Sum numbers

Context

Brainfuck only allows increase / decrease values by 1.
So, to compute a sum A+B, we need to add 1 to A, and do it B times.

Initial state

• Memory: A, B
• Cursor: first cell
• Input: any

Process

• Move to B
• While B cell is not null, decrease B and increase A by 1
• Loop invariant: sum of both cells is A+B
• When second cell equals 0, then first equals A+B
• Move back to A

Code

>[-<+>]<

Final state

• Memory: A+B, 0
• Cursor: first cell
• Input: any
• Output: unchanged