:21 | |

ABSTRACT. We propose CABSC, a system that performs Constraint Acquisition Based on Solution Counting. In order to learn a Constraint Satisfaction Problem (CSP), the user provides positive examples and a Meta-CSP, i.e. a model of a combinatorial problem whose solution is a CSP. It allows listing the potential constraints that can be part of the CSP the user wants to learn. It also allows stating the parameters of the constraints and imposing constraints over these parameters. The CABSC reads the Meta-CSP using an augmented version of the language MiniZinc and returns the CSP that accepts the fewest solutions among the CSPs accepting all positive examples. This is done using a branch and bound where the bounding mechanism makes use of a model counter. Experiments show that CABSC is successful at learning constraints and their parameters from positive examples. | Aug 01 17:23 | |

ABSTRACT. The Boolean Satisfiability Problem (SAT) was the first known NP-complete problem and has a very broad literature focusing on it. It has been applied successfully to various real-world problems, such as scheduling, planning and cryptography. SAT problem feature extraction plays an essential role in this field. SAT solvers are complex, fine-tuned systems that exploit problem structure. The ability to represent/encode a large SAT problem using a compact set of features has broad practical use in instance classification, algorithm portfolios, and solver configuration. The performance of these techniques relies on the ability of feature extraction to convey helpful information. Researchers often craft these features ``by hand'' to capture particular structures of the problem. Instead, in this paper, we extract features using semi-supervised deep learning. We train a convolutional autoencoder (AE) to compress the SAT problem into a limited latent space and reconstruct it minimizing the reconstruction error. The latent space projection should preserve much of the structural features of the problem. We compare our approach to a set of features commonly used for algorithm selection. Firstly, we train classifiers on the projection to predict if the problems are satisfiable or not. If the compression conveys valuable information, a classifier should be able to take correct decisions. In the second experiment, we check if the classifiers can identify the original problem that was encoded as SAT. The empirical analysis shows that the autoencoder is able to represent problem features in a limited latent space efficiently, as well as convey more information than current feature extraction methods. | Aug 01 11:55 | |

ABSTRACT. Constraint Programming (CP) is one of the most flexible approaches for modeling and solving vehicle routing problems (VRP). This paper proposes the sequence variable domain, that is inspired by the insertion graph and the subset bound domain for set variables. This domain representation, which targets VRP applications, allows for an efficient insertion-based search on a partial tour and the implementation of simple, yet efficient filtering algorithms for constraints that enforce time-windows on the visits and capacities on the vehicles. Experiment results demonstrate the efficiency and flexibility of this CP domain for solving some hard VRP problems, including the Dial-A-Ride, the Patient Transportation, and the asymmetric TSP with time windows. | Aug 01 10:23 | |

ABSTRACT. Interactive constraint systems often suffer from infeasibility (no solution) due to conflicting user constraints. A common approach to recover infeasibility is to eliminate the constraints that cause the conflicts in the system. This approach allows the system to provide an explanation as: “if the user is willing to drop out some of their constraints, there exists a solution”. However, one can criticise this form of explanation as not being very informative. A counterfactual explanation is a type of explanation that can provide a basis for the user to recover feasibility by helping them understand which changes can be applied to their existing constraints rather than removing them. This approach has been extensively studied in the machine learning field, but requires a more thorough investigation in the context of constraint satisfaction. We propose an iterative method based on conflict detection and maximal relaxations in over-constrained constraint satisfaction problems to help compute a counterfactual explanation. | Aug 01 14:40 | |

ABSTRACT. We introduce two log-linear-time dispersion propagators---(a) spread (variance, and indirectly standard deviation) and (b) the Gini coefficient---capable of explaining their propagations, thus allowing clause learning solvers to use the propagators. Propagators for (a) exist in the literature but do not explain themselves, while propagators for (b) have not been previously studied. | Aug 01 17:20 | |

ABSTRACT. Backtrack search is a classical complete approach for exploring the search space of a constraint optimization problem. % when optimality is sought to be proved. Each time a new solution is found during search, its associated bound is used to constrain more the problem, and so the remaining search. An extreme (bad) scenario is when solutions are found in sequence with very small differences between successive bounds. In this paper, we propose an aggressive bound descent (ABD) approach to remedy this problem: new bounds are modified exponentially as long as the searching algorithm is successful. We show that this approach can render the solver more robust, especially at the beginning of search. Our experiments confirm this behavior for constraint optimization. | Aug 01 14:00 | |

ABSTRACT. One of the possible approaches for solving a CSP is to encode the input problem into a CNF formula, and then use a SAT solver to solve it. The main advantage of this technique is that it allows to benefit from the practical efficiency of modern SAT solvers, based on the CDCL architecture. However, the reasoning power of SAT solvers is somehow "weak", as it is limited by that of the resolution proof system they use internally. This observation led to the development of so called pseudo-Boolean (PB) solvers, that implement the stronger cutting planes proof system, along with many of the solving techniques inherited from SAT solvers. Additionally, PB solvers can natively reason on PB constraints, i.e., linear equalities or inequalities over Boolean variables. These constraints are more succinct than clauses, so that a single PB constraint can represent exponentially many clauses. In this paper, we leverage both this succinctness and the reasoning power of PB solvers to solve CSPs by designing PB encodings for different common constraints, and feeding them into PB solvers to compare their performance with that of existing CP solvers. | Aug 01 16:40 | |

