Ximin Luo ?????:
> Florent Daigniere wrote:
>> Ximin Luo wrote:
>>> Florent Daigniere wrote:
>>>> Anyway, how do you determine if a file is already compressed or not without
>>>> actually compressing it? Did you do the maths?
>>> An heuristic that should get this right most of the time is just count how
>>> many
>>> times each byte value (0x00, 0x01, 0x02) etc appears in the first few
>>> megabytes. If the distribution is not even then theoretically the data can
>>> be
>>> compressed further.
>>>
>>> X
>> Well, that would typically fail with all the dictionary-based
>> compression algorithms I'm familiar with... As the dictionary can be
>> compressed further using another compression algorithm, possibly not
>> dictionary based.
>
> How big could the dictionary be? We could do it for the last few megabytes
> then, or say, a random interval in the last 3/4 of the file? Or take random
> samples from the file, but that would take more time.
Problem with anything random is that the behaviour would then not be based on
the file. If you insert a file and reinsert it again, you expect to get the
same
blocks (and hopefully the same key all together).
> Also, I should think video/audio compression is somewhat more sophisticated
> than dictionary? JPEG uses discrete cosine transforms...
>
> X
- Volodya
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