How to get a random number in Metal shader?

It looks like there's not one built in. This example code for MetalShaderShowcase/AAPLWoodShader.metal defines its own simple rand function.

// Generate a random float in the range [0.0f, 1.0f] using x, y, and z (based on the xor128 algorithm)
float rand(int x, int y, int z)
{
    int seed = x + y * 57 + z * 241;
    seed= (seed<< 13) ^ seed;
    return (( 1.0 - ( (seed * (seed * seed * 15731 + 789221) + 1376312589) & 2147483647) / 1073741824.0f) + 1.0f) / 2.0f;
}

Instead of computing the random number on the GPU, you can also compute a bunch of random numbers on the CPU and pass them into a the shader using a uniform / MTLBuffer.

Please take a look at [pcg-random], it's very simple and fast, more importantly it's fast. And it's super easy to modify their C code for Metal. https://www.pcg-random.org/

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

void pcg32_srandom_r(thread 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);
}

uint32_t pcg32_random_r(thread pcg32_random_t* rng)
{
    uint64_t oldstate = rng->state;
    rng->state = oldstate * 6364136223846793005ULL + rng->inc;
    uint32_t xorshifted = ((oldstate >> 18u) ^ oldstate) >> 27u;
    uint32_t rot = oldstate >> 59u;
    return (xorshifted >> rot) | (xorshifted << ((-rot) & 31));
}

How do I use it?

float randomF(thread pcg32_random_t* rng)
{
    //return pcg32_random_r(rng)/float(UINT_MAX);
    return ldexp(float(pcg32_random_r(rng)), -32);
}

pcg32_random_t rng;
pcg32_srandom_r(&rng, pos_grid.x*int_time, pos_grid.y*int_time);

auto randomFloat = randomF(&rng);