Roaring bitmaps are found to work well in many important applications:
> Use Roaring for bitmap compression whenever possible. Do not use other bitmap compression methods ([Wang et al., SIGMOD 2017](http://db.ucsd.edu/wp-content/uploads/2017/03/sidm338-wangA.pdf))
This library is used in production in several systems, it is part of the [Awesome Go collection](https://awesome-go.com).
There are also [Java](https://github.com/RoaringBitmap/RoaringBitmap) and [C/C++](https://github.com/RoaringBitmap/CRoaring) versions. The Java, C, C++ and Go version are binary compatible: e.g, you can save bitmaps
from a Java program and load them back in Go, and vice versa. We have a [format specification](https://github.com/RoaringBitmap/RoaringFormatSpec).
This code is licensed under Apache License, Version 2.0 (ASL2.0).
Most alternatives to Roaring are part of a larger family of compressed bitmaps that are run-length-encoded
bitmaps. They identify long runs of 1s or 0s and they represent them with a marker word.
If you have a local mix of 1s and 0, you use an uncompressed word.
There are many formats in this family:
* Oracle's BBC is an obsolete format at this point: though it may provide good compression,
it is likely much slower than more recent alternatives due to excessive branching.
* WAH is a patented variation on BBC that provides better performance.
* Concise is a variation on the patented WAH. It some specific instances, it can compress
much better than WAH (up to 2x better), but it is generally slower.
* EWAH is both free of patent, and it is faster than all the above. On the downside, it
does not compress quite as well. It is faster because it allows some form of "skipping"
over uncompressed words. So though none of these formats are great at random access, EWAH
is better than the alternatives.
There is a big problem with these formats however that can hurt you badly in some cases: there is no random access. If you want to check whether a given value is present in the set, you have to start from the beginning and "uncompress" the whole thing. This means that if you want to intersect a big set with a large set, you still have to uncompress the whole big set in the worst case...
Roaring solves this problem. It works in the following manner. It divides the data into chunks of 2<sup>16</sup> integers
(e.g., [0, 2<sup>16</sup>), [2<sup>16</sup>, 2 x 2<sup>16</sup>), ...). Within a chunk, it can use an uncompressed bitmap, a simple list of integers,
or a list of runs. Whatever format it uses, they all allow you to check for the present of any one value quickly
(e.g., with a binary search). The net result is that Roaring can compute many operations much faster than run-length-encoded
formats like WAH, EWAH, Concise... Maybe surprisingly, Roaring also generally offers better compression ratios.
- Daniel Lemire, Owen Kaser, Nathan Kurz, Luca Deri, Chris O'Hara, François Saint-Jacques, Gregory Ssi-Yan-Kai, Roaring Bitmaps: Implementation of an Optimized Software Library, Software: Practice and Experience 48 (4), 2018 [arXiv:1709.07821](https://arxiv.org/abs/1709.07821)
- Samy Chambi, Daniel Lemire, Owen Kaser, Robert Godin,
Better bitmap performance with Roaring bitmaps,
Software: Practice and Experience 46 (5), 2016.
http://arxiv.org/abs/1402.6407 This paper used data from http://lemire.me/data/realroaring2014.html
- Daniel Lemire, Gregory Ssi-Yan-Kai, Owen Kaser, Consistently faster and smaller compressed bitmaps with Roaring, Software: Practice and Experience 46 (11), 2016. http://arxiv.org/abs/1603.06549
### Dependencies
Dependencies are fetched automatically by giving the `-t` flag to `go get`.
they include
- github.com/willf/bitset
- github.com/mschoch/smat
- github.com/glycerine/go-unsnap-stream
- github.com/philhofer/fwd
- github.com/jtolds/gls
Note that the smat library requires Go 1.6 or better.
#### Installation
- go get -t github.com/RoaringBitmap/roaring
### Example
Here is a simplified but complete example:
```go
package main
import (
"fmt"
"github.com/RoaringBitmap/roaring"
"bytes"
)
func main() {
// example inspired by https://github.com/fzandona/goroar
fmt.Println("I wrote the content to a byte stream and read it back.")
}
// you can iterate over bitmaps using ReverseIterator(), Iterator, ManyIterator()
}
```
Only the 32-bit roaring format is standard and cross-operable between Java, C++, C and Go. There is no guarantee that the 64-bit versions are compatible.
