Subtract CGRect from CGRect — largest piece of one not containing the other

Question

How I can substract one CGRect from another? I want the result R1 - R2 to be the largest subrectangle of R1 that does not intersect R2.

Answer 1

Your definition is fairly ambiguous, what says whether the subtraction is horizontal or vertical? I recommend using a combination of CGRectIntersection and CGRectDivide, along with specifying a direction to remove ambiguity.

(not tested, or even compiled)

CGRect rectSubtract(CGRect r1, CGRect r2, CGRectEdge edge) {
    // Find how much r1 overlaps r2
    CGRect intersection = CGRectIntersection(r1, r2);

    // If they don't intersect, just return r1. No subtraction to be done
    if (CGRectIsNull(intersection)) {
        return r1;
    }

    // Figure out how much we chop off r1
    float chopAmount = (edge == CGRectMinXEdge || edge == CGRectMaxXEdge)
                       ? intersection.size.width
                       : intersection.size.height;

    CGRect r3, throwaway;
    // Chop
    CGRectDivide(r1, &throwaway, &r3, chopAmount, edge);
    return r3;
}