This code shows self-similarity issues with Vigna's xoshiro256**. The original version of this code was written by Tyge Løvset.

xoshiro256-similarity.c

/*
 * Show self-similarity issues with Vigna's xoshiro256**
 *
 * Originally posted in a NumPy developers discussion by Tyge Løvset
 * but simplified to only mix 32 outputs.
 *
 * Compile, and then test with PractRand by running
 *
 * ./xoshiro256-similarity \ 
 *   | ./RNG_test stdin64 -te 1 -tlmin 15 -tlmax 50 -tlmaxonly -multithreaded 
 *
 * It should produce output like this
 * 
 * rng=RNG_stdin64, seed=unknown
 * length= 8 gigabytes (2^33 bytes), time= 189 seconds
 *   Test Name                         Raw       Processed     Evaluation
 *   [Low4/64]BCFN(0+0,13-1,T)         R=  +9.5  p =  1.3e-4   unusual
 *   [Low1/64]FPF-14+6/4:(0,14-0)      R= +18.2  p =  1.7e-16    FAIL
 *   [Low1/64]FPF-14+6/4:(1,14-0)      R= +11.3  p =  4.7e-10  very suspicious
 *   [Low1/64]FPF-14+6/4:all           R= +15.1  p =  1.1e-13    FAIL
 *   ...and 618 test result(s) without anomalies
 * 
 * rng=RNG_stdin64, seed=unknown
 * length= 16 gigabytes (2^34 bytes), time= 345 seconds
 *   Test Name                         Raw       Processed     Evaluation
 *   [Low4/64]BCFN(0+0,13-0,T)         R= +23.1  p =  6.9e-12    FAIL
 *   [Low1/64]FPF-14+6/4:(0,14-0)      R= +39.3  p =  5.9e-36    FAIL !!!
 *   [Low1/64]FPF-14+6/4:(1,14-0)      R= +21.5  p =  1.7e-19    FAIL !
 *   [Low1/64]FPF-14+6/4:(2,14-0)      R=  +8.7  p =  1.2e-7   mildly suspicious
 *   [Low1/64]FPF-14+6/4:all           R= +28.9  p =  1.3e-26    FAIL !!
 *   ...and 655 test result(s) without anomalies
 *
 * Original code here
 *     https://github.com/numpy/numpy/issues/16313#issuecomment-642847859
 */

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

static inline uint64_t rotl(const uint64_t x, int k) {
    return (x << k) | (x >> (64 - k));
}
static inline uint64_t xoshiro256starstar_rand(uint64_t* s) {
    const uint64_t result = rotl(s[1] * 5, 7) * 9;
    const uint64_t t = s[1] << 17;
    s[2] ^= s[0];
    s[3] ^= s[1];
    s[1] ^= s[2];
    s[0] ^= s[3];
    s[2] ^= t;
    s[3] = rotl(s[3], 45);
    return result;
}

int main()
{
    // Only necessary on Windows, but harmless elsewhere.
    // FILE* f = freopen(NULL, "wb", stdout);  
#ifndef NTHREADS
    enum {NTHREADS = 32};
#endif
    uint64_t seed = 1591888413; // <- e.g. this fails. // (uint64_t) time(NULL); 
    fprintf(stderr, "seed: %lu\n", seed);

    static uint64_t xo[NTHREADS][4];

    for (size_t i = 0; i < NTHREADS; ++i) {
        xo[i][0] = xo[i][1] = xo[i][2] = xo[i][3] = seed + (12839732 * i);
        for (int j=0; j<12; ++j)
            xoshiro256starstar_rand(xo[i]);
    }
    static uint64_t buffer[NTHREADS];
    
    while (1) {
        for (int i=0; i<NTHREADS; ++i)
            buffer[i] = xoshiro256starstar_rand(xo[i]);
        fwrite((void*) buffer, sizeof(buffer[0]), NTHREADS, stdout);
    }
    return 0;
}