ok I found why he using non modified(or not so much) fftw

FFT using FFTW I am calling on a key -tm10. Advantages are:

1) a prime number can be anything, not necessarily k.2 ^ n + 1, in my case FFFFFF79

2) the number of points can be anything, not necessarily the 2 ^ n, I select nearest from the top to the N, which is divisible by 2,3,5,7 - but this is a purely arbitrary

3) FFTW is certainly using complex numbers, but there is a transformation to form result of purely real number form. Then from the result residue is taken by mod ffffff79, and that's all. This (mod calc) could be done at the very end.

4) in order to achieve this, you need a good understanding of the issues fft, try ...

http://www.wasm.ru/forum/viewtopic.php?pid=514154#p514154

(forum is offline now I get it though google cache

http://webcache.googleusercontent.com/search?q=cache:wf1yuBMhF_QJ:www.wasm.ru/forum/viewtopic.php%3Fpid%3D514887+&cd=1&hl=ru&ct=clnk&gl=ru&lr=lang_en%7Clang_ru

, for simplicity http://wasm.ru/forum/viewtopic.php?id=29639,

but citation is from page 14 which is not in archive)

it can be seen as developer diary, so if someone wants to spend a lot of time to reread it he could reimplement algorithm.

there is no actual links to articles which author used, but enough information to find them.

А реализовать это дело - это как сыграть по нотам, задача вроде техническая и второстепенная, но требует ведь заметных усилий

To realize this thing - it's like to play on the notes, the problem seems minor and technical, but also requires significant efforts.