Andreas Lobinger wrote:
Aloha,
can anyone here recommend a implementation for bitstreams?
For a project i need to extract bitfields from a file containing
one large bitstream (lenght >> 1000bit). The bitfields (1bit-24bit)
are not aligned to char boundaries (8bit).
Something like
b1 = bitstream.fromfile('m1.bin')
v1 = b1.getbits(startpos=3,length=10) # v ist a reg. int
v2 = b1.getnext(8) # bits 0-7
v3 = b1.getnext(10) # bits 8-17
Hoping for an answer and wishing a happy day
LOBI
You might start with something like the following as a primitive,
tweak it to deal correctly with your byte sex (big-endian vs. little
endian), bit numbering, and such. Then when that works, add a
current position and auto-advance stuff. Then you can decide if you
want to go in bigger "bytes" by something like 'L' instead of 'B'
below. Note that 1000 bits is not so very big that reading the file
into memory is a bad idea. In fact, I'd only think about that above
about a million bits.
import array
class Bitvector(object):
def __init__(self, filename):
datafile = file(filename, 'rb')
self.data = array.array('B', datafile.read())
datafile.close()
self.elbits = self.data.itemsize * 8 # 8 bits/byte
def grab(self, start, length):
word, part = divmod(start, self.elbits)
sofar = self.elbits - part
result = self.data[word] & ((1L << sofar) - 1)
# 1L above so no sign bit hassles below
while sofar < length:
word += 1
result = (result << self.elbits) | self.data[word]
sofar += self.elbits
return result >> sofar - length
--
-Scott David Daniels
Sc***********@Acm.Org 
