Distance between two rectangles

How can I find the distance between two rectangles? Intersects should return 0 in distance.

Here's a quick function for calculating distance between two CGRects represented by a CGSize:

CGSize CGSizeDistanceBetweenRects(CGRect rect1, CGRect rect2)
{
    if (CGRectIntersectsRect(rect1, rect2))
    {
        return CGSizeMake(0, 0);
    }

    CGRect mostLeft = rect1.origin.x < rect2.origin.x ? rect1 : rect2;
    CGRect mostRight = rect2.origin.x < rect1.origin.x ? rect1 : rect2;

    CGFloat xDifference = mostLeft.origin.x == mostRight.origin.x ? 0 : mostRight.origin.x - (mostLeft.origin.x + mostLeft.size.width);
    xDifference = MAX(0, xDifference);

    CGRect upper = rect1.origin.y < rect2.origin.y ? rect1 : rect2;
    CGRect lower = rect2.origin.y < rect1.origin.y ? rect1 : rect2;

    CGFloat yDifference = upper.origin.y == lower.origin.y ? 0 : lower.origin.y - (upper.origin.y + upper.size.height);
    yDifference = MAX(0, yDifference);

    return CGSizeMake(xDifference, yDifference);
}