Angular distance

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In general, the smallest angle (angular distance) between two vectors is given by:

(if and are unit/normalized, then this can be simplified further).

Given two angles and , you could substitute and above, or you could use the following code:

// https://gamedev.stackexchange.com/a/4472
// a0 and a1 are in radians (always positive)
float delta_angle(float a0, float a1)
{
    return M_PI - fabs(fabs(a1 - a0) - M_PI);
}