C Language Permuted Congruential Generator

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Example

Here's a standalone random number generator that doesn't rely on rand() or similar library functions.

Why would you want such a thing? Maybe you don't trust your platform's builtin random number generator, or maybe you want a reproducible source of randomness independent of any particular library implementation.

This code is PCG32 from pcg-random.org, a modern, fast, general-purpose RNG with excellent statistical properties. It's not cryptographically secure, so don't use it for cryptography.

#include <stdint.h>

/* *Really* minimal PCG32 code / (c) 2014 M.E. O'Neill / pcg-random.org
 * Licensed under Apache License 2.0 (NO WARRANTY, etc. see website) */

typedef struct { uint64_t state;  uint64_t inc; } pcg32_random_t;

uint32_t pcg32_random_r(pcg32_random_t* rng) {
    uint64_t oldstate = rng->state;
    /* Advance internal state */
    rng->state = oldstate * 6364136223846793005ULL + (rng->inc | 1);
    /* Calculate output function (XSH RR), uses old state for max ILP */
    uint32_t xorshifted = ((oldstate >> 18u) ^ oldstate) >> 27u;
    uint32_t rot = oldstate >> 59u;
    return (xorshifted >> rot) | (xorshifted << ((-rot) & 31));
}

void pcg32_srandom_r(pcg32_random_t* rng, uint64_t initstate, uint64_t initseq) {
    rng->state = 0U;
    rng->inc = (initseq << 1u) | 1u;
    pcg32_random_r(rng);
    rng->state += initstate;
    pcg32_random_r(rng);
}

And here's how to call it:

#include <stdio.h>
int main(void) {
    pcg32_random_t rng; /* RNG state */
    int i;

    /* Seed the RNG */
    pcg32_srandom_r(&rng, 42u, 54u);

    /* Print some random 32-bit integers */
    for (i = 0; i < 6; i++)
        printf("0x%08x\n", pcg32_random_r(&rng));
    
    return 0;
}
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