compute crc of file in python
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 usin开发者_C百科g zlib.crc32
?
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.
A modified version of kobor42's answer, with performance improved by a factor 2-3 by reading fixed size chunks instead of "lines":
import zlib
def crc32(fileName):
with open(fileName, 'rb') as fh:
hash = 0
while True:
s = fh.read(65536)
if not s:
break
hash = zlib.crc32(s, hash)
return "%08X" % (hash & 0xFFFFFFFF)
Also includes leading zeroes in the returned string.
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')
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.
Python 3.8+ (using the walrus operator):
import zlib
def crc32(filename, chunksize=65536):
"""Compute the CRC-32 checksum of the contents of the given filename"""
with open(filename, "rb") as f:
checksum = 0
while (chunk := f.read(chunksize)) :
checksum = zlib.crc32(chunk, checksum)
return checksum
chunksize
is how many bytes to read from the file at a time. You will get the same CRC for the same file no matter what you set chunksize
to (it has to be > 0
), but setting it too low might make your code slow, too high might use too much memory.
The result is a 32 bit integer. The CRC-32 checksum of an empty file is 0
.
Edited to include Altren's solution below.
A modified and more compact version of CrouZ's answer, with a slightly improved performance, using a for loop and file buffering:
def forLoopCrc(fpath):
"""With for loop and buffer."""
crc = 0
with open(fpath, 'rb', 65536) as ins:
for x in range(int((os.stat(fpath).st_size / 65536)) + 1):
crc = zlib.crc32(ins.read(65536), crc)
return '%08X' % (crc & 0xFFFFFFFF)
Results, in a 6700k, HDD:
(Note: Retested multiple times and it was consistently faster.)
Warming up the machine...
Finished.
Beginning tests...
File size: 90288KB
Test cycles: 500
With for loop and buffer.
Result 45.24728019630359
CrouZ solution
Result 45.433838356097894
kobor42 solution
Result 104.16215688703986
Altren solution
Result 101.7247863946586
Tested in Python 3.6.4 x64 using the script below:
import os, timeit, zlib, random, binascii
def forLoopCrc(fpath):
"""With for loop and buffer."""
crc = 0
with open(fpath, 'rb', 65536) as ins:
for x in range(int((os.stat(fpath).st_size / 65536)) + 1):
crc = zlib.crc32(ins.read(65536), crc)
return '%08X' % (crc & 0xFFFFFFFF)
def crc32(fileName):
"""CrouZ solution"""
with open(fileName, 'rb') as fh:
hash = 0
while True:
s = fh.read(65536)
if not s:
break
hash = zlib.crc32(s, hash)
return "%08X" % (hash & 0xFFFFFFFF)
def crc(fileName):
"""kobor42 solution"""
prev = 0
for eachLine in open(fileName,"rb"):
prev = zlib.crc32(eachLine, prev)
return "%X"%(prev & 0xFFFFFFFF)
def crc32altren(filename):
"""Altren solution"""
buf = open(filename,'rb').read()
hash = binascii.crc32(buf) & 0xFFFFFFFF
return "%08X" % hash
fpath = r'D:\test\test.dat'
tests = {forLoopCrc: 'With for loop and buffer.',
crc32: 'CrouZ solution', crc: 'kobor42 solution',
crc32altren: 'Altren solution'}
count = 500
# CPU, HDD warmup
randomItm = [x for x in tests.keys()]
random.shuffle(randomItm)
print('\nWarming up the machine...')
for c in range(count):
randomItm[0](fpath)
print('Finished.\n')
# Begin test
print('Beginning tests...\nFile size: %dKB\nTest cycles: %d\n' % (
os.stat(fpath).st_size/1024, count))
for x in tests:
print(tests[x])
start_time = timeit.default_timer()
for c in range(count):
x(fpath)
print('Result', timeit.default_timer() - start_time, '\n')
It is faster because for loops are faster than while loops (sources: here and here).
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()
There is faster and more compact way to compute CRC using binascii:
import binascii
def crc32(filename):
buf = open(filename,'rb').read()
hash = binascii.crc32(buf) & 0xFFFFFFFF
return "%08X" % hash
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)))
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="")...
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