ABSTRACT. A component of the Bound Seeker [1] is Boolean formula seeker, a part devoted to search for Boolean formulae. Here, a Boolean formula involves n arithmetic conditions linked by a single commutative logical operator or by a sum. Part of the originality of the Boolean formula seeker is that it was synthesised by a constraint program. This extended abstract includes (i) the type of Boolean formulae we consider, (ii) the importance of allowing Boolean formulae in the context of maps of conjectures, (iii) the components of the Boolean formula seeker, and (iv) a short description of the different steps of the acquisition process of Boolean formulae. | Aug 01 17:00 | |

ABSTRACT. Given a Boolean specification between a set of inputs and outputs, the problem of Boolean functional synthesis is to synthesize each output as a function of inputs such that the specification is met. In the report, we first discussed a state-of-the-art data-driven approach for a Boolean functional synthesis, called Manthan [1]. Motivated by the progress in machine learning, Manthan views functional synthesis as a classification problem and relies on advances in constrained sampling for data generation, and advances in automated reasoning for a novel proof-guided repair and provable verification. We then discussed challenges faced by Manthan during data-driven synthesis and the remedies to overcome those challenges [2]. Finally, to discuss the applications, we showed the reduction of program synthesis to a special variant of Boolean functional synthesis in which we have explicit dependencies on universally quantified variables [3]. We hope that program synthesis will be that killer application that will motivate further research into functional synthesis. [1] Priyanka Golia, Subhajit Roy, and Kuldeep S. Meel. 2020. Manthan: A Data-Driven Approach for Boolean Function Synthesis. In Proceedings of International Conference on Computer-Aided Verification (CAV), 2020. [2] Priyanka Golia, Friedrich Slivovsky, Subhajit Roy, and Kuldeep S. Meel. 2021. Engineering an Efficient Boolean Functional Synthesis Engine. In Proceedings of International Conference On Computer Aided Design (ICCAD), 2021. [3] Priyanka Golia, Subhajit Roy, and Kuldeep S. Meel. Program synthesis as dependency quantified formula modulo theory. In Proceedings of International Joint Conference on Artificial Intelligence (IJCAI), 2021. | Aug 01 16:20 | |

ABSTRACT. In this work, we use Reinforcement Learning to combine Machine Learning (ML) and Constraint Programming (CP). We show that combining ML and CP allows the agent to reflect a pretrained network while taking into account constraints, leading to melodic lines that respect both the corpus' style and the music theory constraints. | Aug 01 10:26 | |

ABSTRACT. Dominance breaking is an effective technique to reduce the time for solving constraint optimization problems. Lee and Zhong propose an automatic dominance breaking framework for a class of constraint optimization problems based on specific forms of objectives and constraints. In this paper, we propose to enhance the framework for problems with nested function calls which can be flattened to functional constraints. In particular, we focus on aggregation functions and exploit such properties as monotonicity, commutativity and associativity to give an efficient procedure for generating effective dominance breaking nogoods. Experimentation also shows orders-of-magnitude runtime speedup using the generated dominance breaking nogoods and demonstrates the ability of our proposal to reveal dominance relations in the literature and discover new dominance relations on problems with ineffective or no known dominance breaking constraints. | Aug 01 15:23 | |

ABSTRACT. Real-world Constraint Satisfaction Problems (CSPs) are subject to uncertainty/dynamism that cannot be known in advance. Some techniques in the literature offer robust solutions for CSPs, but they have low performance in large-scale CSPs. We propose a Large Neighbourhood Search (LNS) algorithm and a value selection heuristic that searches for robust solutions in large-scale CSPs. | Aug 01 15:26 | |

ABSTRACT. This paper deals with the multi-agent path finding (MAPF) problem for a team of tethered robots. When considering point-sized robots, paths may share a same subpath provided that they do not cross, and we have shown in a previous work that this case basically involves solving an assignment problem: The objective is to find a set of non-crossing paths, and the makespan is equal to the length of the longest path. In this work, we extend it to the case of non-point-sized robots where robot paths must be synchronized when they share a same subpath and waiting times are considered when computing the makespan. We prove that the upper bound can be computed by solving the linear sum assignment problem. We introduce a new variable neighborhood search method to improve the upper bound and show that it is robust to different instances. We also introduce a Constraint Programming model for solving the problem to optimality. | Aug 01 10:00 | |

