computer science

VCDIFF

VCDIFF is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand VCDIFF rather than just read about it. In short: 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" written in 1999.

Key takeaways

VCDIFF belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect VCDIFF to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of VCDIFF from memory before moving on to harder problems.

Reference excerpt

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" written in 1999. 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.

Implementations 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. xdelta - A tool, which is an Open Source VCDIFF delta compression implementation google/open-vcdiff - Another Open Source VCDIFF delta compression implementation vczip Archived 2014-02-22 at the Wayback Machine as part of vcodex and ast-open packages (also part of UWIN) diffable - Java implementation, but some kind of original modification of VCDIFF jvcdiff[link removed] - Full decoder/encoder Java implementation of VCDIFF vcdiff-java - another Java implementation of VCDIFF Miscellaneous Utility Library Archived 2013-02-04 at the Wayback Machine - C# implementation, decoding only CyanDelta - Updates CyanogenMod ROMs using Xdelta3 patches.

See also Delta encoding Data differencing

References

External links RFC 3284 - The VCDIFF Generic Differencing and Compression Data Format

Worked examples

Example 1 — a first encounter with VCDIFF

Start with the simplest possible case. Write down what VCDIFF claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to VCDIFF before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about VCDIFF ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of VCDIFF

In research: VCDIFF appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses VCDIFF in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: VCDIFF is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computing stubs, Data differencing, so understanding it makes those chapters shorter.
In everyday life: Look for VCDIFF outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study VCDIFF in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what VCDIFF means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain VCDIFF out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is VCDIFF in simple terms?

Why does VCDIFF matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study VCDIFF?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on VCDIFF.