/** Karazuba multiplication */
private BigIntPolynomial multRecursive(BigIntPolynomial poly2) {
BigInteger[] a = coeffs;
BigInteger[] b = poly2.coeffs;
int n = poly2.coeffs.length;
if (n <= 1) {
BigInteger[] c = Arrays.clone(coeffs);
for (int i = 0; i < coeffs.length; i++) {
c[i] = c[i].multiply(poly2.coeffs[0]);
}
return new BigIntPolynomial(c);
} else {
int n1 = n / 2;
BigIntPolynomial a1 = new BigIntPolynomial(Arrays.copyOf(a, n1));
BigIntPolynomial a2 = new BigIntPolynomial(Arrays.copyOfRange(a, n1, n));
BigIntPolynomial b1 = new BigIntPolynomial(Arrays.copyOf(b, n1));
BigIntPolynomial b2 = new BigIntPolynomial(Arrays.copyOfRange(b, n1, n));
BigIntPolynomial A = (BigIntPolynomial) a1.clone();
A.add(a2);
BigIntPolynomial B = (BigIntPolynomial) b1.clone();
B.add(b2);
BigIntPolynomial c1 = a1.multRecursive(b1);
BigIntPolynomial c2 = a2.multRecursive(b2);
BigIntPolynomial c3 = A.multRecursive(B);
c3.sub(c1);
c3.sub(c2);
BigIntPolynomial c = new BigIntPolynomial(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] = c.coeffs[n1 + i].add(c3.coeffs[i]);
}
for (int i = 0; i < c2.coeffs.length; i++) {
c.coeffs[2 * n1 + i] = c.coeffs[2 * n1 + i].add(c2.coeffs[i]);
}
return c;
}
}
/**
* Adds another polynomial which can have a different number of coefficients, and takes the
* coefficient values mod <code>modulus</code>.
*
* @param b another polynomial
*/
void add(BigIntPolynomial b, BigInteger modulus) {
add(b);
mod(modulus);
}