double root(double a, int n)
{
double d, x = 1;
if (!a) return 0;
if (n < 1 || (a < 0 && !(n&1))) return 0./0.; /* NaN */
do { d = (a / pow_(x, n - 1) - x) / n;
x+= d;
} while (abs_(d) >= abs_(x) * (DBL_EPSILON * 10));
return x;
}
double pow_(double x, int n)
{
int flag = 0;
double r;
if (n == 0) return 1.;
if (n == 1) return x;
if (n < 0)
{
n = -n;
flag = 1;
}
r = pow_(x, n/2);
r = r*r;
if (n%2==1)
r *= x;
if (flag == 1)
return 1/r;
return r;
}
void test_pow_()
{
printf("%f\n", pow_(3,-2));
}
prod(LgM, rhs, M_AB);
}
};
auto M2M = [&](int L, const source_box_type& sbox) {
const point_type& s_center = sbox.center();
const point_type s_scale = 1. / sbox.extents();
for (source_box_type cbox : children(sbox)) {
const point_type& c_center = cbox.center();
const point_type& c_extent = cbox.extents();
// Define the child box chebyshev grid
std::array<point_type,pow_(Q,D)> c_cheb;
auto cbi = c_cheb.begin();
for (auto&& i : TensorIndexGridRange<D,Q>()) {
for (unsigned d = 0; d < D; ++d)
(*cbi)[d] = c_center[d] + Chebyshev<double,Q>::x[i[d]] * c_extent[d];
++cbi;
}
// Precompute Lagrange interp matrix with points transformed to ref grid
// XXX: Avoid computing at all for quad/octrees
auto make_ref = [&](const point_type& c) {
return (c - s_center) * s_scale;
};
auto LgM = LagrangeMatrix<D,Q>(
boost::make_transform_iterator(c_cheb.begin(), make_ref),
boost::make_transform_iterator(c_cheb.end(), make_ref));
