implementation of rand()

Question

I am writing some embedded code in C and need to use the rand() function. Unfortunately, rand() is not supported in the library for the controller. I need a simple implement

Answer 1

Use the C code for LFSR113 from L'écuyer:

unsigned int lfsr113_Bits (void)
{
   static unsigned int z1 = 12345, z2 = 12345, z3 = 12345, z4 = 12345;
   unsigned int b;
   b  = ((z1 << 6) ^ z1) >> 13;
   z1 = ((z1 & 4294967294U) << 18) ^ b;
   b  = ((z2 << 2) ^ z2) >> 27; 
   z2 = ((z2 & 4294967288U) << 2) ^ b;
   b  = ((z3 << 13) ^ z3) >> 21;
   z3 = ((z3 & 4294967280U) << 7) ^ b;
   b  = ((z4 << 3) ^ z4) >> 12;
   z4 = ((z4 & 4294967168U) << 13) ^ b;
   return (z1 ^ z2 ^ z3 ^ z4);
}

Very high quality and fast. Do NOT use rand() for anything. It is worse than useless.

Answer 2

Better yet, use multiple linear feedback shift registers combine them together.

Assuming that sizeof(unsigned) == 4:

unsigned t1 = 0, t2 = 0;

unsigned random()
{
    unsigned b;

    b = t1 ^ (t1 >> 2) ^ (t1 >> 6) ^ (t1 >> 7);
    t1 = (t1 >> 1) | (~b << 31);

    b = (t2 << 1) ^ (t2 << 2) ^ (t1 << 3) ^ (t2 << 4);
    t2 = (t2 << 1) | (~b >> 31);

    return t1 ^ t2;
}

Answer 3

I'd take one from the GNU C library, the source is available to browse online.

http://qa.coreboot.org/docs/libpayload/rand_8c-source.html

But if you have any concern at all about the quality of the random numbers, you should probably look at more carefully written mathematically libraries. It's a big subject and the standard rand implementations aren't highly thought of by experts.

Here's another possibility: http://www.boost.org/doc/libs/1_39_0/libs/random/index.html

(If you find you have too many options, you could always pick one at random.)

Answer 4

I recommend the academic paper Two Fast Implementations of the Minimal Standard Random Number Generator by David Carta. You can find free PDF through Google. The original paper on the Minimal Standard Random Number Generator is also worth reading.

Carta's code gives fast, high-quality random numbers on 32-bit machines. For a more thorough evaluation, see the paper.

Answer 5

Check out this collection of random number generators from George Marsaglia. He's a leading expert in random number generation, so I'd be confident using anything he recommends. The generators in that list are tiny, some requiring only a couple unsigned longs as state.

Marsaglia's generators are definitely "high quality" by your standards of long period and good uniform distribution. They pass stringent statistical tests, though they wouldn't do for cryptography.