private IntegerPolynomial MultRecursive(IntegerPolynomial Factor)
{
int[] a = Coeffs;
int[] b = Factor.Coeffs;
int n = Factor.Coeffs.Length;
if (n <= 32)
{
int cn = 2 * n - 1;
IntegerPolynomial c = new IntegerPolynomial(new int[cn]);
for (int k = 0; k < cn; k++)
{
for (int i = Math.Max(0, k - n + 1); i <= Math.Min(k, n - 1); i++)
c.Coeffs[k] += b[i] * a[k - i];
}
return c;
}
else
{
int n1 = n / 2;
IntegerPolynomial a1 = new IntegerPolynomial(a.CopyOf(n1));
IntegerPolynomial a2 = new IntegerPolynomial(a.CopyOfRange(n1, n));
IntegerPolynomial b1 = new IntegerPolynomial(b.CopyOf(n1));
IntegerPolynomial b2 = new IntegerPolynomial(b.CopyOfRange(n1, n));
// make a copy of a1 that is the same length as a2
IntegerPolynomial A = new IntegerPolynomial(a1.Coeffs.CopyOf(a2.Coeffs.Length));
A.Add(a2);
// make a copy of b1 that is the same length as b2
IntegerPolynomial B = new IntegerPolynomial(b1.Coeffs.CopyOf(b2.Coeffs.Length));
B.Add(b2);
IntegerPolynomial c1 = a1.MultRecursive(b1);
IntegerPolynomial c2 = a2.MultRecursive(b2);
IntegerPolynomial c3 = A.MultRecursive(B);
c3.Subtract(c1);
c3.Subtract(c2);
IntegerPolynomial c = new IntegerPolynomial(2 * n - 1);
for (int i = 0; i < c1.Coeffs.Length; i++)
c.Coeffs[i] = c1.Coeffs[i];
for (int i = 0; i < c3.Coeffs.Length; i++)
c.Coeffs[n1 + i] += c3.Coeffs[i];
for (int i = 0; i < c2.Coeffs.Length; i++)
c.Coeffs[2 * n1 + i] += c2.Coeffs[i];
return c;
}
}
