What is a good random number generator for a game?

Question

What is a good random number generator to use for a game in C++?

My considerations are:

1. Lots of random numbers are needed, so speed is good.
2. P

Answer 1

Sometimes game developers don't want true randomness and a shuffle bag is more appropriate.

If you do want randomness, the Mersenne twister satisfies your requirements. It is fast, statistically random, has a long period and there are plenty of implementations out there.

Edit: rand() is typically implemented as a linear congruential generator. It's probably best if you make an informed choice of whether or not it's good enough for your purposes.

Answer 2

There are much better choices than Mersenne Twister nowadays. Here is a RNG called WELL512, designed by the designers of Mersenne, developed 10 years later, and an all around better choice for games. The code is put in the public domain by Dr. Chris Lomont. He claims this implementation is 40% faster than Mersenne, does not suffer from poor diffusion and trapping when the state contains many 0 bits, and is clearly a lot simpler code. It has a period of 2^512; a PC takes over 10^100 years to cycle through the states, so it is large enough.

Here is a paper overviewing PRNGs where I found the WELL512 implementation. http://www.lomont.org/Math/Papers/2008/Lomont_PRNG_2008.pdf

So - faster, simpler, created by the same designers 10 years later, and produces better numbers than Mersenne. How can you go wrong? :)

UPDATE (11-18-14): Fixed error (changed 0xDA442D20UL to 0xDA442D24UL, as described in the paper linked above).

/* initialize state to random bits */
static unsigned long state[16];
/* init should also reset this to 0 */
static unsigned int index = 0;
/* return 32 bit random number */
unsigned long WELLRNG512(void)
   {
   unsigned long a, b, c, d;
   a = state[index];
   c = state[(index+13)&15];
   b = a^c^(a<<16)^(c<<15);
   c = state[(index+9)&15];
   c ^= (c>>11);
   a = state[index] = b^c;
   d = a^((a<<5)&0xDA442D24UL);
   index = (index + 15)&15;
   a = state[index];
   state[index] = a^b^d^(a<<2)^(b<<18)^(c<<28);
   return state[index];
   }

Answer 3

You know what? Forgive me if you think this answer completely sucks... But I've been (for god only knows what reason...) using DateTime.Now.Milliseconds as a way to get a random number. I know it's not completely random, but it appears to be...

I just couldn't be bothered typing so much JUST to get a random number! :P

Answer 4

Depending on the target OS, you might be able to use /dev/random. It doesn't really require any implementation, and on Linux (and maybe some other operating systems) it's truly random. The read blocks until sufficient entropy is available, so you might want to read the file and store it in a buffer or something using another thread. If you can't use a blocking read call, you can use /dev/urandom. It generates random data almost as well as /dev/random, but it reuses some random data to give output instantly. It's not as secure, but it could work fine depending on what you plan to do with it.

Answer 5

I'm a fan of Isaac, unlike mersense twister, it's crypographically secure (you *can't crack the period by observing the rolls)

IBAA (rc4?) is also one that is used by blizzard to prevent people from predicting the random number used for loot rolls.. I imagine something similar is done w/ diablo II when you are playing off of a battle.net server.

*can't within any reasonable timeframe (centuries?)

Answer 6

I'd vote for the Mersenne Twister as well. Implementations are widely available, it has a very large period of 2^19937 -1, is reasonably fast and passes most randomness tests including the Diehard tests developed by Marsaglia. rand() and Co., being LCGs, produce lower quality deviates and their successive values can be easily inferred.

One point of note, however, is to properly seed MT into a state that passes randomness tests. Usually a LCG like drand48() is used for that purpose.

I'd say the MT satisfies all the requirements you've set (provably), and it'd be an overkill to go for something like MWCG imo.