Question: Python - "xor"ing each byte in "bytes" in the most efficient way


Python - "xor"ing each byte in "bytes" in the most efficient way

Answers 2
Added at 2017-10-03 10:10

I have a "bytes" object and an "int" mask, I want to do a xor over all the bytes with my mask. I do this action repeatedly over big "bytes" objects (~ 4096 KB).

This is the code I have which does the work well, only it is very CPU intensive and slows down my script:

# 'data' is bytes and 'mask' is int
bmask = struct.pack('!I', mask) # converting the "int" mask to "bytes" of 4 bytes 
a = bytes(b ^ m for b, m in zip(data, itertools.cycle(bmask)))

The best I could come up with is this, which is about 20 times faster:

# 'data' is bytes and 'mask' is int
# reversing the bytes of the mask
bmask = struct.pack("<I", mask)
mask = struct.unpack(">I", bmask)[0]

# converting from bytes to array of "int"s
arr = array.array("I", data)

# looping over the "int"s
for i in range(len(arr)):
    arr[i] ^= mask

# must return bytes
a = bytes(arr)

My questions are:

  1. Is there a more efficient way to do this (CPU-wize)?
  2. Is there a "cleaner" way to do this (without hurting performance)?

P.S. if it is of any importance, I'm using Python 3.5

Answers to

Python - &quot;xor&quot;ing each byte in &quot;bytes&quot; in the most efficient way

nr: #1 dodano: 2017-10-03 11:10

I don't think you can get much faster than your algorithm, using pure Python. (But Fabio Veronese's answer shows that's not true). You can shave off a tiny bit of time by doing the looping in a list comprehension, but then that list needs to be converted back into an array, and the array has to be converted to bytes, so it uses more RAM for a negligible benefit.

However, you can make this much faster by using Numpy. Here's a short demo.

from time import perf_counter
from random import randrange, seed
import array
import numpy as np


def timed(func):
    ''' Timing decorator '''
    def wrapped(*args):
        start = perf_counter()
        result = func(*args)
        stop = perf_counter()
        print('{}: {:.6f} seconds'.format(func.__name__, stop - start))
        return result
    wrapped.__name__ = func.__name__
    wrapped.__doc__ = func.__doc__
    return wrapped

def do_mask_arr1(data, mask):
    arr = array.array("I", data)
    # looping over the "int"s
    for i in range(len(arr)):
        arr[i] ^= mask
    return arr.tobytes()

def do_mask_arr2(data, mask):
    arr = array.array("I", data)
    return array.array("I", [u ^ mask for u in arr]).tobytes()

def do_mask_numpy(data, mask):
    return (np.fromstring(data, dtype=np.uint32) ^ mask).tobytes()

def make_data(datasize):
    ''' Make some random bytes '''
    return bytes(randrange(256) for _ in range(datasize))

datasize = 100000
mask = 0x12345678
data = make_data(datasize)

d1 = do_mask_arr1(data, mask)
d2 = do_mask_arr2(data, mask)
print(d1 == d2)

d3 = do_mask_numpy(data, mask)
print(d1 == d3)

typical output

make_data: 0.751557 seconds
do_mask_arr1: 0.026865 seconds
do_mask_arr2: 0.025110 seconds
do_mask_numpy: 0.000438 seconds

Tested using Python 3.6.0 on an old single core 32 bit 2GHz machine running on Linux.

I just did a run with datasize = 4000000 and do_mask_numpy took 0.0422 seconds.

nr: #2 dodano: 2017-10-03 13:10

An alternative in case you don't want to use numpy. The advantage comes from making a single comparison, while extending the mask size to the needed (depending on the datasize).

def do_mask_int(data, mask):
    intdata = int.from_bytes(data, byteorder='little', signed=False)
    strmask = format(mask,'0x')
    strmask = strmask * ((intdata.bit_length() + 31) // 32)
    n = intdata ^ int(strmask, 16)
    return n.to_bytes(((n.bit_length() + 7) // 8), 'little') or b'\0'

results are as it follows:

make_data: 8.288754 seconds
do_mask_arr1: 0.258530 seconds
do_mask_arr2: 0.253095 seconds
do_mask_numpy: 0.010309 seconds
do_mask_int: 0.060408 seconds

Still credits to numpy for being faster, but maybe one doesn't want to include it in production environment.

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