Re: Block-level CRC checks

On Mon, Nov 10, 2008 at 11:31:33PM +0000, Gregory Stark wrote:
> No, I just meant that you could calculate the CRC by scanning the whole buffer
> efficiently using one of the good word-wise CRC algorithms, then look at the
> line pointers to find the hint bits and subtract them out of the CRC. The
> result should be zero after adjusting for the hint bits.

If you're going to look at the line pointers anyway, couldn't you just
do it in one pass, like:

n = 0
next = &tuple[n].hintbits
pos = 0
while pos < BLOCK_SIZE:
if pos == next:
CRC_ADD( block[pos] & mask )
n++
next = &tuple[n].hintbits # If n == numtups, next = BLOCK_SIZE
else:
CRC_ADD( block[pos]
pos++

This only handles one byte of hintbits but can easily be extended. No
need to actually *store* the hintbit free version anywhere...

Have a nice day,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> Please line up in a tree and maintain the heap invariant while
> boarding. Thank you for flying nlogn airlines.