Not a bitmap[1], but a bitmap[2].

[1]: https://en.wikipedia.org/wiki/Bitmap

[2]: https://en.wikipedia.org/wiki/Bit_array

Featurebase[1] is a database based on bitmaps. (Formerly known as Piloasa) I'm not sure if they actually use Roaring Bitmaps [2] or they implemented it themselves. Either way it's based on the idea of compressed bitmaps.

[1] https://github.com/FeatureBaseDB/featurebase [2] https://roaringbitmap.org/

But on most of my unicode use-cases slower. When you can encode ranges of ints or bools, a simple binary search in such ranges is usually faster than roaring bitmaps. https://github.com/rurban/libu8ident/blob/master/perf.c

I still have no idea what they mean by "bitmap". Obviously it is not the same thing as I think of.

Roaring bitmaps are very interesting implementation. It's a mix of using arrays or bitsets (equivalent would be [1]) depending upon which are more efficient. I've used them in the past, and these are very effective, both for storage and for unions and intersections.

[1]: https://en.cppreference.com/w/cpp/utility/bitset

The meaning of bitmap here is essentially "bit array". Roaring bitmaps does a bunch of tricks to e.g. compress long runs of identical bits, which also happens to make bitwise operations a lot faster.

They're used to make very compact sets when the number of possible items is relatively small.