Yesterday I wrote about the terrible quality of the default pseudo random number generators in the C++ standard library. The article came to the conclusion they're all around terrible, and should generally be avoided, yet, it didn't provide any alternatives. Therefore, today I'll provide you with three very good alternatives to the existing generators in the C++ standard library.

splitmix

xorshift

pcg

They're almost fully compliant to the standard, and should be an almost drop-in replacement for the old generators.

There is a minor change in the interface, related how the generator is seeded. To seed any of the generators, you need to pass in the std::random_device (or any class which implements operator() and returns an unsigned int ) you want to use, instead of just the seed. By doing so, I can make sure each generator is seeded accordingly.

Each of these is passing both the PractRand and BigCrush test. The raw PractRand test results can be found here and the code can be found at the end of the article.

Comparison

For comparison, I've reproduced the comparison tables from the previous article. The newly added generators are highlighted in bold.

generator 512M 1G 2G 4G 8G 16G 32G 64G 128G 256G 512G 1T 2T ranlux24_base ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ranlux48_base ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ minstd_rand ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ minstd_rand0 ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ knuth_b ✓ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ mt19937 ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✗ ✗ ✗ ✗ mt19937_64 ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✗ ✗ ✗ ranlux24 ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ranlux48 ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ splitmix ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ xorshift ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ pcg ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

All three generators have excellent statistical quality, while having a smaller footprint and being faster. As a bonus, each of the three generators can be implemented in less than 10 lines of code, making them super easy to port to other environments or languages.

Name Predictability Performance State Period Mersenne Twister Trivial Ok 2 KiB 2^19937 LCG (minstd) Trivial Ok 4-8 byte <= 2^32 LFG (ranluxXX_base) Trivial Slow 8-16 byte <= 2^32 Knuth Trivial Ok 1KiB <= 2^32 ranlux Unknown? Super Slow ~120 byte 10^171 splitmix Trivial Fast 8 byte 2^64 xorshift Yes Fast 8 byte 2^64 pcg Unknown Fast 16 byte 2^128

Example

Following is an example of how to generate 10 random die rolls using the supplied pcg generator:

#include <stdio.h> #include "random.h" int main() { std::random_device rd; pcg rand(rd); std::uniform_int_distribution<> u(1, 6); for (int i = 0; i < 10; ++i) printf("%d

", u(rand)); }

Code

Code can be found in the following gist.