[im3w1l]

Let's say you have two indices into an array: a and b. You want to know how much earlier a is than b so you compute b-a, but as b was in fact the earlier one you get a negative number.

You can deal with this by casting before doing the subtraction, or you can deal with it by storing the indices as signed integers at all times. The latter is more ergonomic at the cost of wasted capacity.