ABSTRACT. Modern electrical power utilities must maintain their electrical equipment and replace them as they reach the end of their useful life. The Transmission Maintenance Scheduling (TMS) problem consists in generating an annual maintenance plan for electric power transportation equipment while maintaining the stability of the network and ensuring a continuous power flow for customers. Each year, a list of equipment that needs to be maintained or replaced is available and the goal is to generate an optimal maintenance plan. This paper proposes a constraint-based scheduling approach to solve the TMS problem. The model considers two types of constraints: (1) constraints that can be naturally formalized inside a constraint programming model, and (2) complex constraints that do not have a proper formalization from the field specialists. The latter cannot be integrated inside the model due to their complexity. Their satisfaction is thus verified by a black box tool, which is a simulator mimicking the impact of a maintenance schedule on the real power network. The simulator is based on complex differential power-flow equations. Experiments are carried out at five strategic points of Hydro-Québec power grid infrastructure, which involves more than 200 electrical equipment and 300 withdrawal requests. Results show that our model is able to comply with most of the formalized and unformalized constraints. It also generates maintenance schedules within an execution time of few minutes. The generated schedules are similar to the ones proposed by a field specialist and can be used to simulate maintenance programs for the next 10 years. | Aug 01 10:20 | |

ABSTRACT. The branch-and-cut algorithm for integer programming has a wide variety of tunable parameters that have a huge impact on its performance, but which are challenging to tune by hand. An increasingly popular approach is to use machine learning to configure these parameters based on a training set of integer programs from the application domain. We bound how large the training set should be to ensure that for any configuration, its average performance over the training set is close to its expected future performance. Our guarantees apply to parameters that control the most important aspects of branch-and-cut: node selection, branching constraint selection, and cut selection, and are sharper and more general than those from prior research. | Aug 01 15:20 | |

ABSTRACT. Decision diagrams are an increasingly important tool in cutting-edge solvers for discrete optimization. However, the field of decision diagrams is relatively new, and is still incorporating the library of techniques that conventional solvers have had decades to build. We drew inspiration from the warm-start technique used in conventional solvers to address one of the major challenges faced by decision diagram based methods. Decision diagrams become more useful the wider they are allowed to be, but also become more costly to generate, especially with large numbers of variables. We present a method of peeling off a sub-graph of previously constructed diagrams and using it as the initial diagram for subsequent iterations that we call peel-and-bound. We test the method on the sequence ordering problem, and our results indicate that our peel-and-bound scheme generates stronger bounds than a branch-and-bound scheme using the same propagators, and at significantly less computational cost. | Aug 01 12:18 | |

ABSTRACT. Software masking, a software mitigation against power-side channel attacks, aims at removing the secret dependencies from the power traces that may leak cryptographic keys. However, high-level software mitigations often depend on general purpose compilers, which do not preserve non-functional properties. What is more, microarchitectural features, such as the memory bus and register reuse, may also reveal secret information. These abstraction are not visible at the high-level implementation of the program. Instead, they are decided at compile time. To remedy these problems, security engineers often turn off compiler optimization and/or perform local, post-compilation transformations. However, theses solution lead to inefficient code. To deal with this issues, we propose Secure by Construction Code Generation (SecConCG), a secure constraint-based compiler backend to generate code that is secure. SecConCG can control the quality of the mitigated program by efficiently searching the best possible low-level implementation according to a processor cost model. In our experiments with ten masked implementations on MIPS and ARM Cortex M0, SecConCG improves the generated code from 10% to 9x compared to non-optimized secure code at a small overhead of up to 8% compared to non-secure optimized code. | Aug 01 16:00 | |

ABSTRACT. In this extended abstract, we summarise the work from our main conference paper. We explore the problem of selecting encodings for pseudo-Boolean and linear constraints using a supervised machine learning approach. We show that it is possible to select encodings effectively using a standard set of features for constraint problems; however we obtain better performance with a new set of features specifically designed for the pseudo-Boolean and linear constraints. In fact, we achieve good results when selecting encodings for unseen problem classes. | Aug 01 12:15 | |

ABSTRACT. Ordered Multi-valued Decision Diagrams (MDDs) have been shown to be useful to represent finite domain functions/relations. For example, various constraints can be modelled with MDD constraints. Recently, a new representation called Binary Constraint Tree (BCT), which is a (special) tree structure binary Constraint Satisfaction Problem, has been proposed to encode MDDs and shown to outperform existing MDD constraint propagators in Constraint Programming solvers. BCT is a compact representation, and it can be exponentially smaller than MDD for representing some constraints. Here, we also show that BCT is compact for representing non-deterministic finite state automaton (NFA) constraints. In this paper, we investigate how to encode BCT into CNF form, making it suitable for SAT solvers. We present and investigate five BCT CNF encodings. We compare the propagation strength of the BCT CNF encodings and experimentally evaluate the encodings on a range of existing benchmarks. We also compare with seven existing CNF encodings of MDD constraints. Experimental results show that the CNF encodings of BCT constraints can outperform those of MDD constraints on various benchmarks. | Aug 01 12:24 |