Question

What is the best seeded random number generator on ObjC?

I have to generate the same random number sequences on 2 different iPhones, therefore it has to be seeded. This is why I can’t use arc4rand().

NOTE: by best I mean fastest/most unpredictable relation.

Answer 1

The Mersenne Twister implementation that comes with the C++ standard library is very good. As it’s C++, you’ll need to create a wrapper so that you can call it from C and ObjC code, or alternatively rename the file that uses it to have a .mm (Objective-C++) extension.

I’m thinking of something like this, in the header:

#ifdef __cplusplus
extern "C" {
#endif

struct rng_state;
struct rng_state* create_rng(unsigned seed);
void destroy_rng(struct rng_state* rng);
unsigned long long rng_random_unsigned(struct rng_state* rng, unsigned long long max);

#ifdef __cplusplus
}
#endif

And then, in a .cpp file that includes the above:

#include <random>
struct rng_state
{
  std::mt19937* rng;
};
struct rng_state* create_rng(unsigned seed)
{
  std::mt19937* engine = new std::mt19937(seed);
  rng_state* state = new rng_state;
  state->rng = engine;
}
void destroy_rng(struct rng_state* rng)
{
  delete rng->rng;
  delete rng;
}
unsigned long long rng_random_unsigned(struct rng_state* rng, unsigned long long max)
{
  std::uniform_int_distribution<unsigned long long> distribution(0, max);
  return distribution(*rng->rng);
}

I haven’t tested the above, but it should be pretty close. You can then include the header in your C/ObjC files as usual, create a rng with a seed, get a bunch of random numbers, and destroy the rng when you’re done. You can also add more generator functions if needed – the library comes with different random distributions you can use.