VCDIFF is a format and an algorithm

Algorithm

In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...

 for delta encoding

Delta encoding

Delta encoding is a way of storing or transmitting data in the form of differences between sequential data rather than complete files; more generally this is known as data differencing...

, described in RFC 3284. The algorithm is based on Jon Bentley and Douglas McIlroy's paper "Data Compression Using Long Common Strings" written in 1999. VCDIFF is used as one of the delta encoding algorithm in "Delta encoding in HTTP" (RFC 3229).

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.

Implementations

Free software

Free software

Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...

 implementations include Xdelta (version 3) and open-vcdiff.

• Xdelta
  Xdelta
  xdelta is a command line program for delta encoding, which generates two file difference. This is similar to diff and patch, but it is targeted for binary files and does not generate human readable output....
  
  - A tool, which is an Open Source VCDIFF delta compression implementation
• open-vcdiff - Another Open Source VCDIFF delta compression implementation
• diffable - Java implementation, but some kind of original modification of VCDIFF
• Miscellaneous Utility Library - C# implementation, decoding only