Introduction - If you have any usage issues, please Google them yourself
//The real sequence of FFT algorithm// When isign = 1, the sequence of real data [1 .. n] do FFT,// Return positive frequency coefficients stored in the data [1 .. n] in// When isign =-1, the actual sequence to do IFFT, the result is multiplied by 2/n for the original sequence// n must be a power of 2