zsync internals

[Phillip Susi]

Paul Sladen wrote:

>Whereas it *should* instead use the sizes from the Zmap to compress each
>block at a time.  This would cope with dpkg chunking stuff in 8kB and then
>the only guessing would be to get the compression level and search window
>size (these will be constant in 99.97% of cases).
>
>  
>

You can't recompress the uncompressed data blocks and get the same 
compressed stream unless you do so using the same options.  The zsync 
look inside algorithm relies on being able to reconstruct the compressed 
data stream so it doesn't have to download the parts that correspond to 
unchanged uncompressed blocks. 

If you don't get the same compressed stream when you recompress, you 
won't be able to correctly patch the file without downloading every 
compressed byte from the first change to the end, because they have all 
changed as a result of that one byte in the uncompressed stream that 
changed.