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Re: Whither delta combiner

From: Branko Čibej <brane_at_xbc.nu>
Date: 2002-02-09 22:09:19 CET

Daniel Berlin wrote:

>On Fri, 8 Feb 2002, Daniel Berlin wrote:
>>One thing we should also be doing is fulltext compression. Rather than
>>give up, or store full fulltexts, we could at least delta them against an
>>empty string.
>>Since vdelta is based off lempel-ziv, it'll give you some compression
>>(better than compress, worse than gzip, according to the paper).
>>Right now, we just leave it completely fulltext.
>Strike that.
>I forgot the compression comes mainly from their encoding/folding, rather
>than anything else.
>Since we do neither, ...
Not at all. Vdelta itself compresses. That has nothing to do with the
encoding of the block-copy instructions.

For instance, even thouth svndiff is a simple, non-compressing encoding,
gzip -9 only manages an additional 15% compression over what we get with
vdelta. The reason vdelta doesn't compress better is that it saves time
by not searching for the optimal sequence of copies, but uses an
heuristic to choose a good average. So, instead of taking quadratic
time, it's linear.

Brane Čibej   <brane_at_xbc.nu>   http://www.xbc.nu/brane/
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Received on Sat Oct 21 14:37:06 2006

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