I know how to find out angle between 2 vectors, but it always gives me internal angle, but I want it to give me always the angle in anticlockwise direction, even if it is greater then 180. I'm using C++ but it is not really important because I need to get the theory.
This is what I am using now
You're looking for the atan2(y,x) function (http://en.wikipedia.org/wiki/Atan2). If you give it the two components of a 2D vector, it will give you the angle of the vector from the x axis, in the counter-clockwise direction. To solve your specific problem try:
atan2(v_y, v_x) - atan2(u_y, u_x)
Then you can add or subtract 360 degrees if the answer is out of the range of angles you desire.
来源：https://stackoverflow.com/questions/15054593/how-to-find-out-if-the-angle-between-two-vectors-is-external-or-internal