What is the need of this outer while loop while finding upper tangent of two convex Hull

Question

Below is the code, where inda and indb are rightmost point of polygon a and leftmost point of polygon b accordingly. Both the polygons are sorted in counter clockwise.

Orientation is a function that checks orientation of 3 point.

bool done = 0;

while (!done)
{
    done = 1;
    while (orientation(b[indb], a[inda], a[(inda+1)%n1]) >=0)
        inda = (inda + 1) % n1;

    while (orientation(a[inda], b[indb], b[(n2+indb-1)%n2]) <=0)
    {
        indb = (n2+indb-1)%n2;
        done = 0;
    }
}

I don't seem to understand the use of outer while loop using done for check, I think we shall find out the tangent without the outer loop.

Do correct me if I'm wrong.