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A compressed bitmap class in C++.
C++ Makefile
Branch: master
Latest commit fbb7dd @lemire Avoiding subtraction.

Compressed bitset in C++

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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


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.


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

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


make 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.

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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