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-LZ4 Block Format Description
-============================
-Last revised: 2015-05-07.
-Author : Yann Collet
-
-
-This specification is intended for developers
-willing to produce LZ4-compatible compressed data blocks
-using any programming language.
-
-LZ4 is an LZ77-type compressor with a fixed, byte-oriented encoding.
-There is no entropy encoder back-end nor framing layer.
-The latter is assumed to be handled by other parts of the system (see [LZ4 Frame format]).
-This design is assumed to favor simplicity and speed.
-It helps later on for optimizations, compactness, and features.
-
-This document describes only the block format,
-not how the compressor nor decompressor actually work.
-The correctness of the decompressor should not depend
-on implementation details of the compressor, and vice versa.
-
-[LZ4 Frame format]: lz4_Frame_format.md
-
-
-
-Compressed block format
------------------------
-An LZ4 compressed block is composed of sequences.
-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.
-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.
-
-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 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".
-(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
-  - 33 : (=48-15) remaining length to reach 48
-
-Example 2 : A length of 280 will be represented as :
-
-  - 15  : value for the 4-bits High field
-  - 255 : following byte is maxed, since 280-15 >= 255
-  - 10  : (=280 - 15 - 255) ) remaining length to reach 280
-
-Example 3 : A length of 15 will be represented as :
-
-  - 15 : value for the 4-bits High field
-  - 0  : (=15-15) yes, the zero must be output
-
-Following the token and optional length bytes, are the literals themselves.
-They are exactly as numerous as previously decoded (length of literals).
-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
-(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".
-The maximum offset value is 65535, 65536 cannot be coded.
-Note that 0 is an invalid value, not used.
-
-Then we need to extract the match length.
-For this, we use the second token field, the low 4-bits.
-Value, obviously, ranges from 0 to 15.
-However here, 0 means that the copy operation will be minimal.
-The minimum length of a match, called minmatch, is 4.
-As a consequence, a 0 value means 4 bytes, and a value of 15 means 19+ bytes.
-Similar to literal length, on reaching the highest possible value (15),
-we output additional bytes, one at a time, with values ranging from 0 to 255.
-They are added to total to provide the final match length.
-A 255 value means there is another byte to read and add.
-There is no limit to the number of optional bytes that can be output this way.
-(This points towards a maximum achievable compression ratio of about 250).
-
-With the offset and the matchlength,
-the decoder can now proceed to copy the data from the already decoded buffer.
-On decoding the matchlength, we reach the end of the compressed sequence,
-and therefore start another one.
-
-
-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.
-
-These rules are in place to ensure that the decoder
-will never read beyond the input buffer, nor write beyond the output buffer.
-
-Note that the last sequence is also incomplete,
-and stops right after literals.
-
-
-Additional notes
------------------------
-There is no assumption nor limits to the way the compressor
-searches and selects matches within the source data block.
-It could be a fast scan, a multi-probe, a full search using BST,
-standard hash chains or MMC, well whatever.
-
-Advanced parsing strategies can also be implemented, such as lazy match,
-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.