Sort points by angle from given axis?

Question

How can I sort an array of points/vectors by counter-clockwise increasing angle from a given axis vector?

For example:

Answer 1

Assuming they are all the same length and have the same origin, you can sort on

struct sorter { 
    operator()(point a, point b) const {  
        if (a.y > 0) { //a between 0 and 180
            if (b.y < 0)  //b between 180 and 360
                return false;
            return a.x < b.x; 
        } else { // a between 180 and 360
            if (b.y > 0) //b between 0 and 180
                return true;
            return a.x > b.x;
        }
    }
    //for comparison you don't need exact angles, simply relative.
}

This will quickly sort them from 0->360 degress. Then you find your vector 0 (at position N), and std::rotate the results left N elements. (Thanks TomSirgedas!)