possible maxrects optimizations?

[3b]

After splitting free rects, it needs to check for free rects contained in other rects and remove them. Currently it filters out rects that don't touch the newly placed rect, and then checks all the other rects after splitting against each other.
Instead of having a single new set, it might be better to have a separate set for each face of the placed rect, since free rects touching different faces can't contain each other?
Also, with a strict bottom-left placement, I'm not sure the rects touching the bottom and left faces can have any new redundant rects, so if that is true it might be able to skip those 2 sets completely?