A C interface to Cassowary - with some reduction, cleaning, orthogonalization.
All of the credit to the whiz-bang's due to the original authors of Cassowary: respectively, Greg J. Badros; Alan Borning; Peter J. Stuckey. The sharp edges and hole-riddled hacks should, on the other hand, be laid directly at my doormat.
This library's a fairly simple wrapper layer that exposes the user-facing object hierarchy within Cassowary in a C-compatible (and C-interfacing FFI-compatible) interface. (There is in fact one significant difference that'll arouse a great deal of suspicion if you're a Cassowary veteran: ClLinearEquations and ClLinearInequalities have been combined into the same class with the help of the abstract relation operator facility ClConstraint exposes - and ClLinearInequalities assiduously uses as well; even to the point of obviating the need for ClLinearEquations.)
I hope I've done a fair job of providng something predictable and pleasing to the touch.
ClC.h ostensibly does the same job, but I'd argue that the interface it exposes (ClConstraints must be instantiated from strings to be parsed, and very little of the composing abstractions underneath are shown) requires we round-trip through an expensive intermediate representation if we wish to go straight from meaningful data already represented in structures, to Expressions in Cassowary's own.
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Copy libcassowary's static libraries and the "c++" subdirectory from Cassowary's source distribution into the root of the source tree; you may need help getting Cassowary to compile, and so I've documented my experiences below.
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make; libcassowary-lite should pop up in both shared and dynamic forms.
That's it - you'll have more fun wrangling with the interface I've cooked up; cassowary-lite.h should be spare enough to be easily understood. Bear in mind the standard narrative of a Cassowary application: Variables are either directly involved in StayOrEditConstraints, which assert invariants (as long as Cassowary can assert the invariants, at the very least); or shoved into Expressions, and in turn into Relations, which are then put into SimplexSolvers to have their constraints met - at which point the values you need'll be back in the Variables.
If you so wish to code UI event-handling patterns around this library, there exists a mutable-state annex that allows you to edit the variables within StayOrEditConstraints, and re-solve with alterations made as required - the original "SuggestValues()" indicates that alterations may occur, but I'd argue more important is to also indicate that they occur when edits are made!
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Extract the C++ source directory from the rest of the Cassowary source distribution. You'll find it under the 'c++' folder - we're in luck, since this subdirectory's self-contained: we can build precisely what we need with only it. Copy that into the root of this source tree.
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Add -fPIC to CPPFLAGS in Makefile.linux - the Makefile target we're using builds both static and shared libraries, while the proper directives are only issued for the former, and
*. Surgically remove ClReader from the compile process - the directives issued to flex and yacc no longer do what they should, and you'll end up with issues with both that you shouldn't quite frankly have to care about. Do both steps in one fell swoop - patch Makefile.linux
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ln -s . cassowary - Cl.h requires that headers be in a non-existent subdirectory called 'cassowary'; since they're in the current directory, though, we'll just fake up the subdirectory's existence.
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make -f Makefile.linux all-notests: 'notests' because the functionality that ClReader provides is being tested in those as well - which we've just stripped out. At this point you should have libcassowary.[a | so] and libcccassowary.[a | so] built in the directory - at which point you should be able to copy the libraries back up to the top, and proceed with the rest of the instructions.
Public domain. I don't trust myself enough to believe that even a ceremony establishing my rights is required.