This code implements a UTFSet, a set containing Unicode characters (‘runes’) in a tree structure that mirrors the UTF-8 encoding format. The result is an exceedingly simple data structure, implemented here in under 75 lines of C, which nevertheless takes up less space when storing smaller (and generally more common) runes, and more when storing larger ones. It may not be the most efficient, but it is kind of neat.
The code is very well commented; read utfset.c in order to understand it.