Efficient Bit addressing in Python.

[Ross Ridge]

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
back:

import string
import binascii
import array

_tr_16 = string.maketrans("0123456789abcdef",
			  "\x00\x01\x02\x03"
			  "\x10\x11\x12\x13"
			  "\x20\x21\x22\x23"
			  "\x30\x31\x32\x33")
_tr_4 = string.maketrans("0123",
			 "\x00\x01"
			 "\x10\x11")
_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"
			     "\x10\x11",
			     "0123")
_tr_rev_16 = string.maketrans("\x00\x01\x02\x03"
			      "\x10\x11\x12\x13"
			      "\x20\x21\x22\x23"
			      "\x30\x31\x32\x33",
			      "0123456789abcdef")
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/ 
 db  //