InfInt karatsubaMult(string x, string y)
{
InfInt *result = new InfInt(0);
int m = makeEqualLength(x, y);
if(m == 0)
return *result;
size_t sz = 0;
if( x.length() == 1 || y.length() == 1 ) {
result = new InfInt(stoull(x, &sz, 0) * stoull(y, &sz, 0));
return *result;
}
// m2 + m2Rest = m
int m2 = m/2;
int m2Rest = m - m2;
string xl = x.substr(0, m2);
string xr = x.substr(m2, m2Rest);
string yl = y.substr(0, m2);
string yr = y.substr(m2, m2Rest);
InfInt z0 = karatsubaMult(xr, yr);
InfInt z1 = karatsubaMult(add(xl, xr), add(yl, yr));
InfInt z2 = karatsubaMult(xl, yl);
*result = (z2 * basePower(*INFINTBASE10, 2*m2) + ((z1-z2-z0) * basePower(*INFINTBASE10, m2)) + z0);
return *result;
}
BigInt BigInt::karatsubaMult(const BigInt& a, const BigInt& b)
{
int minSize = std::min(a.size(), b.size());
if (minSize < 30) {
return a * b;
}
BigInt x0, x1, y0, y1;
BigInt z0, z1, z2;
int mid = minSize / 2;
x0.digits.resize(mid);
x1.digits.resize(a.digits.size() - mid);
y0.digits.resize(mid);
y1.digits.resize(b.digits.size() - mid);
copy(a.digits.begin(), a.digits.begin() + mid, x0.digits.begin());
copy(a.digits.begin() + mid, a.digits.end(), x1.digits.begin());
copy(b.digits.begin(), b.digits.begin() + mid, y0.digits.begin());
copy(b.digits.begin() + mid, b.digits.end(), y1.digits.begin());
z0 = karatsubaMult(x0, y0);
z1 = karatsubaMult(x1, y1);
z2 = (karatsubaMult(x0 + x1, y0 + y1) - z1) - z0;
padZeros(z2, mid);
padZeros(z1, 2*mid);
return z0 + z1 + z2;
}
