How to calculate both positive and negative angle between two lines?

Question

There is a very handy set of 2d geometry utilities here.

The angleBetweenLines has a problem, though. The result is always positive. I need to detect both positive and negative angles, so if one line is 15 degrees "above" or "below" the other line, the shape obviously looks different.

The configuration I have is that one line remains stationary, while the other line rotates, and I need to understand what direction it is rotating in, by comparing it with the stationary line.

EDIT: in response to swestrup's comment below, the situation is actually that I have a single line, and I record its starting position. The line then rotates from its starting position, and I need to calculate the angle from its starting position to current position. E.g if it has rotated clockwise, it is positive rotation; if counterclockwise, then negative. (Or vice versa.)

How to improve the algorithm so it returns the angle as both positive or negative depending on how the lines are positioned?

Answer 1

Here's the implementation of brainjam's suggestion. (It works with my constraints that the difference between the lines is guaranteed to be small enough that there's no need to normalize anything.)

CGFloat angleBetweenLinesInRad(CGPoint line1Start, CGPoint line1End, CGPoint line2Start, CGPoint line2End) {
    CGFloat a = line1End.x - line1Start.x;
    CGFloat b = line1End.y - line1Start.y;
    CGFloat c = line2End.x - line2Start.x;
    CGFloat d = line2End.y - line2Start.y;

    CGFloat atanA = atan2(a, b);
    CGFloat atanB = atan2(c, d);

    return atanA - atanB;
}

I like that it's concise. Would the vector version be more concise?