tweety's blog

By tweety, history, 5 weeks ago, In English,

Hello fellow people of Codeforces

I hope everyone doing well in these uncertain and unprecedented times. I wanted to share this cool C++20 trick that I learned recently.

Check out this cool implementation of is_prime(n) to test if n integer is prime in O(1) runtime (without any runtime precomputation!)

bool is_prime(int n);

This code computes primes up to MAXN during compilation and stores them in a table right in the compiled binary. On a runtime is_prime(n) call, it queries the compiled table to find the result. Check this compiled assembly to find the sieve mask baked in the binary: https://godbolt.org/z/G6o84x.

This trick is achieved by passing the constructed Sieve as a template argument, which is always evaluated on compile-time. If you compile this code using older C++ standard, you will get the error:

error: a non-type template parameter cannot have type 'Sieve<MAXN>'

However, since C++20, non-type template parameter can have any LiteralType (note). In short, it is possible to compile-time compute an instance of any class with a constexpr constructor.

Here is a time test of compile-time vs run-time sieve computation:

main.cpp

Compiled with GCC 9.3.0 using the command g++ -std=c++2a main.cpp. This is the execution output on my laptop:

Runtime ans: 90. Elapsed (ms): 1008
Compiletime ans: 90. Elapsed (ms): 0

Of course no one re-computes sieve 1000 times, this is done here for the sole purpose of showing that the algorithm is not computing sieve in run time 1000 times.

While such tricks won't help you cheat pass Codeforces problems, they might will help your submission stand out for fastest runtimes. Hope you found this interesting!

 
 
 
 
  • Vote: I like it
  • +166
  • Vote: I do not like it

»
5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Nice :) I wonder if you can do this in earlier versions of C++ by having just the is_prime array be compile-time computed (instead of the Sieve instance)

It might be worth it to try space-saving tricks like bit-packing and wheel sieve to increase max_N, the runtime penalty should be small, bit-packing should increase max_n by 8x, the simplest wheel sieve (omit all even numbers) should increase it further by 2x.

»
5 weeks ago, # |
Rev. 2   Vote: I like it +27 Vote: I do not like it

I think this can be done with C++14. Also you don't need a Wrapper class:

bool fast_is_prime(int n) {
    static constexpr Sieve<MAXN> s;
    return s.is_prime[n];
}

Runtime ans: 100. Elapsed (ms): 583
Compiletime ans: 100. Elapsed (ms): 0

See C++14 on ideone

  • »
    »
    5 weeks ago, # ^ |
      Vote: I like it +10 Vote: I do not like it

    That's cool! I thought declaring something as constexpr only hints the compiler to compute it during compilation and I had to pass a constexpr constructor as a template argument to force it to be computed compile-time. But seems like declaring static constexpr works too

    • »
      »
      »
      5 weeks ago, # ^ |
      Rev. 2   Vote: I like it +1 Vote: I do not like it

      constexpr on a function declaration is a hint that (under certain conditions) it may be evaluated at compile time and the result be stored in the read-only part of the binary. However, if you declare a "variable" constexpr the compiler is forced to compute the right-hand side of the assignment. You cannot change the value afterwards (constexpr implies const). If you want an actual variable you may use constinit from C++20. If you always want compile time evaluation of a function, you can declare it consteval with the latest standard. I'd say, these new keywords may be rarely of any use in competitive programming.

»
5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Do you have more tricks up your sleeve? If so please make a series or something like that. I found this really helpful.

»
5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Amazing, provide a new idea to cost compile-time instead of exec-time ! It may useful for particluar questions since CP only care about exec-time.

But: ‘constexpr’ loop iteration count has limit of 262144 (use ‘-fconstexpr-loop-limit=’ to increase the limit), and we can't use -fconstexpr-loop-limit= parameter in CP, sadly this ideal may can't put prime sieve in practice even ...._.._ provide C++14(So C++17 works) version of your idea.

»
5 weeks ago, # |
  Vote: I like it -27 Vote: I do not like it
  • »
    »
    5 weeks ago, # ^ |
      Vote: I like it +14 Vote: I do not like it

    Unless I’m misreading something, this post is about moving the computation into the compilation, not about how to actually write a prime sieve (which is what the paper is about).