Comment by tyromaniac

4 days ago

I haven't tried it and am afk rn but I'd imagine you just define a constexpr function that implements collatz and use that in your contract.

(Edit: if the input is constexpr itself) Its also quire easy to think about specializing a few templates and doing it that way. If you look up an example of the fib sequence in c++ templates you'll get the general idea.

8 comments

tyromaniac

Reply
kelseyfrog  4 days ago

I feel like I'd have something to learn from seeing the code.

Won't you just run into a constexpr evaluation depth error with a suggestion to set -fconstexpr-depth? I don't see how this is the gotcha it's made out to be.

  • tyromaniac  4 days ago

    I'd exepct something like that to happen. Worth reiterating that the whole scenario is for a hypothetical different implementation of contract that would execute comptime, as noted in the parent thread.

    If you wanted a c++26 preconditon for collatz tho, here it is. ```cpp bool collatz(unsigned long long n) { while (n != 1) n = n % 2 ? 3 * n + 1 : n / 2; return true; }

    void f(unsigned long long n) pre(collatz(n)); ``` That's basically what you'd write if you were doing it with asserts, as the linked article states, just now you add it to the declaration.

    • Someone  3 days ago

      > If you wanted a c++26 preconditon for collatz tho, here it is.

        cpp bool collatz(unsigned long long n)
  {
    while (n != 1)
      n = n % 2 ? 3 * n + 1 : n / 2;
    return true;
  }

      You’ll need to add code to detect cycles other than 1, 4, 2, 1, 4, 2,… to that. That can be done in constant space (https://en.wikipedia.org/wiki/Cycle_detection#Floyd’s_tortoi...)

      Ideally, you also need a way to detect runs towards infinity. That is harder.

      Otherwise, an optimising compiler can use the forward progress guarantee and optimize that to:

        cpp bool collatz(unsigned long long n)
  {
    return true;
  }

      and then completely remove the precondition.

      3 replies →