The Cassowary linear arithmetic constraint solving algorithm

  • Authors:

  • Greg J. Badros;Alan Borning;Peter J. Stuckey

  • Affiliations:

  • University of Washington, Seattle, WA;University of Washington, Seattle, WA;University of Melbourne, Parkville, Victoria, Australia

  • Venue:
  • ACM Transactions on Computer-Human Interaction (TOCHI)
  • Year:
  • 2001

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Abstract

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.