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Re: [Discuss-gnuradio] digital_correlate_access_code_bb

From: Nowlan, Sean
Subject: Re: [Discuss-gnuradio] digital_correlate_access_code_bb
Date: Sun, 15 Apr 2012 22:30:49 +0000

Sorry to bump this... It appears the set_access_code method only occurs once  
from within the constructor code. I'm not arguing that the old method isn't 
faster, but it's functionally imprecise and the overhead of the if-else 
statement isn't huge over the life of the object instance. If a time-variable 
access code scheme were implemented, I could see it adding up, though. But 
set_access_code isn't even SWIGged up as a public method, so I assume there 
hasn't been demand for such a use case...


-----Original Message-----
From: address@hidden [mailto:address@hidden On Behalf Of Tom Rondeau
Sent: Monday, April 09, 2012 5:23 PM
To: Nick Foster
Cc: address@hidden
Subject: Re: [Discuss-gnuradio] digital_correlate_access_code_bb

On Mon, Apr 9, 2012 at 2:02 PM, Nick Foster <address@hidden> wrote:
> On Mon, Apr 9, 2012 at 10:48 AM, Marcus D. Leech <address@hidden> wrote:
>> On 04/09/2012 01:38 PM, Tom Rondeau wrote:
>>> On Sat, Apr 7, 2012 at 10:12 PM, Marcus D. Leech<address@hidden>
>>>  wrote:
>>>> Just looking at this function:
>>>> correlate_access_code_bb
>>>> In the method set_access_code, it takes a string.  Which should be 
>>>> ASCII '1'
>>>> and '0' characters to represent the binary sequence being
>>>>  correlated against.
>>>> Here's a little beauty of a code snippet:
>>>>  d_access_code = 0;
>>>>  for (unsigned i=0; i<  64; i++){
>>>>    d_access_code<<= 1;
>>>>    if (i<  len)
>>>>      d_access_code |= access_code[i]&  1;    // look at LSB only
>>>>  }
>>>> This relies on the fact that ASCII '1' and '0' happen to have 
>>>> low-order bits of the right "flavour".  This is insanely dirty and 
>>>> gross and I can't
>>>>  believe we let this nonsense in the code base.
>>>> There's no reason not to do the right thing here.
>>>> --
>>>> Marcus Leech
>>>> Principal Investigator
>>>> Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org
>>> Want to submit a patch?
>>> Tom
>> Attached.
> While you're patching correlate_access_code_bb, please patch 
> correlate_access_code_tag_bb with the attached patch.
> --n

So my guess is that the use of the binary & operator is to avoid the need for 
an if/if else/else branching check. It was most likely done for efficiency. So 
while this patch might be the "right" way to do it from a code perspective, it 
could result in slower code (on certain machines that don't handle branch 
prediction well). It does make assumptions about the correctness of the access 
code, though.


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