Transform method
Implementation
List<Complex> Transform(List<num> x) {
int len = x.length;
if (!isPowerOf2(len)) throw "length must be power of 2";
_twiddles = new _Twiddles(len);
var xcp =
x.map((num d) => new Complex.cartesian(d, 0.0)).toList(growable: false);
return _transform(xcp, xcp.length, 1).toList(growable: false);
}