[vorbis-dev] OGG header

[Erik Stenborg]

Thank you. I have no idea on whether the algorithm in framing.c is a
proper CRC algo (I hope it is), but now at least my code workes after
copying the table and algo straight off.

/Erik

On Thu, 29 Aug 2002, Keith Wright wrote:

> 
> Err.  Except I begin to doubt my own words.  The algorithm
> I described above would be
> 
> for(i=0;i<og->header_len;i++)
>   crc_reg=(crc_reg<<8)^crc_lookup[((crc_reg >> 24)&0xff)]^og->header[i];
> 
> whereas in framing.c we find
>   crc_reg=(crc_reg<<8)^crc_lookup[((crc_reg >> 24)&0xff)^og->header[i]];
> 
> (note change in position of left bracket of subscript.)
> 
> That is, if you have a 32-bit shift register shifting in one bit at
> a time into the low order, then the new bits shifted in do not
> affect the addition of the generator "polynomial" until they
> have been shifted through the intervening 24 bits, while the 
> Ogg algorithm makes the new data byte affect the XORed generator
> immediately.
> 
> I do not know if this difference is deliberate, or whether it
> affects the error checking properties of the algorithm.  You might
> just say the algorithm is what it is (the Popeye defense) and
> it doesn't matter so long as it is the same every time, but
> this might piss off somebody who is trying to do the checksum
> in hardware with a real shift register.
> 
> Or is it so late at night that I am hallucinating without drugs?
> 
> -- 
>      -- Keith Wright  <kwright at free-comp-shop.com>
> 
> Programmer in Chief, Free Computer Shop <http://www.free-comp-shop.com>
>          ---  Food, Shelter, Source code.  ---
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