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VCDIFF is a format and an algorithm for delta encoding, described in IETF's RFC 3284. The algorithm is based on Jon Bentley and Douglas McIlroy's paper "Data Compression Using Long Common Strings"^[1] written in 1999.^{[citation needed]} VCDIFF is used as one of the delta encoding algorithms in "Delta encoding in HTTP" (RFC 3229) and was employed in Google's Shared Dictionary Compression Over HTTP technology, formerly used in their Chrome browser.

Delta instructions

VCDIFF has 3 delta instructions. ADD, COPY, and RUN. ADD adds a new sequence, COPY copies from an old sequence, and RUN adds repeated data.

Free software implementations include xdelta (version 3) and open-vcdiff.

Google's Shared Dictionary Compression Over HTTP proposal uses this algorithm, and was included in the Google Chrome browser, up to version 58.^[2]
xdelta - A tool, which is an Open Source VCDIFF delta compression implementation
google/open-vcdiff - Another Open Source VCDIFF delta compression implementation
vczip as part of vcodex and ast-open packages (also part of UWIN)
diffable - Java implementation, but some kind of original modification of VCDIFF
jvcdiff - Full decoder/encoder Java implementation of VCDIFF
vcdiff-java - another Java implementation of VCDIFF
Miscellaneous Utility Library - C# implementation, decoding only
CyanDelta - Updates CyanogenMod ROMs using Xdelta3 patches.

^ Bentley, Jon; McIlroy, Douglas (1999). Data compression using long common strings. DCC '99: Proceedings of the Conference on Data Compression. IEEE Computer Society. CiteSeerX 10.1.1.11.8470. doi:10.1109/DCC.1999.755678.
^ "Intent to Unship: SDCH". Retrieved 2017-08-08.

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