I have some bytearray with length of 2*n:
a1 a2 b1 b2 c1 c2
I need to switch bytes endian in each 2-byte word, and make:
a2 a1 b2 b1 c2 c1
Now I use next approach but it is very slow for my task:
converted = bytearray([])
for i in range(int(len(chunk)/2)):
converted += bytearray([ chunk[i*2+1], chunk[i*2] ])
Is it possible to switch endian of bytearray by calling some system/libc function?
Ok, thanks to all, I timed some suggestions:
import timeit
test = [
"""
converted = bytearray([])
for i in range(int(len(chunk)/2)):
converted += bytearray([ chunk[i*2+1], chunk[i*2] ])
""",
"""
for i in range(0, len(chunk), 2):
chunk[i], chunk[i+1] = chunk[i+1], chunk[i]
""",
"""
byteswapped = bytearray([0]) * len(chunk)
byteswapped[0::2] = chunk[1::2]
byteswapped[1::2] = chunk[0::2]
""",
"""
chunk[0::2], chunk[1::2] = chunk[1::2], chunk[0::2]
"""
]
for t in test:
print(timeit.timeit(t, setup='chunk = bytearray([1]*10)'))
and result is:
$ python ti.py
11.6219761372
2.61883187294
3.47194099426
1.66421198845
So in-pace slice assignment with a step of 2 now is fastest. Also thanks to Mr. F for detailed explaining but I not yet tried it because of numpy
You could use slice assignment with a step of 2:
byteswapped = bytearray(len(original))
byteswapped[0::2] = original[1::2]
byteswapped[1::2] = original[0::2]
Or if you want to do it in-place:
original[0::2], original[1::2] = original[1::2], original[0::2]
Timing shows that slicing massively outperforms a Python-level loop for large arrays:
>>> timeit.timeit('''
... for i in range(0, len(chunk), 2):
... chunk[i], chunk[i+1] = chunk[i+1], chunk[i]''',
... 'chunk=bytearray(1000)')
81.70195105159564
>>>
>>> timeit.timeit('''
... byteswapped = bytearray(len(original))
... byteswapped[0::2] = original[1::2]
... byteswapped[1::2] = original[0::2]''',
... 'original=bytearray(1000)')
2.1136113323948393
>>>
>>> timeit.timeit('chunk[0::2], chunk[1::2] = chunk[1::2], chunk[0::2]', 'chunk=
bytearray(1000)')
1.79349659994989
For small arrays, slicing still beats the explicit loop, but the difference isn't as big:
>>> timeit.timeit('''
... for i in range(0, len(chunk), 2):
... chunk[i], chunk[i+1] = chunk[i+1], chunk[i]''',
... 'chunk=bytearray(10)')
1.2503637694328518
>>>
>>> timeit.timeit('''
... byteswapped = bytearray(len(original))
... byteswapped[0::2] = original[1::2]
... byteswapped[1::2] = original[0::2]''',
... 'original=bytearray(10)')
0.8973060929306484
>>>
>>> timeit.timeit('chunk[0::2], chunk[1::2] = chunk[1::2], chunk[0::2]', 'chunk=
bytearray(10)')
0.6282232971918802