CODEWITHSUNDEEP
c++c++11randomentropy
Manelicus
Manelicus

Reputation: 61

Which machines support nondeterministic random_device?

I need to obtain data from different C++ random number generation algorithms, and for that purpose I created some programs. Some of them use pseudo-random number generators and others use random_device (nondeterministic random number generator). The following program belongs to the second group:

#include <iostream>
#include <vector>
#include <cmath>
#include <random>
using namespace std;

const int N = 5000;
const int M = 1000000;
const int VALS = 2;
const int ESP = M / VALS;

int main() {
    for (int i = 0; i < N; ++i) {
        random_device rd;
        if (rd.entropy() == 0) {
            cout << "No support for nondeterministic RNG." << endl;
            break;
        } else {
            mt19937 gen(rd());
            uniform_int_distribution<int> distrib(0, 1);
            vector<int> hist(VALS, 0);
            for (int j = 0; j < M; ++j) ++hist[distrib(gen)];
            int Y = 0;
            for (int j = 0; j < VALS; ++j) Y += abs(hist[j] - ESP);
            cout << Y << endl;
        }
    }
}

As you can see in the code, I check for the entropy to be greater than 0. I do this because:

Unlike the other standard generators, this [random_device] is not meant to be an engine that generates pseudo-random numbers, but a generator based on stochastic processes to generate a sequence of uniformly distributed random numbers. Although, certain library implementations may lack the ability to produce such numbers and employ a random number engine to generate pseudo-random values instead. In this case, entropy returns zero. Source

Checking the value of the entropy allows me to abort de data obtaining if the resulting data is going to be pseudo-random (not nondeterministic). Please note that I assume that if rd.entropy() == 0 is true, then we are in pseudo-random mode.

Unfortunately, all my trials result in a file with no data because of entropy being 0. My question is: what can I do to my computer, or where can I find a machine that allows me to obtain the data?

Upvotes: 3

Views: 552

Answers (1)

Caleth
Caleth

Reputation: 63142

The source you cite is misleading you. The standard says that 

double entropy() const noexcept;

Returns: If the implementation employs a random number engine, returns 0.0. Otherwise, returns an entropy estimate for the random numbers returned by operator(), in the range min() to log2(max()+1).

And a better reference has some empirical observations

Notes

This function is not fully implemented in some standard libraries. For example, LLVM libc++ always returns zero even though the device is non-deterministic. In comparison, Microsoft Visual C++ implementation always returns 32, and boost.random returns 10.

In practice, nearly all the main implementations (targeting general purpose computers) have non-deterministic std::random_devices. Your test has a very high false negative rate.

Upvotes: 3

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