Which C++ random number engines have a O(1) discard function?

Question

Since C++11 there are a number of std random number engines. One of the member functions they implement is void discard(int long long z) which skips over z randomly

Answer 1

Well, if you use precomputed jump points, O(1) will work for each and every RNG in existence. Please, remember, that there are algorithm which might have better than O(z), but not O(1) - say, O(log₂ z).

If we're talking about jump to arbitrary point, things get interesting. For example, for linear congruential generator there is known O(log₂ z) jump ahead algorithm, based upon paper by F. Brown, "Random Number Generation with Arbitrary Stride," Trans. Am. Nucl. Soc. (Nov. 1994). Code example is here.

There is LCG RNG in the C++11 standard, not sure how fast jump ahead is done in particular implementation (http://en.cppreference.com/w/cpp/numeric/random/linear_congruential_engine)

PCG family of RNGs share the same property, I believe

Answer 2

I don't think such things exists at all. My heuristic conclusion is that O(1)-jump RNG is basically a hash, with all that this implies (e.g. it might not be "good" RNG at all).

But even if you are asking about O(log z) I don't see that implemented in the STL. In GCC's all the discard functions I was able to grep are all simple loops.

      discard(unsigned long long __z)
      {
        for (; __z != 0ULL; --__z)
          (*this)();
      }

Which is not only sad but also misleading since discard should exists only if there is an efficient way to do it.

The only non trivial one is mersenne (below) but it is still O(z).

    discard(unsigned long long __z)
    {
      while (__z > state_size - _M_p)
    {
      __z -= state_size - _M_p;
      _M_gen_rand();
    }
      _M_p += __z;
    }

Boost's Mersenne, has a skip function but it is only called for skips larger than a (default of) 10000000 (!?). Which already tells me that that the skip is very heavy computationally (even if it is O(log z)). https://www.boost.org/doc/libs/1_72_0/boost/random/mersenne_twister.hpp

Finally, Trust has an efficient discard for linear congruential apparently, but only in the case c=0. (Which I am not sure if it makes it less useful as a RNG.) https://thrust.github.io/doc/classthrust_1_1random_1_1linear__congruential__engine_aec05b19d2a85d02f1ff437791ea4dd68.html#aec05b19d2a85d02f1ff437791ea4dd68

Answer 3

Fact is that std::linear_congruential_engine<UIntType,a,c,m>::discard(unsigned long long z) is definitely possible to implement very efficiently. It is mostly equivalent to exponentiation of a to the power of z modulo m (for both zero and non-zero c) - it means most basic software implementation executes in O(log(z % phi(m))) UIntType multiplications (phi(m)=m-1 for prime number and <m in general) and can be implemented to execute much faster with parallel hardware exponentiation algorithm.

^^^NOTE that in a way O(log(z % phi(m))) is O(1) because log2(z % phi(m)) < log2(m) < sizeof(UIntType)*CHAR_BIT - though in practice it is more often like O(log(z)).

Also probably there are efficient algorithms for most other engines' discard functions that would satisfy O(P(size of state)) constraint (P(x) - some low degree polynomial function, most likely 1+epsilon degree or something even smaller like x*log(x), given that log(z) < sizeof(unsigned long long)*CHAR_BIT may be considered as constant).

Somehow for some unknown reason C++ standard (as of ISO/IEC 14882:2017) does not require to implement discard in more efficient way than just z operator()() calls for any PRNG engine including those that definitely allow this.

To me personally it is baffling and makes no sense whatsoever. This directly contradicts to previous C++ principle to standardize only reasonable functionality in terms of performance - e.g. there is NO std::list<T>::operator[](size_type n) though it is as "easy" as to call operator++() n times with begin() iterator - naturally so because O(n) execution time would make this function unreasonable choice in practice. For this obvious reason a[n] and a.at(n) is not part of mandatory Sequence container requirements but instead is part of Optional sequence container operations. Why in the world then e.discard(z) is part of mandatory Random number engine requirements instead of some optional operations section entry with adequate complexity requirement like O(size of state) or O(P(size of state))?

Even more baffling was to actually find in my GCC this real-world implementation:

void discard(unsigned long long __z)
{
    for (; __z != 0ULL; --__z)
        (*this)(); //<--  Wait what? Are you kidding me?
}

So once again as it was before we have no other choice than to implement the needed functionality ourselves...