compute crc of file in python

Question

I want to calculate the CRC of file and get output like: E45A12AC. Here's my code:

#!/usr/bin/env python 
import os, sys
import zlib

def crc(fileName):
    fd = open(fileName,"rb")
    content = fd.readlines()
    fd.close()
    for eachLine in content:
        zlib.crc32(eachLine)

for eachFile in sys.argv[1:]:
    crc(eachFile)

This calculates the CRC for each line, but its output (e.g. -1767935985) is not what I want.

Hashlib works the way I want, but it computes the md5:

import hashlib
m = hashlib.md5()
for line in open('data.txt', 'rb'):
    m.update(line)
print m.hexdigest()

Is it possible to get something similar using zlib.crc32?

Answer 1

To show any integer's lowest 32 bits as 8 hexadecimal digits, without sign, you can "mask" the value by bit-and'ing it with a mask made of 32 bits all at value 1, then apply formatting. I.e.:

>>> x = -1767935985
>>> format(x & 0xFFFFFFFF, '08x')
'969f700f'

It's quite irrelevant whether the integer you are thus formatting comes from zlib.crc32 or any other computation whatsoever.

Answer 2

Merge the above 2 codes as below:

try:
    fd = open(decompressedFile,"rb")
except IOError:
    logging.error("Unable to open the file in readmode:" + decompressedFile)
    return 4
eachLine = fd.readline()
prev = 0
while eachLine:
    prev = zlib.crc32(eachLine, prev)
    eachLine = fd.readline()
fd.close()

Answer 3

You can use base64 for getting out like [ERD45FTR]. And zlib.crc32 provides update options.

import os, sys
import zlib
import base64


def crc(fileName):
  fd = open(fileName,"rb")
  content = fd.readlines()
  fd.close()
  prev = None
  for eachLine in content:
   if not prev:
     prev = zlib.crc32(eachLine)
   else:
     prev = zlib.crc32(eachLine, prev)
  return prev

for eachFile in sys.argv[1:]:
  print base64.b64encode(str(crc(eachFile)))

Answer 4

solution:

import os, sys
import zlib

def crc(fileName, excludeLine="", includeLine=""):
  try:
        fd = open(fileName,"rb")
  except IOError:
        print "Unable to open the file in readmode:", filename
        return
  eachLine = fd.readline()
  prev = None
  while eachLine:
      if excludeLine and eachLine.startswith(excludeLine):
            continue   
      if not prev:
        prev = zlib.crc32(eachLine)
      else:
        prev = zlib.crc32(eachLine, prev)
      eachLine = fd.readline()
  fd.close()    
  return format(prev & 0xFFFFFFFF, '08x') #returns 8 digits crc

for eachFile in sys.argv[1:]:
    print crc(eachFile)

don't realy know for what is (excludeLine="", includeLine="")...

Answer 5

hashlib-compatible interface for CRC-32 support:

import zlib

class crc32(object):
    name = 'crc32'
    digest_size = 4
    block_size = 1

    def __init__(self, arg=''):
        self.__digest = 0
        self.update(arg)

    def copy(self):
        copy = super(self.__class__, self).__new__(self.__class__)
        copy.__digest = self.__digest
        return copy

    def digest(self):
        return self.__digest

    def hexdigest(self):
        return '{:08x}'.format(self.__digest)

    def update(self, arg):
        self.__digest = zlib.crc32(arg, self.__digest) & 0xffffffff

# Now you can define hashlib.crc32 = crc32
import hashlib
hashlib.crc32 = crc32

# Python > 2.7: hashlib.algorithms += ('crc32',)
# Python > 3.2: hashlib.algorithms_available.add('crc32')

Answer 6

A little more compact and optimized code

def crc(fileName):
    prev = 0
    for eachLine in open(fileName,"rb"):
        prev = zlib.crc32(eachLine, prev)
    return "%X"%(prev & 0xFFFFFFFF)

PS2: Old PS is deprecated - therefore deleted -, because of the suggestion in the comment. Thank you. I don't get, how I missed this, but it was really good.