I do not know the term "-ve".
You cannot determine the sign of an angle between two vectors without further information. Imagine two 3D vectors: You can measure the enclosed angle only, which is always 0 <= angle <= pi/2. If you want a sign, you need a third vector to define an orientation. If you talk about 2D vectors, the global Z-axis [0, 0, 1] is this third vector usually. Then using ATAN2 is recommended, but replies a signed angle also: -pi <= angle < pi.
To get 0 <= angle < 2*pi, you could add the wanted shift value:
index = (angle < 0);
angle(index) = 2*pi + angle(index);
