125 lines
4.8 KiB
Markdown
125 lines
4.8 KiB
Markdown
LZ4 Block Format Description
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============================
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Last revised: 2015-03-26.
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Author : Yann Collet
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This small specification intents to provide enough information
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to anyone willing to produce LZ4-compatible compressed data blocks
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using any programming language.
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LZ4 is an LZ77-type compressor with a fixed, byte-oriented encoding.
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The most important design principle behind LZ4 is simplicity.
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It helps to create an easy to read and maintain source code.
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It also helps later on for optimizations, compactness, and speed.
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There is no entropy encoder back-end nor framing layer.
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The latter is assumed to be handled by other parts of the system.
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This document only describes the block format,
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not how the LZ4 compressor nor decompressor actually work.
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The correctness of the decompressor should not depend
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on implementation details of the compressor, and vice versa.
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Compressed block format
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-----------------------
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An LZ4 compressed block is composed of sequences.
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A sequence is a suite of literals (not-compressed bytes),
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followed by a match copy.
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Each sequence starts with a token.
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The token is a one byte value, separated into two 4-bits fields.
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Therefore each field ranges from 0 to 15.
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The first field uses the 4 high-bits of the token.
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It provides the length of literals to follow.
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If the field value is 0, then there is no literal.
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If it is 15, then we need to add some more bytes to indicate the full length.
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Each additional byte then represent a value from 0 to 255,
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which is added to the previous value to produce a total length.
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When the byte value is 255, another byte is output.
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There can be any number of bytes following the token. There is no "size limit".
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(Side note : this is why a not-compressible input block is expanded by 0.4%).
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Example 1 : A length of 48 will be represented as :
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- 15 : value for the 4-bits High field
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- 33 : (=48-15) remaining length to reach 48
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Example 2 : A length of 280 will be represented as :
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- 15 : value for the 4-bits High field
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- 255 : following byte is maxed, since 280-15 >= 255
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- 10 : (=280 - 15 - 255) ) remaining length to reach 280
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Example 3 : A length of 15 will be represented as :
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- 15 : value for the 4-bits High field
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- 0 : (=15-15) yes, the zero must be output
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Following the token and optional length bytes, are the literals themselves.
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They are exactly as numerous as previously decoded (length of literals).
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It's possible that there are zero literal.
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Following the literals is the match copy operation.
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It starts by the offset.
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This is a 2 bytes value, in little endian format
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(the 1st byte is the "low" byte, the 2nd one is the "high" byte).
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The offset represents the position of the match to be copied from.
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1 means "current position - 1 byte".
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The maximum offset value is 65535, 65536 cannot be coded.
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Note that 0 is an invalid value, not used.
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Then we need to extract the match length.
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For this, we use the second token field, the low 4-bits.
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Value, obviously, ranges from 0 to 15.
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However here, 0 means that the copy operation will be minimal.
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The minimum length of a match, called minmatch, is 4.
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As a consequence, a 0 value means 4 bytes, and a value of 15 means 19+ bytes.
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Similar to literal length, on reaching the highest possible value (15),
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we output additional bytes, one at a time, with values ranging from 0 to 255.
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They are added to total to provide the final match length.
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A 255 value means there is another byte to read and add.
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There is no limit to the number of optional bytes that can be output this way.
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(This points towards a maximum achievable compression ratio of ~250).
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With the offset and the matchlength,
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the decoder can now proceed to copy the data from the already decoded buffer.
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On decoding the matchlength, we reach the end of the compressed sequence,
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and therefore start another one.
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Parsing restrictions
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-----------------------
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There are specific parsing rules to respect in order to remain compatible
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with assumptions made by the decoder :
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1. The last 5 bytes are always literals
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2. The last match must start at least 12 bytes before end of block.
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Consequently, a block with less than 13 bytes cannot be compressed.
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These rules are in place to ensure that the decoder
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will never read beyond the input buffer, nor write beyond the output buffer.
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Note that the last sequence is also incomplete,
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and stops right after literals.
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Additional notes
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-----------------------
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There is no assumption nor limits to the way the compressor
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searches and selects matches within the source data block.
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It could be a fast scan, a multi-probe, a full search using BST,
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standard hash chains or MMC, well whatever.
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Advanced parsing strategies can also be implemented, such as lazy match,
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or full optimal parsing.
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All these trade-off offer distinctive speed/memory/compression advantages.
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Whatever the method used by the compressor, its result will be decodable
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by any LZ4 decoder if it follows the format specification described above.
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