Consistency techniques have been studied extensively in the past as a way of tackling constraint satisfaction problems (CSP). In particular, various arc-consistency algorithms have been proposed, originating from Waltz's filtering algorithm [26] and culminating in the optimal algorithm AC-4 of Mohr and Henderson [15]. AC-4 runs in O(ed 2 ) in the worst case, where e is the number of arcs (or constraints) and d is the size of the largest domain. Being applicable to the whole class of (binary) CSP, these algorithms do not take into account the semantics of constraints. In this paper, we present a new generic arc-consistency algorithm AC-5. This algorithm is parametrized on two specified procedures and can be instantiated to reduce to AC-3 and AC-4. More important, AC-5 can be instantiated to produce an O(ed) algorithm for a number of important classes of constraints: functional, anti-functional, monotonic and their generalization to (functional, anti-functional, and monotonic) piecewise constraints. We also show that AC-5 has an important application in constraint logic programming over finite domains [23]. The kernel of the constraint solver for such a programming language is an arc-consistency algorithm for a set of basic constraints. We prove that AC-5, in conjunction with node consistency, provides a decision procedure for these constraints running in time O(ed).

We describe the design and implementation of a finite domain constraint solver embedded in a Prolog system using an extended unification mechanism via attributed variables as a generic constraint interface. The solver is essentially a scheduler for indexicals, i.e. reactive functional rules encoding local consistency methods performing incremental constraint solving or entailment checking, and global constraints, i.e. general propagators which may use specialized algorithms to achieve a higher degree of consistency or better time and space complexity. The solver has an open-ended design: the user can introduce new constraints, either in terms of indexicals by writing rules in a functional notation, or as global constraints via a Prolog programming interface. Constraints defined in terms of indexicals can be linked to 0/1-variables modeling entailment; thus indexicals are used for constraint solving as well as for entailment testing. Constraints can be arbitrarily combined using the ...

We present the clp(FD) system: a Constraint Logic Programming language with finite domain constraints...

We develop a model for timed, reactive computation by extending the asynchronous, untimed concurrent constraint programming model in a simple and uniform way. In the spirit of process algebras, we develop some combinators expressible in this model, and reconcile their operational, logical and denotational character. We show how programs may be compiled into finite-state machines with loop-free computations at each state, thus guaranteeing bounded response time. 1 Introduction and Motivation Reactive systems [12,3,9] are those that react continuously with their environment at a rate controlled by the environment. Execution in a reactive system proceeds in bursts of activity. In each phase, the environment stimulates the system with an input, obtains a response in bounded time, and may then be inactive (with respect to the system) for an arbitrary period of time before initiating the next burst. Examples of reactive systems are controllers and signal-processing systems. The primary issu...

We present an abstract instruction set for a constraint solver over finite domains, which can be smoothly integrated in the WAM architecture. It is based on the use of a single primitive constraint X in r which embeds the core propagation mechanism. Complex user constraints such as linear equations or inequations are compiled into X in r expressions which encode the propagation scheme chosen to solve the constraint. The uniform treatment of a single primitive constraint leads to a better understanding of the overall constraint solving process and makes possible three main global optimizations which encompass many previous particular optimizations of "black box" finite domains solvers. Implementation results show that this approach combines both simplicity and efficiency. Our clp(FD) system is more than twice as fast as CHIP on average, with peak speedup reaching seven. 1 Introduction Constraint Logic Programming (CLP) has shown to be a very active field of research over recent years, ...

Synchronous programming (Berry (1989)) is a powerful approach to programming reactive systems. Following the idea that "processes are relations extended over time" (Abramsky (1993)), we propose a simple but powerful model for timed, determinate computation, extending the closure-operator model for untimed concurrent constraint programming (CCP). In (Saraswat et al. 1994a) we had proposed a model for this called tcc--- here we extend the model of tcc to express strong time-outs: if an event A does not happen through time t, cause event B to happen at time t. Such constructs arise naturally in practice (e.g. in modeling transistors) and are supported in synchronous programming languages. The fundamental conceptual difficulty posed by these operations is that they are nonmonotonic. We provide a compositional semantics to the non-monotonic version of concurrent constraint programming (Default cc) obtained by changing the underlying logic from intuitionistic logic to Reiter's default logic...

This paper explores Lhc expressive power of Lhc tcc paradigm. The origin of Lhc work in Lhc inLcgraLion of synchronous and consLrainL programming is described. The basic conceptual and maLhcmaLical framework developed in Lhc spirk of Lhc model-based approach characLcrisLic of LhcorcLical compuLcr science is reviewed. Wc show LhaL a range of consLrucLs for expressing LimcouLs, prccmpLion and oLhcr complicaLcd paLLcrns of Lcmporal acLivky arc expressible in the basic model and language-framework. Indeed, we present a single construct on processes, definable in the language, that can simulate the effect of other preemption constructs

In this paper we describe the design and the implementation of the GNU Prolog system. This system draws on our previous experience of compiling Prolog to C in the wamcc system and of compiling finite domain constraints in the clp(FD) system. The compilation scheme has however been redesigned in order to overcome the drawbacks of compiling to C. In particular, GNU-Prolog is based on a low-level mini-assembly platform-independent language that makes it possible to avoid compiling C code, and thus drastically reduces compilation time. It also makes it possible to produce small stand-alone executable files as the result of the compilation process. Interestingly, GNU Prolog is now compliant to the ISO standard, includes several extensions (OS interface, sockets, global variables, etc) and integrates a powerful constraint solver over finite domains. The system is efficient and in terms of performance is comparable with commercial systems for both the Prolog and constraint aspects.

) Vineet Gupta Radha Jagadeesan Prakash Panangaden y vgupta@mail.arc.nasa.gov radha@cs.luc.edu prakash@cs.mcgill.ca Caelum Research Corporation Dept. of Math. and Computer Sciences School of Computer Science NASA Ames Research Center Loyola University--Lake Shore Campus McGill University Moffett Field CA 94035, USA Chicago IL 60626, USA Montreal, Quebec, Canada Abstract This paper describes a stochastic concurrent constraint language for the description and programming of concurrent probabilistic systems. The language can be viewed both as a calculus for describing and reasoning about stochastic processes and as an executable language for simulating stochastic processes. In this language programs encode probability distributions over (potentially infinite) sets of objects. We illustrate the subtleties that arise from the interaction of constraints, random choice and recursion. We describe operational semantics of these programs (programs are run by sampling random choices), deno...

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