Zoltán Szabadka

Brotli is a data format specification for data streams compressed with a specific combination of the general-purpose LZ77 lossless compression algorithm, Huffman coding and 2nd order context modelling. Brotli was initially developed to decrease the size of transmissions of WOFF2 web fonts, and in that context was a continuation of the development of zopfli, which is a zlib-compatible implementation of the standard gzip and deflate specifications. Brotli allows a denser packing than gzip and deflate because of several algorithmic and format level improvements: the use of context models for literals and copy distances, describing copy distances through past distances, use of move-to-front queue in entropy code selection, joint-entropy coding of literal and copy lengths, the use of graph algorithms in block splitting, and a larger backward reference window are example improvements. The Brotli specification was generalized in September 2015 for HTTP stream compression (content-encoding type 'br'), and can now be used to encode any data sent by a web server to a web browser if both client and server support the format. This generalized iteration also improved the compression ratio by using a pre-defined dictionary of frequently-used words and phrases.

The Brotli specification was developed in 2013–2016 by Google employees Jyrki Alakuijala and Zoltan Szabadka, and was accompanied by a reference implementation developed by the two authors of the specification together with Evgenii Kliuchnikov and Lode Vandevenne, who had previously developed Google's zopfli reimplementation of deflate/gzip compression formats in 2013. Unlike zopfli, which was a reimplementation of an existing data format specification, Brotli was a new data format, and allowed the authors to improve compression ratios even further.

Brotli was designed for use on a sequentially-processed data stream (a bitstream), rather than on discrete random-access files. This makes Brotli particularly suitable for compressing data as it is sent over a network connection. Under ideal circumstances, this reduces the volume of data being transmitted. The transmission of a compressed stream may then also complete sooner than would be the case for an uncompressed stream, or a stream compressed with a less efficient stream compressor such as gzip or deflate. While gzip and deflate are comparatively light-weight compressors (i.e. less processor- and memory-intensive than Brotli), and are already widely supported by many web servers, Brotli has not yet been implemented as widely. The Brotli compressed data format was submitted to the Internet Engineering Task Force with a request for comment (RFC 7932) in July 2016. The Brotli data format is an integral part of the 2nd iteration of the Web Open Font Format.

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