Has anyone got a Xojo implementation for the inverse FFT?
Do you have the forward FFT algorithm? If so just flip the sign of the imaginary input (complex conjugation), do the forward fft, then flip the sign of the imaginary output. You also need to handle the scaling, one way is to divide the input by the number of points (since the fft scales up this amount).
For Mac I have declares to the vDSP framework that does fft super fast.
Another technique I just learned about here is to simply swap the real and imaginary components of the input and then swap again the output. If your complex data is in 2 arrays, 1 for real, 1 for imaginary, then all you have to do is swap those arrays.