Copy value

Context

We already wrote some code to move a value - copy that destroys source.
To copy a value, we need to "destroy-copy twice" the target value, and optionally move one of the copies to the initial location.
So, to copy A we need to add 1 to the next 2 cells, A times - first cell will then be null; and move contents of third cell to first one.

Initial state

• Memory: A, 0, 0
• Cursor: first cell
• Input: any

Process

• While first cell is not null, decrease first and increase second / third by 1, and move back to first
• Loop invariant: second cell = third cell, and first+second = A
• When first cell equals 0, then second and third equals A
• Move third cell value to first one

Code

[->+>+<<]>>[-<<+>>]<<

Final state

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