computer science

LZ4 (compression algorithm)

LZ4 (compression algorithm) 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 LZ4 (compression algorithm) rather than just read about it. In short: LZ4 is a lossless data compression algorithm optimized for fast compression and decompression. It belongs to the LZ77 family of byte-oriented compression schemes.

Key takeaways

LZ4 (compression algorithm) 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 LZ4 (compression algorithm) to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of LZ4 (compression algorithm) from memory before moving on to harder problems.

Reference excerpt

LZ4 is a lossless data compression algorithm optimized for fast compression and decompression. It belongs to the LZ77 family of byte-oriented compression schemes.

Features The LZ4 algorithm provides a good trade-off between speed and compression ratio. Typically, it has a smaller (i.e., worse) compression ratio than the similar LZO algorithm, which in turn is worse than algorithms like DEFLATE. However, LZ4 compression speed is similar to LZO and several times faster than DEFLATE, while decompression speed is significantly faster than LZO.

Design LZ4 only uses a dictionary-matching stage (LZ77) and, unlike other common compression algorithms, does not combine it with an entropy coding stage (e.g. Huffman coding in DEFLATE). The LZ4 algorithm represents the data as a series of sequences. Each sequence begins with a one-byte token that is broken into two 4-bit fields. The first field represents the number of literal bytes that are to be copied to the output. The second field represents the number of bytes to copy from the already decoded output buffer (with 0 representing the minimum match length of 4 bytes). A value of 15 in either of the bitfields indicates that the length is larger and there is an extra byte of data that is to be added to the length. A value of 255 in these extra bytes indicates that yet another byte is to be added. Hence, arbitrary lengths are represented by a series of extra bytes containing the value 255. The string of literals comes after the token and any extra bytes needed to indicate string length. This is followed by an offset that indicates how far back in the output buffer to begin copying. The extra bytes (if any) of the match-length come at the end of the sequence. Compression can be carried out in a stream or in blocks. Higher compression ratios can be achieved by investing more effort in finding the best matches. This results in both a smaller output and faster decompression. LZ4 has two frame formats. The legacy format was very restrictive and relied on an external end-of-file signal, which proved to be a problem in Linux initramfs requiring a workaround to handle zero-padding. The new format is a lot more flexible and has its own end-of-frame marker. It resembles the Zstd frame format in design.

Implementation The reference implementation in C by Yann Collet is licensed under a BSD license. There are ports and bindings in various languages including Java, C#, Rust, and Python. The Apache Hadoop system uses this algorithm for fast compression. LZ4 was also implemented natively in the Linux kernel 3.11. The FreeBSD, Illumos, ZFS on Linux, and ZFS-OSX implementations of the ZFS filesystem support the LZ4 algorithm for on-the-fly compression. Linux supports LZ4 for SquashFS since 3.19-rc1. LZ4 is also supported by the newer zstd command line utility by Yann Collet, as well as a 7-Zip fork called 7-Zip-zstd.

References

External links Official website

Worked examples

Example 1 — a first encounter with LZ4 (compression algorithm)

Start with the simplest possible case. Write down what LZ4 (compression algorithm) 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 LZ4 (compression algorithm) 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 LZ4 (compression algorithm) 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 LZ4 (compression algorithm)

In research: LZ4 (compression algorithm) 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 LZ4 (compression algorithm) 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: LZ4 (compression algorithm) is common in secondary-school and first-year university syllabi. It links to neighbouring topics C (programming language) libraries, Data compression, Free data compression software, so understanding it makes those chapters shorter.
In everyday life: Look for LZ4 (compression algorithm) 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 LZ4 (compression algorithm) in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what LZ4 (compression algorithm) 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 LZ4 (compression algorithm) out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is LZ4 (compression algorithm) in simple terms?

LZ4 is a lossless data compression algorithm optimized for fast compression and decompression. It belongs to the LZ77 family of byte-oriented compression schemes.

Why does LZ4 (compression algorithm) 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 LZ4 (compression algorithm)?

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 LZ4 (compression algorithm).