20080905, 15:47  #1 
Mar 2007
Austria
456_{8} 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^{4}×733 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
7^{3}·23 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
17321_{8} 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
6731_{8} 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
456_{8} 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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