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. 

A technical report is included in the distribution that describes the
algorithm, interface, and implementation of the Cassowary solver. 
Additionally, the distribution contains toy sample applications
written in Smalltalk, C++, Java, and Python, and a more complex
example Java applet, the "Constraint Drawing Application".