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added a paragraph on overlap matches

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Yann Collet 2017-06-06 11:20:36 -07:00
parent 03d8586fca
commit 7e15e240ab
1 changed files with 12 additions and 4 deletions
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doc/lz4_Block_format.md
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@ -90,10 +90,18 @@ 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.
Decoding the matchlength reaches the end of current sequence.
Next byte will be the start of another sequence.
But before moving to next sequence,
it's time to use the decoded match position and length.
The decoder copies matchlength bytes from match position to current position.
In some cases, matchlength is larger than offset.
Therefore, match pos + match length > current pos,
which means that later bytes to copy are not yet decoded.
This is called an "overlap match", and must be handled with special care.
The most common case is an offset of 1,
meaning the last byte is repeated matchlength times.
Parsing restrictions