We report the design, implementation, and deployment of Lepton, a fault-tolerant system that losslessly compresses JPEG images to 77% of their original size on average. Lepton replaces the lowest layer of baseline JPEG compression-a Huffman code-with a parallelized arithmetic code, so that the exact bytes of the original JPEG file can be recovered quickly. Lepton matches the compression efficiency of the best prior work, while decoding more than nine times faster and in a streaming manner. Lepton has been released as open-source software and has been deployed for a year on the Dropbox file-storage backend. As of February 2017, it had compressed more than 203 PiB of user JPEG files, saving more than 46 PiB.
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