Chapter 2. Subversion’s Delta Editor: Interface As Ontology

Karl Fogel

Examples of beautiful code tend to be local solutions to well-bounded, easily comprehensible problems, such as Duff’s Device ('s_device) or rsync’s rolling checksum algorithm ( This is not because small, simple solutions are the only beautiful kind, but because appreciating complex code requires more context than can be given on the back of a napkin.

Here, with the luxury of several pages to work in, I’d like to talk about a larger sort of beauty—not necessarily the kind that would strike a passing reader immediately, but the kind that programmers who work with the code on a regular basis would come to appreciate as they accumulate experience with the problem domain. My example is not an algorithm, but an interface: the programming interface used by the open source version control system Subversion ( to express the difference between two directory trees, which is also the interface used to transform one tree into the other. In Subversion, its formal name is the C type svn_delta_editor_t, but it is known colloquially as the delta editor.

Subversion’s delta editor demonstrates the properties that programmers look for in good design. It breaks down the problem along boundaries so natural that anyone designing a new feature for Subversion can easily tell when to call each function, and for what purpose. It ...

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