Brotli was first released in 2015 for off-line compression of web fonts. The version of Brotli released in September 2015 by the Google software engineers contained enhancements in generic lossless data compression, with particular emphasis on use for HTTP compression. The encoder was partly rewritten, with the result that the compression ratio improved, both the encoder and the decoder have been sped up, the streaming API was improved, more compression quality levels have been added. Additionally, the new release shows performance improvements across platforms, with decoding memory reduction.Unlike most general purpose compression algorithms, Brotli uses a pre-defined 120 kilobyte dictionary, in addition to the dynamically populated ("sliding window") dictionary. The pre-defined dictionary contains over 13000 common words, phrases and other substrings derived from a large corpus of text and HTML documents. Using a pre-defined dictionary has been shown to increase compression where a file mostly contains commonly-used words.
