/** 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; } }