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.

It is important to keep in mind that Cassowary is not a traditional linear programming solver! Cassowary is optimized for user interface applications, and uses a modified (incremental) version of the simplex algorithm which might yield different results compared with traditional linear programming tools. I have been contacted before by people who were using Cassowary.net as a regular LP solver, but were getting strange results. For more details about the underlying algorithm, have a look at the Cassowary TOCHI paper by Badros, Borning and Stuckey.