S16 provides a FFT implementation and related functions for signed 16 bit integer samples.
To use this call S16 for allocating a S16 instance with the appropriate parameters and then call fft or inverse_fft to perform the FFT or inverse FFT on a buffer of samples.
After use free the S16 instance with free.
For the best performance use fft_next_fast_length to get a number that is
entirely a product of 2, 3 and 5 and use this as the len parameter for
S16.
The len parameter specifies the number of samples in the time domain that will be processed or generated. The number of
samples in the frequency domain is len/2 + 1. To get n samples in the frequency domain use 2*n - 2 as len.
Before performing the FFT on time domain data it usually makes sense to apply a window function to it. For this window can comfortably be used.
Be aware, that you can't simply run inverse_fft on the resulting frequency data of fft to get the original data back. The relation between them is iFFT (FFT (x)) = x / nfft where nfft is the length of the FFT. This also has to be taken into account when calculation the magnitude of the frequency data.