static double xAx(
const VEC& x, // in
const MAT& mat) //in: must be symmetric
{
const int n = NSIZE(x);
CV_Assert(mat.rows == n && mat.cols == n && x.isContinuous());
const double* px = Buf(x);
double diagsum = 0, sum = 0;
int i = n;
while (i--)
{
const double xi = px[i];
const double* const rowbuf = mat.ptr<double>(i);
diagsum += rowbuf[i] * SQ(xi); // sum diag elements
for (int j = i+1; j < n; j++) // sum upper right triangle elements
sum += rowbuf[j] * xi * px[j];
}
return diagsum + 2 * sum; // "2 *" to include lower left triangle elements
}
