Efficient Bit addressing in Python.

Ross Ridge rridge at csclub.uwaterloo.ca
Fri Oct 10 11:57:46 CEST 2008

Hendrik van Rooyen <mail at microcorp.co.za> wrote:
>Is there a canonical way to address the bits in a structure
>like an array or string or struct?
>Or alternatively, is there a good way to combine eight
>ints that represent bits into one of the bytes in some
>array or string or whatever?

This is the code I use to convert large bit arrays to byte strings and

import string
import binascii
import array

_tr_16 = string.maketrans("0123456789abcdef",
_tr_4 = string.maketrans("0123",
_tr_2 = string.maketrans("01", "\x00\x01")

def string_to_bit_array(s):
	"""Convert a string to an array containing a sequence of bits."""
	s = binascii.hexlify(s).translate(_tr_16)
	s = binascii.hexlify(s).translate(_tr_4)
	s = binascii.hexlify(s).translate(_tr_2)
	a = array.array('B', s)
	return a

_tr_rev_2 = string.maketrans("\x00\x01", "01")
_tr_rev_4 = string.maketrans("\x00\x01"
_tr_rev_16 = string.maketrans("\x00\x01\x02\x03"
def bit_array_to_string(a):
	"""Convert an array containing a sequence of bits to a string."""
	remainder = len(a) % 8
	if remainder != 0:
		a.fromlist([0] * (8 - remainder))
	s = a.tostring()
	s = binascii.unhexlify(s.translate(_tr_rev_2))
	s = binascii.unhexlify(s.translate(_tr_rev_4))	
	return binascii.unhexlify(s.translate(_tr_rev_16))

I don't think you can do anything faster with standard modules, although
it might not be effecient if you're only working with a single byte.

				Ross Ridge

 l/  //	  Ross Ridge -- The Great HTMU
[oo][oo]  rridge at csclub.uwaterloo.ca
-()-/()/  http://www.csclub.uwaterloo.ca/~rridge/ 
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