Here’s a little challenge I had on my mind. I was wondering if it were possible to build an adder that would look like if we would write it on paper : two horizontally laid out registers (low-weight bit on the right) and storing the result in the bottom register and do the carry using the color of the next ball. I currently have a solution for a 2-bit version but a 3-bit version must be possible and 4-bit version might be possible.

An intermediary puzzle / necessary first step would be to create a full adder :

if red means no carry ( C = 0) and blue a carry (C = 1), A and B both bits then the full adder would do :

B = A xor B xor C

C = (A and B) or (C and (A xor B))