Angular distance

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

$a=\cos ^{-1}{\frac {A\cdot B}{\left|A\right|\cdot \left|B\right|}}$

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

Given two angles $a_{0}$ and $a_{1}$, you could substitute $A=\left[\cos a_{0},\sin a_{0}\right]$ and $B=\left[\cos a_{1},\sin a_{1}\right]$ 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);
}