README.md

JavaEWAH

(c) 2009-2014 Daniel Lemire (http://lemire.me/en/), Cliff Moon (https://github.com/cliffmoon), David McIntosh (https://github.com/mctofu), Robert Becho (https://github.com/RBecho), Colby Ranger (https://github.com/crangeratgoogle) Veronika Zenz (https://github.com/veronikazenz) Owen Kaser (https://github.com/owenkaser) Gregory Ssi-Yan-Kai (https://github.com/gssiyankai) and Rory Graves (https://github.com/rorygraves)

This code is licensed under Apache License, Version 2.0 (ASL2.0). (GPL 2.0 derivatives are allowed.)

This is a word-aligned compressed variant of the Java Bitset class. We provide both a 64-bit and a 32-bit RLE-like compression scheme. It can be used to implement bitmap indexes.

The goal of word-aligned compression is not to achieve the best compression, but rather to improve query processing time. Hence, we try to save CPU cycles, maybe at the expense of storage. However, the EWAH scheme we implemented is always more efficient storage-wise than an uncompressed bitmap (as implemented in the java BitSet class by Sun).

JavaEWAH offers competitive speed. In an exhaustive comparison, Guzun et al. (ICDE 2014) found that "EWAH offers the best query time for all distributions."

For better performance, use a 64-bit JVM over 64-bit CPUs when using the 64-bit scheme (javaewah.EWAHCompressedBitmap). The 32-bit version (javaewah32.EWAHCompressedBitmap32) should compress better but be comparatively slower.

Java 6 or better is required.

Real-world usage

JavaEWAH is part of Apache Hive, Apache Spark and Eclipse JGit. It has been used in production systems for many years.

Data format

For more details regarding the compression format, please see Section 3 of the following paper:

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

(The PDF file is freely available on the arXiv site.)

Benchmark

For a simple comparison between this library and other libraries such as WAH, ConciseSet, BitSet and other options, please see

https://github.com/lemire/simplebitmapbenchmark

Unit testing

As of October 2011, this packages relies on Maven. To test it:

mvn test

See http://maven.apache.org/guides/introduction/introduction-to-the-lifecycle.html for details.

Usage

See example.java.

Maven central repository

You can download JavaEWAH from the Maven central repository: http://repo1.maven.org/maven2/com/googlecode/javaewah/JavaEWAH/

You can also specify the dependency in the Maven "pom.xml" file:

<dependencies>
     <dependency>
     <groupId>com.googlecode.javaewah</groupId>
     <artifactId>JavaEWAH</artifactId>
     <version>0.9.1</version>
     </dependency>
 </dependencies>

Naturally, you should replace "version" by the version you desire.

Travis (Continuous integration)

You can check whether the latest version builds on your favorite version of Java using Travis: https://travis-ci.org/lemire/javaewah/builds/

Clojure

Joel Boehland wrote Clojure wrappers:

https://github.com/jolby/clojure-ewah-bitmap

Frequent questions

Question: Will JavaEWAH support long values?

Answer: It might, but it does not at the moment.

Question: How do I check the value of a bit?

Answer: If you need to routinely check the value of a given bit quickly, then EWAH might not be the right format. However, if you must do it, you can proceed as follows:

      /**
       * Suppose you have the following bitmap:
       */
      EWAHCompressedBitmap b = EWAHCompressedBitmap.bitmapOf(0,2,64,1<<30);
      /**
       * We want to know if bit 64 is set:
       */
      boolean is64set = b.get(64);

API Documentation

http://lemire.me/docs/javaewah/

Further reading

Owen Kaser and Daniel Lemire, Compressed bitmap indexes: beyond unions and intersections, Software: Practice and Experience, 2014. http://arxiv.org/abs/1402.4466

Acknowledgement

Special thanks to Shen Liang for optimization advice.

This work was supported by NSERC grant number 26143.