Cassowary is an incremental constraint solving toolkit that efficiently solves systems of linear equalities and inequalities. Constraints may be either requirements or preferences. Client code specifies the constraints to be maintained, and the solver updates the constrained variables to have values that satisfy the constraints.
Cassowary was developed by Greg Badros and Alan Borning, and was optimized for user interface applications. Badros used Cassowary amongst others for implementing Constraint Cascading Style Sheets (CCSS), an extension to Cascading Style Sheets (CSS). CCSS adds support for layout constraints. These allow designers to describe the layout of a web page in a more flexible manner. Cassowary is used to solve these constraints and calculate the final layout.
Cassowary is used by Scwm, the Scheme Constraints Window Manager. As of 2011, Cassowary is being used as the layout engine for Mac OS X Lion.