A long time ago I did something similar but rather than sorting I just kept index lists for each direction and the objects sorted themselves. Meaning like there are 4 lists `objectIndicesSortedByLeftEdge/RightEdge/TopEdge/BottomEdge` If an object moves horizontally then it updates its own indices in the leftEdge and rightEdge arrays. This is because as it moves it likely only has to swap 1 or 2 indices at most to move itself.
objects are unlikely to jump from one side the screen to the other so comparing them to stuff on the other side of the screen seems like a waste (which is what a generic sort will do).
