Bitpacked Data

[none]

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

[Bjoern Schliessmann]

Re: Bitpacked Data

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
}

Strange data types. What language is this?

how can read the struct and convert data into dictionary

I'd use the module struct, with bit shifting operators to extract
the bits.


Regards,


Björn

--
BOFH excuse #232:

Ionization from the air-conditioning

[bearophileHUGS@lycos.com]

Re: Bitpacked Data

Bjoern Schliessmann:

I'd use the module struct, with bit shifting operators to extract
the bits.

Maybe Pyrex can be used to create a small module that can manage
similar bitfields quickly and with a syntax not too much far from the
Erlang bit syntax.

Bye,
bearophile

[Nick Vatamaniuc]

Re: Bitpacked Data

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@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('11 00101011111110' ,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=unpa ck('>H',packed)
>>unpacked

(51966,)

>>odata=unpacke d[0]
>>vars=[]
>>for mask in (m4,m3,m2,m1):

....: vars.append(oda ta & int(mask,2) )
....: odata>>=len(mas k)
....:

>>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: 110010101111111 0 you get the original
idata...

Hope this helped,
Nick Vatamaniuc

[Nick Craig-Wood]

Re: Bitpacked Data

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

You could try Construct

http://construct.wikispaces.com/

This allows you to build a python class which will translate to and
from that datastructure.

--
Nick Craig-Wood <nick@craig-wood.com-- http://www.craig-wood.com/nick