Rapidhash RNG algorithm

[JayceFayne]

Replaces the current wyhash RNG algorithm with rapidhash.

The last commit can be dropped if the output (given the same seed) should be stable across different platforms.

closes #69

[JayceFayne]

rapidhash's MSRV is 1.71. If there is interest in moving from wyhash to rapidhash I can port the code to the current fastrand MSRV

[ofek]

Has there been any progress on this?

[taiki-e]

• We want to avoid situations where the output differs across platforms even when the seed is the same, because there are use cases like #20.

• How does the portable Rapidhash RNG differ from wyrand? The constants and the processing performed there are the same, right?

  • https://github.com/hoxxep/rapidhash/blob/v4.4.1/rapidhash/src/rng.rs

    const RAPID_SECRET: [u64; 3] = [0x2d358dccaa6c78a5, 0x8bb84b93962eacc9, 0x4b33a62ed433d4a3];

    #[inline]
    pub fn rapidrng_fast(seed: &mut u64) -> u64 {
       *seed = seed.wrapping_add(RAPID_SECRET[0]);
       rapid_mix::<false>(*seed, *seed ^ RAPID_SECRET[1])
    }

  • https://github.com/hoxxep/rapidhash/blob/v4.4.1/rapidhash/src/util/mix.rs

    #[inline(always)]
    #[must_use]
    pub(crate) const fn rapid_mix<const PROTECTED: bool>(a: u64, b: u64) -> u64 {
        let r = (a as u128).wrapping_mul(b as u128);
    
        if !PROTECTED {
            (r as u64) ^ (r >> 64) as u64
        } else {
            (a ^ r as u64) ^ (b ^ (r >> 64) as u64)
        }
    }

  fastrand/src/lib.rs

  Lines 148 to 159 in 6c4e800

  	#[inline]
  	fn gen_u64(&mut self) -> u64 {
  	// Constants for WyRand taken from: https://github.com/wangyi-fudan/wyhash/blob/master/wyhash.h#L151
  	// Updated for the final v4.2 implementation with improved constants for better entropy output.
  	const WY_CONST_0: u64 = 0x2d35_8dcc_aa6c_78a5;
  	const WY_CONST_1: u64 = 0x8bb8_4b93_962e_acc9;
  	let s = self.0.wrapping_add(WY_CONST_0);
  	self.0 = s;
  	let t = u128::from(s) * u128::from(s ^ WY_CONST_1);
  	(t as u64) ^ (t >> 64) as u64
  	}

To be honest, it’s odd if they’re claiming things have improved when nothing has actually changed.