Functions to generate random numbers from rapidhash

[wigging]

The wyhash repo (which rapidhash evolved from) provides wyrand, wy2u01, and wy2gau functions for generating random numbers. The code for these functions is shown below. What are the equivalent functions for rapidhash?

// The wyrand PRNG that pass BigCrush and PractRand
static inline uint64_t wyrand(uint64_t *seed) {
    *seed += 0x2d358dccaa6c78a5ull;
    return _wymix(*seed, *seed ^ 0x8bb84b93962eacc9ull);
}

// Convert any 64 bit pseudo random numbers to uniform distribution [0,1). It can be combined with
// wyrand, wyhash64 or wyhash.
static inline double wy2u01(uint64_t r) {
    const double _wynorm = 1.0 / (1ull << 52);
    return (r >> 12) * _wynorm;
}

// Convert any 64 bit pseudo random numbers to APPROXIMATE Gaussian distribution. It can be combined
// with wyrand, wyhash64 or wyhash.
static inline double wy2gau(uint64_t r) {
    const double _wynorm = 1.0 / (1ull << 20);
    return ((r &0x1fffff) + ((r >> 21) &0x1fffff) + ((r >> 42) &0x1fffff)) * _wynorm - 3.0;
}

[hoxxep]

The wy2 uniform and Gaussian are standalone functions that don't rely on any secrets.

As for wyrand I'd built a similar method on the Rust crate months ago using rapidhash V1 secrets. @Nicoshev if you're happy with the construction of the rapidrng_fast method with V1 secrets?

https://github.com/hoxxep/rapidhash/blob/master/src/rng.rs

[wigging]

In the code below I replaced the RAPID_SECRET with the actual values. The rapidrng_fast function looks exactly like the wyrand function so I guess they will produce the same results and have similar performance.

pub fn rapidrng_fast(seed: &mut u64) -> u64 {
    *seed = seed.wrapping_add(0x2d358dccaa6c78a5);
    rapid_mix(*seed, *seed ^ 0x8bb84b93962eacc9)
}

[hoxxep]

Yeah I could change the rust code to use the wyhash RNG values as a breaking change. Using the wyhash values could save us testing the RNG with new secrets (although I'm happy to test this later). Whichever @Nicoshev prefers so that we're in sync?

[Nicoshev]

@wigging @hoxxep wyrand is provided in secret.h; behaviour is the same as in wyhash repo.

I can add wy2u01 and wy2gau too, if the yare actually used

[wigging]

So how are these RNG functions related to the hash function? The wyrand function uses _wymix and the rapidrng_fast function uses rapid_mix. Neither of these use the hash function itself. The only relation I see is that the hash and RNG functions both use the mix function. But the hash and RNG functions are not dependent on each other.

[hoxxep]

Correct, they simply use the same folded multiply as a mixing function to distribute entropy evenly in the output. It's useful as a built in PRNG for generating seeds or secrets.

You can also use the hash function directly as a PRNG, but the RNG-specific functions are faster and deal with uint64 input types directly. Doing a single mix step distributes entropy less well than the full hash function though, but it was apparently enough to pass some of the PRNG benchmarks judging by the wyrand comment.