Fast Fourier Transform in double precision. There is no reverse() method
because it can be done by swapping Re/Im inputs, doing the forward
transform and then swapping back again, also dividing by n (see
net.grelf.image.Image64 for an example of an inverse FFT using this class).
Construct a transformer for arrays of the given length. Initialises
tables of trigonometrical functions so they are not recomputed for
multiple FFTs of the same length.
Requires n to be a power of 2 - throws RuntimeException if it is not.
public void forward(double xRe,
Compute the in-place FFT of a length n complex sequence.
Uses a non-recursive version of the Cooley-Tukey FFT.
Runs in O (n log n) time.
n must equal the value set in the constructor.
The offsetToStart is the starting position for the n values in either
array, so that successive ROWS of an image can be transformed in-place.
To do columns of an image it must be rotated through 90 degrees first
and back again after.
For performance, does not use objects of class Complex.