[ninjin]

For the general case, I am pretty sure what you describe is the halting problem [1]. This does not mean that I believe some approximation is impossible (your “write to file” comment is particularly true). Just feeling the need to highlight that a clean, general solution is most likely not something that gets done in an afternoon.

[1]: https://en.wikipedia.org/wiki/Halting_problem

[Someone]

Yeah, that “left as an exercise” was tongue-in-cheek. Even past executions do not tell you much. Change “n=10” to “n=12”, and who knows what will happen to execution times? The code might be O(n⁴), or contain an “if n=12” clause.

Looking at the world’s best best reactive system, I think it never automatically fetches external data, and only recalculates stuff it knows it can cancel, and also has a decent idea about how much time each step will take.

Now that Excel has lambdas, that latter part has gone. You can write an Ackermann function in it (https://www.spreadsheetweb.com/how-to-calculate-the-ackerman...)