public static Polinomials operator *(Polinomials a, Polinomials b)
{
int n = 1;
while (n < Math.Max(a.Length, b.Length))
{
n <<= 1;
}
n <<= 1;
double[] res = new double[n];
Complex[] p1 = a.Numbers.Select(x => new Complex(x, 0)).ToArray(); Array.Resize <Complex>(ref p1, n);
Complex[] p2 = b.Numbers.Select(x => new Complex(x, 0)).ToArray(); Array.Resize <Complex>(ref p2, n);
Fourier.FFT(ref p1, false);
Fourier.FFT(ref p2, false);
for (int i = 0; i < n; i++)
{
p1[i] *= p2[i];
}
Fourier.FFT(ref p1, true);
for (int i = 0; i < n; i++)
{
res[i] = Math.Round((p1[i].Real), 2);
}
return(new Polinomials(res));
}
public static void FFT(ref Complex[] polinomial, bool invert)
{
int l = polinomial.Length;
if (l == 1)
{
return;
}
Complex[] p1 = new Complex[l / 2];
Complex[] p2 = new Complex[l / 2];
for (int i = 0; i < l / 2; i++)
{
p1[i] = polinomial[2 * i];
p2[i] = polinomial[2 * i + 1];
}
Fourier.FFT(ref p1, invert);
Fourier.FFT(ref p2, invert);
double angle = 2 * Math.PI / l * (invert ? -1 : 1);
Complex w = new Complex(1, 0);
Complex wn = new Complex(Math.Cos(angle), Math.Sin(angle));
for (int i = 0; i < l / 2; i++)
{
polinomial[i] = p1[i] + w * p2[i];
polinomial[i + l / 2] = p1[i] - w * p2[i];
if (invert)
{
polinomial[i] /= 2; polinomial[i + l / 2] /= 2;
}
w *= wn;
}
}