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Re^2: Unpacking variable length bit fields

by puterboy (Scribe)
on Jun 19, 2012 at 04:15 UTC ( #976953=note: print w/ replies, xml ) Need Help??


in reply to Re: Unpacking variable length bit fields
in thread Unpacking variable length bit fields

Oops, I spoke to soon. The field length specifies the number of bits (multiple of 8) but I want to extract the fields themselves as bytes not as bit strings of 1/0's.

My problems is that using something like: unpack('C/C'...) would unpack C bytes for each filed rather than C bits for each field which is what I want. So I really would like to divide by 8 but I'm not sure how to do that. I imagine I could unpack as a bit string of 1/0's then split, then repack but that seems klugey.

It's a screwy format, but I don't have control over it....


Comment on Re^2: Unpacking variable length bit fields
Re^3: Unpacking variable length bit fields
by BrowserUk (Pope) on Jun 19, 2012 at 05:30 UTC

    The the easiest way is to convert the ascii-ised binary back to real numbers:

    $bitstream = "\x18\xf0\xff\x0f\x10\xaa\x55\x20\xde\xad\xbe\xef";; ( $f1, $f2, $f3 ) = map unpack( 'L', pack( 'b32', $_ )), unpack 'C/b C +/b C/b', $bitstream;; print for $f1, $f2, $f3;; 1048560 21930 4022250974

    This assumes maximum 32-bit values. You could use Q if you are on a 64-bit platform and Perl and need that.

    By converting them all to the maximum sized integer the platform can handle you save having to make decisions and they'd end up that way anyway.

    You might need to adjust the L (or the 'b32' to 'B32' or 'B64') for the endianness of the originating platform.


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