Reverse order of bit in repeating seqence of byte string

MRAB google at mrabarnett.plus.com
Sat Jan 3 00:00:54 CET 2009


Francesco Bochicchio wrote:
> imageguy ha scritto:
>> I am looking for the most efficient method of replacing a repeating
>> sequence in a byte string returned from a imaging .dll, connected via
>>
>> I receive the byte string with the following sequence 'bgrbgrbgrbgr'
>> and I would like to convert this to 'rbgrbgrbgrbg'
>> FWIW, the string is created using ctypes.create_string_buffer function
>>
>> The following code works but feels a bit clunk and is rather slow too.
>>
>> blist = list(buffer)
>> for start in xrange(0,len(blist), 3):
>>    try:
>>         blue = blist[start]
>>         red = blist[start+2]
>>         blist[start] = red
>>         blist[start+2] = blue
>>    except IndexError:
>>        pass
>>
>> new_buffer = ''.join(blist)
>>
>> new_buffer is then passed to a wx program to create and image.
>>
>> Any thoughts comments would be appreciated.
>>
>> geoff.
>>
>> PS:  I started this post earlier, but I think I hit the send button
>> too soon.  My apologies to the group for sloppy typing.
>>
> You could try the same algorithm on an array.array object : it might be 
> faster.
> 
 >>> s = "012345678"
 >>> from array import array
 >>> a = array("b", s)
 >>> a
array('b', [48, 49, 50, 51, 52, 53, 54, 55, 56])
 >>> a[0::3], a[2::3] = a[2::3], a[0::3]
 >>> a
array('b', [50, 49, 48, 53, 52, 51, 56, 55, 54])
 >>> a.tostring()
'210543876'




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