20080905, 15:47  #1 
Mar 2007
Austria
2×151 Posts 
Questions on coding my FFT&LLR implementation
I know that there is a fftthread in this forum, but it's still to hard for me. I'm currently coding an implementation of the LLR(Lucaslehmerriesel) test.It's currently using the GMP and I want to implement FFT for it. Can you please explain me the way from the x^2 calculation(I think that's the point where FFT helps) to the FFT. Also,if possible,can you show me an example on a small mersenne number(LL test)? This might help me a lot in understanding.
Thanks, nuggetprime 
20080905, 18:02  #2 
∂^{2}ω=0
Sep 2002
República de California
2^{3}·3·487 Posts 
Search for the thread titled "FFT explanation for non math guys".
Last fiddled with by ewmayer on 20080905 at 18:02 
20080905, 23:29  #3 
"Richard B. Woods"
Aug 2002
Wisconsin USA
2^{2}×3×641 Posts 
... in the Math subforum. Start at its post #5.

20080905, 23:58  #4 
P90 years forever!
Aug 2002
Yeehaw, FL
2×3×5×7×37 Posts 

20080906, 10:03  #5 
Mar 2007
Austria
2·151 Posts 
Thanks,I'm now understanding DFT in basics.
How does then the IBDWT work(different)? There aren't many resources on the web to this. Thanks, nugget 
20080906, 13:29  #6 
P90 years forever!
Aug 2002
Yeehaw, FL
1E5A_{16} Posts 

20080907, 14:08  #7 
Mar 2007
Austria
2·151 Posts 
Question: What's the best way to store the transformed numbers? floating point,or?
Thanks, nuggetprime 
20080908, 17:41  #8 
Mar 2007
Austria
2·151 Posts 
Otherwise formulated:is double enough for,say, FFT length 500K?
Thanks, nuggetprime 
20080909, 14:01  #9 
Tribal Bullet
Oct 2004
3·1,181 Posts 
Yes, double precision should be plenty. It may be just insufficient if you are not careful with how you generate the trig factors for the FFT; references often use a recursion for them which causes increased roundoff error for very long transform lengths.

20080910, 15:19  #10 
Mar 2007
Austria
2×151 Posts 
Thank you for your answer jasonp.
But how can I calculate in binary in C? I probably will have to create arrays of 1bit fields,or? Thanks, nugget 
20080910, 17:32  #11 
Mar 2007
Austria
2·151 Posts 
The above was not stated precisely, I think.
I want to know if in C, when i define a ,say, unsigned short int, how can I access a certain bit of it,in reasonable CPU time? Thanks, nuggetprime Last fiddled with by nuggetprime on 20080910 at 17:33 
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