Alan Borning

The Programming Language Aspects of ThingLab, a Constraint-Oriented Simulation Laboratory

The programming language aspects of a graphic simulation laboratory named ThingLab are presented. The design and implementation of ThingLab are extensions to Smalltalk. In ThingLab, constraints are used to specify the relations that must hold among the parts of the simulation. The system is object-oriented and employs inheritance and part-whole hierarchies to describe the structure of a simulation. An interactive, graphic user interface is provided that allows the user to view and edit a simulation.

Microsimulation of urban development and location choices : design and implementation of UrbanSim

UrbanSim is a new urban simulation model, developed over the past several years, which is now operational in three urban areas in the United States. The model system is designed to address emerging needs to better coordinate transportation and land use planning as a result of recognition of the strong interactions between land use and transportation, increasing pressure from federal transportation and environmental legislation, and growing adoption of state growth management programs. The model system is implemented as a set of interacting model components that represent the major actors and choices in the urban system, including household moving and residential location, business choices of employment location, and developer choices of locations and types of real estate development, all subject to the influence of governmental transportation and land use policy scenarios. The model design is unusual in the degree of disaggregation of space, time, and agents, and in the adoption of a dynamic disequilibrium approach. The objective of this paper is to describe the entire system at a sufficient level of detail to convey the key specification and design choices made in implementing the system.

An incremental constraint solver

An incremental constraint solver, the DeltaBlue algorithm maintains an evolving solution to the constraint hierarchy as constraints are added and removed. DeltaBlue minimizes the cost of finding a new solution after each change by exploiting its knowledge of the last solution.

Constraint-based tools for building user interfaces

A constraint describes a relation that must be maintained. Constraints provide a useful mechanism to aid in the construction of interactive graphical user interfaces. They can be used to maintain consistency between data and a view of the data, to maintain consistency among multiple views, to specify layout, and to specify relations between events and responses for describing animations of interactive systems and event-driven simulations. Object-oriented techniques for constraint representation and satisfaction are presented, and a range of examples that demonstrate the practical use of static and temporal constraints for such purposes is presented. These examples include animations of algorithms and physics simulations, and constructing user-interface elements such as file browsers, views onto statistical data, and an interactive monitor or a simulated operating system.

The Cassowary linear arithmetic constraint solving algorithm

Linear equality and inequality constraints arise naturally in specifying many aspects of user interfaces, such as requiring that one window be to the left of another, requiring that a pane occupy the leftmost third of a window, or preferring that an object be contained within a rectangle if possible. Previous constraint solvers designed for user interface applications cannot handle simultaneous linear equations and inequalities efficiently. This is a major limitation, as such systems of constraints arise often in natural declarative specifications. We describe Cassowary---an incremental algorithm based on the dual simplex method, which can solve such systems of constraints efficiently. We have implemented the algorithm as part of a constraint-solving toolkit. We discuss the implementation of the toolkit, its application programming interface, and its performance.

OneBusAway: results from providing real-time arrival information for public transit

Public transit systems play an important role in combating traffic congestion, reducing carbon emissions, and promoting compact, sustainable urban communities. The usability of public transit can be significantly enhanced by providing good traveler information systems. We describe OneBusAway, a set of transit tools focused on providing real-time arrival information for Seattle-area bus riders. We then present results from a survey of OneBusAway users that show a set of important positive outcomes: strongly increased overall satisfaction with public transit, decreased waiting time, increased transit trips per week, increased feelings of safety, and even a health benefit in terms of increased distance walked when using transit. Finally, we discuss the design and policy implications of these results and plans for future research in this area.

A prototype electronic encyclopedia

We describe a prototype electronic encyclopedia implemented on a powerful personal computer, in which user interface, media presentation, and knowledge representation techniques are applied to improving access to a knowledge resource. In itself, an electronic encyclopedia is an important information resource, but this work also illustrates the issues and approaches for many types of electronic information retrieval environments. In the prototype we make dynamic use of the structure and semantics of the text articles and index of an existing encyclopedia, while experimenting with other forms of representation, such as simulation and videodisc images. We present a long-term vision of an intelligent user-interface agent; summarize previous work related to futuristic encyclopedias, electronic books, decision support systems, and knowledge libraries; and outline current and potential research directions.

Multi-way versus one-way constraints in user interfaces: experience with the DeltaBlue algorithm

The efficient satisfaction of constraints is essential to the performance of constraint‐based user interfaces. In the past, most constraint‐based user interfaces have used one‐way rather than multi‐way constraints because of a widespread belief that one‐way constraints were more efficient. In this paper we argue that many user interface construction problems are handled more naturally and elegantly by multi‐way constraints than by one‐way constraints. We present pseudocode for an incremental multi‐way constraint satisfaction algorithm, DeltaBlue, and describe experience in using the algorithm in two user interface toolkits. Finally, we provide performance figures demonstrating that multi‐way constraint solvers can be entirely competitive in performance with one‐way constraint solvers.

Defining constraints graphically

A number of constraint-oriented, interactive graphical systems have been constructed. A typical problem in such systems is that, to define a new kind of constraint, the user must leave the graphical domain and write code in the underlying implementation language. This makes it difficult for less experienced users to add new kinds of constraints. As a step toward solving this problem, the system described here allows the graphical definition of constraints. An interface has been built in which a user can conveniently construct a new kind of object, annotating it with the relations that it must obey.

Solving linear arithmetic constraints for user interface applications

Linear equality and inequality constraints arise naturally in specifying many aspects of user interfaces, such as requiring that onewindowbe to the left of another, requiring that a pane occupy the leftmost 1/3 of a window, or preferring that an object be contained within a rectangle if possible. Current constraint solvers designed for UI applications cannot efficiently handle simultaneous linear equations and inequalities. This is amajor limitation. We describe incremental algorithms based on the dual simplex and active set methods that can solve such systems of constraints efficiently.