Bitpacked Data

[Nick Vatamaniuc]

On Mar 11, 5:49 pm, none <""luca\"@(none)"> wrote:
> i need to interface python with a bitpacked data file,
> the structure recorded in the file is the following:
>
> struct {
> var_1 4bit
> var_2 6bit
> var_3 2bit
> var_3 4bit
>
> }
>
> how can read the struct and convert data into dictionary
>
> Thnx

I noticed that you have a 4+6+2+4=16 bits total. That's a short
(unsigned) integer that you'd have to unpack using the stuct module's
unpack method.

The format string would be 'H'. Unpack using big endian. Here is a
quick example for you:
------------------------------
vatamane at nthink:~$ ipython
>>> from struct import pack,unpack
>>> m1='1'*4
>>> m2='1'*6
>>> m3='1'*2
>>> m4='1'*4
>>> m1,m2,m3,m4
('1111', '111111', '11', '1111')
>>> #define some data
>>> idata=int('1100101011111110',2)
>>> idata
51966
>>> #pack as a 16 bit integer (usigned int)
>>> packed=pack('>H',idata)
>>> #note > means big endian
>>> packed
'\xca\xfe'
>>> #oh, wow it spells cafe! ;-)
>>> #now unpack
>>> unpacked=unpack('>H',packed)
>>> unpacked
(51966,)
>>> odata=unpacked[0]
>>> vars=[]
>>> for mask in (m4,m3,m2,m1):
   ....:     vars.append(odata & int(mask,2) )
   ....:     odata>>=len(mask)
   ....:
>>> vars
[14, 3, 43, 12]
>>> vars.reverse()
>>> vars
[12, 43, 3, 14]

-------------------------------------------------
So in binary v1='1100' v2='101011' v3='11' v4='1110'
If you concatenate them: 1100101011111110 you get the original
idata...

Hope this helped,
Nick Vatamaniuc