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A compressed bitmap class in C++.
C++ Makefile
Branch: master

README.md

Compressed bitset in C++

Build Status

What is this?

The class EWAHBoolArray is a compressed bitset data structure.

It also provides a basic BoolArray class which can serve as a traditional bitmap.

This library is used by database and information retrieval engines such as Hustle -- A column oriented, embarrassingly distributed relational event database (see https://github.com/chango/hustle).

Licensing

Apache License 2.0.

Update (May 20th, 2013): though by default I use the Apache License 2.0 (which is compatible with GPL 3.0), you can also consider this library licensed under GPL 2.0.

Dependencies

None. (Will work under MacOS, Windows or Linux.)

Compilers tested: clang++, g++, Intel compiler, Microsoft Visual Studio

Usage

make
./unit
make example
./example

Example

Please see example.cpp. For an example with tabular data, please see example2.cpp.

Ruby wrapper

Josh Ferguson wrote a wrapper for Ruby. The implementation is packaged and installable as a ruby gem.

You can install it by typing:

    gem install ewah-bitset

Further reading

Please see

  • Daniel Lemire, Owen Kaser, Kamel Aouiche, Sorting improves word-aligned bitmap indexes. Data & Knowledge Engineering 69 (1), pages 3-28, 2010. http://arxiv.org/abs/0901.3751

Persistent storage

If you save the bitmap to disk using "write" and then try to read it again with "read" on a different machine, it may crash. The file format is specific to the machine you are using. E.g., using two 64-bit Windows PC might work, but if you send the same data to a 32-bit Linux box it may fail.

You can get better persistence with the readBuffer and writeBuffer methods. They will be consistent across machines except for bit endianess: you may need to manually check bit endianess. Moreover, these methods require you to save some metadata (i.e., buffersize and sizeinbits) yourself in some portable way.

For saving in a persistent way:

 size_t sb = mybitmap.sizeInBits(); // save sb somewhere safe, your responsability!
 size_t bs = mybitmap.bufferSize(); // save bs somewhere safe, your responsability!
 mybitmap.writeBuffer(out); // this writes the internal buffer of the bitmap

For loading:

 mybitmap.readBuffer(in,bs); // recover the saved internal buffer, you are responsable for endianess
 mybitmap.setSizeInBits(sb); // you have to do this to get a proper bitmap size
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