> As a random tangent, whilst memcpy in the general case is going to be O(n) given that in this program the n is a fixed size the operation in context is always going to be O(1).
Well, with a fixed n, every operation trivially becomes O(1).
> This isn't really trying to emulate copy-on-write though. It's intentionally making a copy temporarily to mutate it further whilst keeping the original ready to be mutated on the next tick. They share superficial similarity but that's about it.
Having to copy everything is exactly what you need to achieve this effect, when you don't have copy-on-write. You need to make a copy somewhere. (Or otherwise, explicitly roll-back any changes you made.)
As a random tangent, whilst memcpy in the general case is going to be O(n) given that in this program the n is a fixed size the operation in context is always going to be O(1). Fundamentally though it's not a very good way to compare the performance profiles of the approaches to this problem on real hardware.