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Google's new squeeze: Brotli compression open-sourced

Chocolate Factory turns Pied Piper to Deflate expectations

Google wants to bring to life the HBO series Silicon Valley: it's pitching a new open source compression algorithm into the world, with the hope that it can eventually end-of-life the venerable Deflate.

Brotli (“small bread” in Swiss German, apparently) follows on from Zopfli (“little braid,” also bread-themed), but with between 20 and 26 per cent better compression ratios.

That, software engineer Zoltán Szabadka of Google's Compression Team writes, is because while Zopfli maintained Deflate compatibility, Brotli uses new data structures.

Szabadka lists the key changes as “a 2nd order context modeling, re-use of entropy codes, larger memory window of past data, and joint distribution codes.”

In this paper, Szabadka and collaborators Jyrki Alakuijala, Evgenii Kliuchnikov, and Lode Vandevenne also explain that Brotli adds a static dictionary to the compression algorithm:

“It contains 13,504 words or syllables of English, Spanish, Chinese, Hindi, Russian, and Arabic, as well as common phrases used in machine readable languages, particularly HTML and JavaScript. The total size of the static dictionary is 122,784 bytes. The static dictionary is extended by a mechanism of transforms that slightly change the words in the dictionary. A total of 1,633,984 sequences, although not all of them unique, can be constructed by using the 121 transforms.”

In tests under Linux 3.13.0 on an Intel Xeon Eg-1650 v2 machine running at 3.5 GHz, the researchers claimed Brotli running at 3.381:1 compression ratio could compress at 98.3 MB/s and decompress at 334 MB/s.

The fastest Deflate could run at a low-quality setting of 2.913:1 compression was 93.5 MB/s compression and 323 MB/s decompression.

Google notes that better compression would particularly benefit mobile users, who in addition to faster page loads would get savings on their data charges and hopefully lower battery consumption.

And, of course, if Brotli is a success, there's no chance that Google will end up torching a basement.

The Register would note that Deflate's endurance demonstrates just how effective the original work was: it was described in RFC 1951 in 1996.

Brotli is on GitHub here. ®

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