Fred Hemery

Random constraint satisfaction: Easy generation of hard (satisfiable) instances

In this paper, we show that the models of random CSP instances proposed by Xu and Li [K. Xu, W. Li, Exact phase transitions in random constraint satisfaction problems, Journal of Artificial Intelligence Research 12 (2000) 93-103; K. Xu, W. Li, Many hard examples in exact phase transitions with application to generating hard satisfiable instances, Technical report, CoRR Report cs.CC/0302001, Revised version in Theoretical Computer Science 355 (2006) 291-302] are of theoretical and practical interest. Indeed, these models, called RB and RD, present several nice features. First, it is quite easy to generate random instances of any arity since no particular structure has to be integrated, or property enforced, in such instances. Then, the existence of an asymptotic phase transition can be guaranteed while applying a limited restriction on domain size and on constraint tightness. In that case, a threshold point can be precisely located and all instances have the guarantee to be hard at the threshold, i.e., to have an exponential tree-resolution complexity. Next, a formal analysis shows that it is possible to generate forced satisfiable instances whose hardness is similar to unforced satisfiable ones. This analysis is supported by some representative results taken from an intensive experimentation that we have carried out, using complete and incomplete search methods.

Backjump-based techniques versus conflict-directed heuristics

We present a general algorithm which gives a uniform view of several state-of-the-art systematic backtracking search algorithms for solving both binary and nonbinary CSP instances. More precisely, this algorithm integrates the most usual or/and sophisticated look-back and look-ahead schemes. By means of this algorithm, our purpose is then to study the interest of backjump-based techniques with respect to conflict-directed variable ordering heuristics.

Exploiting multidirectionality in coarse-grained arc consistency algorithms

Arc consistency plays a central role in solving Constraint Satisfaction Problems. This is the reason why many algorithms have been proposed to establish it. Recently, an algorithm called AC2001 and AC3.1 has been independently presented by their authors. This algorithm which is considered as a refinement of the basic algorithm AC3 has the advantage of being simple and competitive. However, it does not take into account constraint bidirectionality as AC7 does. In this paper, we address this issue, and, in particular, introduce two new algorithms called AC3.2 and AC3.3 which benefit from good properties of both AC3 and AC7. Indeed, AC3.2 and AC3.3 are as easy to implement as AC3 and take advantage of bidirectionality as AC7 does. More precisely, AC3.2 is a general algorithm which partially exploits bidirectionality whereas AC3.3 is a binary algorithm which fully exploits bidirectionality. It turns out that, when Maintaining Arc Consistency during search, MAC3.2, due to a memorization effect, is more efficient than MAC3.3 both in terms of constraint checks and cpu time. Compared to MAC2001/3.1, our experimental results show that MAC3.2 saves about 50% of constraint checks and, on average, 15% of cpu time.

Representation of extended RBAC model using UML language

This paper presents an extension of the standard role-based access control (RBAC) model together with its representation using the Unified Modeling Language (UML). The presented model is developed for the role engineering in the security of information system. The presented implementation of the RBAC model consists in role creation via defining appropriate permissions. The entire procedure is performed in two stages: defining the permissions assigned to a function and providing the definitions of functions assigned to a particular role.

Sliced Table Constraints: Combining Compression and Tabular Reduction

Many industrial applications require the use of table constraints (e.g., in configuration problems), sometimes of significant size. During the recent years, researchers have focused on reducing space and time complexities of this type of constraint. Static and dynamic reduction based approaches have been proposed giving new compact representations of table constraints and effective filtering algorithms. In this paper, we study the possibility of combining both static and dynamic reduction techniques by proposing a new compressed form of table constraints based on frequent pattern detection, and exploiting it in STR (Simple Tabular Reduction).

Verification of access control coherence in information system during modifications

The paper deals with management of access control in an information system. It is suggested that the security of an information system should be a task solved on two principal levels: system development level and security administration level. Consequently, the responsibility for creating effective security measures for an information system ought to lie with both the application developer and the global administration. Moreover, sets of security constraints should be formulated also on those two levels. The paper defines requirements and obligations of each level using adapted tools based on the role-based access control (RBAC) model and employing the object-oriented conception method with UML (Unified Modeling Language). It is shown how the process of addition of a new application to an information system may be automated and how the administrator can be assisted in detecting incoherences or/and determining new relations between the elements existing in a system, such as roles or permissions.

Implicit random constraint satisfaction problem

Random CSPs (constraint satisfaction problems) provide interesting benchmarks for experimental evaluation of algorithms. From a theoretical point of view, a lot of recent works have contributed to guarantee the existence of a so-called phase transition and, consequently, of hard and large problem instances. From a practical point of view, due to exponential space complexity, a vast majority of experiments based on random CSPs concerns binary problems. In this paper, we introduce a model of implicit random CSPs, i.e., of random CSPs where constraints are not given in extension but defined by a predicate. This new model involves an easy implementation, no space requirement and the possibility to perform experiments with large arity constraints.

Efficient Constraint Propagation for Graph Coloring

Abstract In this paper, we investigate constraint propagation, a mechanism that is run at each basic step of a backtrack search algorithm such as the popular MAC. From a statistical analysis of some relevant features concerning propagation on a large set of graph coloring instances, we show that it is possible to make reasonable predictions about the capability of constraint propagation to detect inconsistency. Using this observation in order to control propagation effort, we show its practical effectiveness.