Problem Statement

The task is to: Note: the task is not to create an RNG, but to report on the languages in-built RNG that would be the most likely RNG used. The main types of pseudo-random number generator (PRNG) that are in use are the Linear Congruential Generator (LCG), and the Generalized Feedback Shift Register (GFSR), (of which the Mersenne twister generator is a subclass). The last main type is where the output of one of the previous ones (typically a Mersenne twister) is fed through a cryptographic hash function to maximize unpredictability of individual bits. Note that neither LCGs nor GFSRs should be used for the most demanding applications (cryptography) without additional steps.

Let's start with the solution:

typedef unsigned long long u8;
typedef struct ranctx { u8 a; u8 b; u8 c; u8 d; } ranctx;

#define rot(x,k) (((x)<<(k))|((x)>>(64-(k))))
u8 ranval( ranctx *x ) {
    u8 e = x->a - rot(x->b, 7);
    x->a = x->b ^ rot(x->c, 13);
    x->b = x->c + rot(x->d, 37);
    x->c = x->d + e;
    x->d = e + x->a;
    return x->d;
}

void raninit( ranctx *x, u8 seed ) {
    u8 i;
    x->a = 0xf1ea5eed, x->b = x->c = x->d = seed;
    for (i=0; i<20; ++i) {
        (void)ranval(x);
    }
}