Function cross

code/geometry/primitives.cpp:7–7

typedef complex<double> point;
double dot(P(a), P(b)) { return real(conj(a) * b); }
double cross(P(a), P(b)) { return imag(conj(a) * b); }
point rotate(P(p), double radians = pi / 2,
             P(about) = point(0,0)) {
  return (p - about) * exp(point(0, radians)) + about; }