[FFmpeg-devel] [PATCH] AAC fatetests

[Måns Rullgård]

Michael Niedermayer <michaelni at gmx.at> writes:

> On Sat, Jul 10, 2010 at 10:44:45PM +0200, Michael Niedermayer wrote:
>> On Sat, Jul 10, 2010 at 09:39:29PM +0100, M?ns Rullg?rd wrote:
>> > Michael Niedermayer <michaelni at gmx.at> writes:
>> > 
>> > > On Sat, Jul 10, 2010 at 11:49:41AM +0200, Reimar D?ffinger wrote:
>> > >> On Sat, Jul 10, 2010 at 10:24:26AM +0100, M?ns Rullg?rd wrote:
>> [...]
>> > >> This would also allow testing of non-seekable input without needing
>> > >> http or piping from stdin...
>> > >
>> > > one cant seek in bz2 ?
>> > > did gnu butcher the _block_ based bw transform that much?
>> > 
>> > There is likely no way to find the block boundaries other than by
>> > decoding the entire thing.
>> 
>> if so that would be a very lame design, because the basic algorithm doesnt
>> use the previous blocks being able to recover in case of damage would
>> have been a nice feature
>
> tfm says:
> RECOVERING DATA FROM DAMAGED FILES
>    bzip2 compresses files in blocks, usually 900kbytes long.   Each  block
>    is  handled  independently.   If a media or transmission error causes a
>    multi-block .bz2 file to become damaged, it may be possible to  recover
>    data from the undamaged blocks in the file.
>
>    The  compressed  representation  of each block is delimited by a 48-bit
>    pattern, which makes it possible to find the block boundaries with rea-
>    sonable certainty.  Each block also carries its own 32-bit CRC, so dam-
>    aged blocks can be distinguished from undamaged ones.
>
>    bzip2recover is a simple program whose purpose is to search for  blocks
>    in  .bz2  files,  and write each block out into its own .bz2 file.  You
>    can then use bzip2 -t to test the integrity of the resulting files, and
>    decompress those which are undamaged.
>
> so my assumtation that one should be able to seek in bz2 is not all
> that wrong

Yes, it seems that it should be possible, albeit with some overhead
scanning for startcodes and with only 900k granularity.

-- 
M?ns Rullg?rd
mans at mansr.com