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

Here is a link to a ANSI C implementation of a few random number generators.

Answer 2

Mersenne twister

A bit from Wikipedia:

• It was designed to have a period of 2¹⁹⁹³⁷ − 1 (the creators of the algorithm proved this property). In practice, there is little reason to use a larger period, as most applications do not require 2¹⁹⁹³⁷ unique combinations (2¹⁹⁹³⁷ is approximately 4.3 × 10⁶⁰⁰¹; this is many orders of magnitude larger than the estimated number of particles in the observable universe, which is 10⁸⁰).
• It has a very high order of dimensional equidistribution (see linear congruential generator). This implies that there is negligible serial correlation between successive values in the output sequence.

• It passes numerous tests for statistical randomness, including the Diehard tests. It passes most, but not all, of the even more stringent TestU01 Crush randomness tests.

• source code for many languages available on the link.

Answer 3

I found this: Simple Random Number Generation, by John D. Cook.

It should be easy to adapt to C, given that it's only a few lines of code.

Edit: and you could clarify what you mean by "relatively high-quality". Are you generating encryption keys for nuclear launch codes, or random numbers for a game of poker?

Answer 4

The standard solution is to use a linear feedback shift register.

Answer 5

There is one simple RNG named KISS, it is one random number generator according to three numbers.

/* Implementation of a 32-bit KISS generator which uses no multiply instructions */ 

static unsigned int x=123456789,y=234567891,z=345678912,w=456789123,c=0; 

unsigned int JKISS32() { 
    int t; 

    y ^= (y<<5); y ^= (y>>7); y ^= (y<<22); 

    t = z+w+c; z = w; c = t < 0; w = t&2147483647; 

    x += 1411392427; 

    return x + y + w; 
}

Also there is one web site to test RNG http://www.phy.duke.edu/~rgb/General/dieharder.php

Answer 6

I've made a collection of random number generators, "simplerandom", that are compact and suitable for embedded systems. The collection is available in C and Python.

I've looked around for a bunch of simple and decent ones I could find, and put them together in a small package. They include several Marsaglia generators (KISS, MWC, SHR3), and a couple of L'Ecuyer LFSR ones.

All the generators return an unsigned 32-bit integer, and typically have a state made of 1 to 4 32-bit unsigned integers.

Interestingly, I found a few issues with the Marsaglia generators, and I've tried to fix/improve all those issues. Those issues were:

• SHR3 generator (component of Marsaglia's 1999 KISS generator) was broken.
• MWC low 16 bits have only an approx 2^29.1 period. So I made a slightly improved MWC, which gives the low 16 bits a 2^59.3 period, which is the overall period of this generator.

I uncovered a few issues with seeding, and tried to make robust seeding (initialisation) procedures, so they won't break if you give them a "bad" seed value.