The cosine of the angle between two vectors (in any number of dimensions) is equal to the dot product of the unit vectors. This would probably be less efficient than atan in 2d, but it ought to come in handy in 3d.

(untested and unoptimised)

  double Angle(Vector v1, Vector v2){    double v1a = sqrt(v1.x*v1.x + v1.y*v1.y);    double v2a = sqrt(v2.x*v2.x + v2.y*v2.y);    Vector v1u = {v1.x/v1a, v1y/v1a};    Vector v2u = {v2.x/v2a, v2y/v2a};    double dotPr = v1u.x*v2u.x + v1u.y*v2u.y;    return acos(dotPr);}

What's the fastest way to calculate a vector angle?

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What's the fastest way to calculate a vector angle?

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