Martin Aronsson

The programming language GCLA—a definitional approach to logic programming

We present a logic programming language, GCLA*** (Generalized horn Clause LAnguage), that is based on a generalization of Prolog. This generalization is unusual in that it takes a quite different view of the meaning of a logic program—a “definitional” view rather than the traditional logical view.GCLA has a number of noteworthy properties, for instance hypothetical and non-monotonic reasoning. This makes implementation of reasoning in knowledge-based systems more direct in GCLA than in Prolog. GCLA is also general enough to incorporate functional programming as a special case.GCLA and its syntax and semantics are described. The use of various language constructs are illustrated with several examples.

Maintaining tracks and traffic flow at the same time

In an ideal world, all railway tracks would be available to trains at all times. In reality, track sections need to be closed every now and again for track maintenance and upgrades in order to ensure a satisfactory level of safety and comfort. In this paper, we present a MIP model that optimizes a production plan with regard to both trains and preventive maintenance. The planned maintenance activities may not be canceled, but may be moved in time within pre-defined time windows. Trains may be moved in time, redirected to other parts of the geography, or even canceled. The goal for the optimization is to find the best possible traffic flow given a fixed set of planned maintenance activities. In addition to presenting the model, we discuss the current maintenance planning process in Sweden, and exemplify the usefulness of our model in practice by applying it to two typical scenarios.

A survey of GCLA: a definitional approach to logic programming

A GCLA program consists of two parts: a declarative part which is a definition presenting declarative knowledge about a domain and a procedural part setting up the machinery for proof search in the local logic generated by the definition. So the first part gives the meaning of propositions, defines functions etc., while the second part specializes the builtin procedures for proving propositions or computing expressions.

Using timetabling optimization prototype tools in new ways to support decision making

The Swedish infrastructure manager Trafikverket is funding research for timetabling optimization tools as part of their overall mission to utilize the existing infrastructure more efficiently. Currently, Trafikverket is modernizing both planning processes and the IT architecture, and will soon be ready to start using optimization tools on a broad scale. Meanwhile, innovative uses of a prototype developed at SICS have shown how a prototype does not necessarily merely serve to pave way for a future, large-scale implementation. This paper shows how computers in railway planning, coupled with OR techniques, relevant data and apt modeling, can help provide a future user with valuable insights even before the full-fledged tool is in place.

Methodology and Programming Techniques in GCLA II

We will demonstrate various implementation techniques in the language GCLA. First an introduction to GCLA is given, followed by some examples of program developments, to demonstrate the development methodology. Other examples are also given to show various implementation techniques and properties of the system.

Implementational Issues in GCLA: A-Sufficiency and Definiens Operation

We present algorithms for computing A-sufficient substitutions and constraint sets together with the definiens operation. These operations are primitive operations in the language GCLA. The paper first defines those primitives, which together form a dual rule to SLD resolution, and then describes the different algorithms and some of their properties together with examples. One of the algorithms shows how a definition can be compiled into a representation holding all possible A-sufficient substitutions/constraint sets together with their corresponding definiens. This representation makes the computation at runtime of a definiens and an A-sufficient substitution/constraint set have the same complexity as the table lookup operation clause/2 in Prolog. The paper also describes the generalisation from unification (sets of equalities) to constraint sets and satisfiability of systems of equalities and inequalities.

Towards shorter lead times in railway timetabling in Sweden

Railway timetabling in Sweden has traditionally been associated with long lead times. In order for the railway sector to be able to compete with other transport modes, the infrastructure manager must become better at meeting the demand for railway services whose need for infrastructure capacity is not well-defined 6-18 months in advance. Research at SICS is changing the Swedish timetabling process so that it will accommodate late capacity requests in a significantly better way than before. This paper describes the principle behind the new method, briefly describes the optimization model that has been developed to support the new process, and presents examples that show how optimization tools can support the new process.

Heterogeneous Scheduling and Rotation

This article highlights an application in the area of decision support for planning transports in a railway company utilising constraint logic programming and a flexible design which has been successfully tested on real world data.We discuss the problem formulation for the co-ordination of distinct sub-problems, the allocation of track resources to transports, the allocation of vehicles to transports, and the allocation of personnel to perform the transportation tasks in a railway company and the development of a heterogeneous constraint model which is usable also for other production planning problems.Using constraints as the key technology, we discuss approaches to find interfacing principles to combine several solvers.

An efficient MIP model for locomotive routing and scheduling

This paper presents a MIP model for a locomotive routing and scheduling problem from the domain of freight railways. Innovative features of the model include the use of binary variables to separate the integer and continuous parts of the problem to maintain the flow character of the integer part of the problem. The model has been developed with, and has found practical Green Cargo, the largest rail freight operator in Sweden.

Towards a comprehensive model for track allocation and roll-time scheduling at marshalling yards☆

This paper considers multi-stage train formation with mixed usage tracks at a marshalling yard without departure yard. A novel integer programming model for scheduling shunting tasks as well as allocating arrival yard tracks and classification bowl tracks is presented. By taking a comprehensive view of the marshalling yard operations, more effective schedules can be found, and a variety of characteristics can be optimised, including shunting work effort, number or cost of tracks, and shunting task start times. Two different objective functions are evaluated: minimising work effort in terms of wagon pull-backs and minimising track costs. A procedure for finding a hot-start solution with few wagon pull-backs is also presented. The proposed model is tested on real data from Savenas marshalling yard in Sweden. The results show that the method is able to return an optimal schedule for a planning period of 4 days if the hot-start solution is optimal or the remaining problem is tractable for the heuristics in CPLEX.