diff options
Diffstat (limited to 'lz4_Block_format.md')
-rw-r--r-- | lz4_Block_format.md | 25 |
1 files changed, 14 insertions, 11 deletions
diff --git a/lz4_Block_format.md b/lz4_Block_format.md index e248fd9..b933a6a 100644 --- a/lz4_Block_format.md +++ b/lz4_Block_format.md @@ -1,10 +1,9 @@ LZ4 Block Format Description ============================ -Last revised: 2015-03-26; +Last revised: 2015-03-26. Author : Yann Collet - This small specification intents to provide enough information to anyone willing to produce LZ4-compatible compressed data blocks using any programming language. @@ -26,7 +25,8 @@ on implementation details of the compressor, and vice versa. Compressed block format ----------------------- An LZ4 compressed block is composed of sequences. -Schematically, a sequence is a suite of literals, followed by a match copy. +A sequence is a suite of literals (not-compressed bytes), +followed by a match copy. Each sequence starts with a token. The token is a one byte value, separated into two 4-bits fields. @@ -35,14 +35,14 @@ Therefore each field ranges from 0 to 15. The first field uses the 4 high-bits of the token. It provides the length of literals to follow. -(Note : a literal is a not-compressed byte). + If the field value is 0, then there is no literal. If it is 15, then we need to add some more bytes to indicate the full length. -Each additionnal byte then represent a value from 0 to 255, +Each additional byte then represent a value from 0 to 255, which is added to the previous value to produce a total length. When the byte value is 255, another byte is output. There can be any number of bytes following the token. There is no "size limit". -(Sidenote this is why a not-compressible input block is expanded by 0.4%). +(Side note : this is why a not-compressible input block is expanded by 0.4%). Example 1 : A length of 48 will be represented as : - 15 : value for the 4-bits High field @@ -65,7 +65,8 @@ It's possible that there are zero literal. Following the literals is the match copy operation. It starts by the offset. -This is a 2 bytes value, in little endian format. +This is a 2 bytes value, in little endian format +(the 1st byte is the "low" byte, the 2nd one is the "high" byte). The offset represents the position of the match to be copied from. 1 means "current position - 1 byte". @@ -95,9 +96,12 @@ Parsing restrictions ----------------------- There are specific parsing rules to respect in order to remain compatible with assumptions made by the decoder : -1) The last 5 bytes are always literals -2) The last match must start at least 12 bytes before end of block -Consequently, a block with less than 13 bytes cannot be compressed. + +1. The last 5 bytes are always literals +2. The last match must start at least 12 bytes before end of block. + + Consequently, a block with less than 13 bytes cannot be compressed. + These rules are in place to ensure that the decoder will never read beyond the input buffer, nor write beyond the output buffer. @@ -118,4 +122,3 @@ or full optimal parsing. All these trade-off offer distinctive speed/memory/compression advantages. Whatever the method used by the compressor, its result will be decodable by any LZ4 decoder if it follows the format specification described above. - |