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

Cascading Style Sheets

Cascading Style Sheets is a style sheet language used to describe the presentation semantics of a document written in a markup language...

 (CSS). CCSS adds support for layout

Page layout

Page layout is the part of graphic design that deals in the arrangement and style treatment of elements on a page.- History and development :...

 constraints. These allow designers to describe the layout of a web page

Web page

A web page or webpage is a document or information resource that is suitable for the World Wide Web and can be accessed through a web browser and displayed on a monitor or mobile device. This information is usually in HTML or XHTML format, and may provide navigation to other web pages via hypertext...

 in a more flexible manner. Cassowary is used to solve these constraints and calculate the final layout.

Smalltalk

Smalltalk

Smalltalk is an object-oriented, dynamically typed, reflective programming language. Smalltalk was created as the language to underpin the "new world" of computing exemplified by "human–computer symbiosis." It was designed and created in part for educational use, more so for constructionist...

, C++

C++

C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...

 and Java

Java (programming language)

Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...

 versions are available in the main distribution. Furthermore, there are bindings for GNU Guile

GNU Guile

GNU Guile is an interpreter/virtual machine for the Scheme programming language. It was first released in 1993. Guile includes modularized extensions for POSIX system calls, APL array functionality, and others packaged as an object library...

, Python

Python (programming language)

Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...

, and STk.

Other people have ported the solver to Squeak

Squeak

The Squeak programming language is a Smalltalk implementation. It is object-oriented, class-based and reflective.It was derived directly from Smalltalk-80 by a group at Apple Computer that included some of the original Smalltalk-80 developers...

 and the .NET framework

.NET Framework

The .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...

 (Cassowary.net).

Cassowary is used by Scwm

Scwm

Scwm or Scheme Constraints Window Manager is a window manager for the X Window System. Its main features are dynamic configurability and programmability via a language based on GNU Guile and the embedded arithmetic Cassowary constraint solver. The primary developers were Greg Badros and Maciej...

, the Scheme Constraints Window Manager.

External links

• project page at the University of Washington
• Cassowary.net (a .NET port of Cassowary)