add SplitMix64::next_u32 from dsiutils

[TheIronBorn]

dsiutils has a 32-bit method for SplitMix64 as well as 64-bit. Perhaps not very useful (and also a breaking change?) but it'd be nice have.

// 2^10 calls to each
test splitmix32 ... bench:       2,929 ns/iter (+/- 869)
test splitmix64 ... bench:       3,630 ns/iter (+/- 516)

[vks]

Interesting, I think this makes sense. This is technically not a breaking change, since this does not affect compilation. It breaks reproducibility, but we are still at version 0.0.1, so this is fine.

The link you provided is dead. Do you have some reference? It would be nice to add a test comparing it with the results of a reference implementation.

[vks]

I can't find your constants in the splitmix paper either. These are the only references I found:

• https://github.com/crystal-lang/crystal-random/blob/master/src/crystal-random/splittable.cr
• http://zimbry.blogspot.com/2011/09/better-bit-mixing-improving-on.html
• https://git.gildedgames.com/GildedGames/OrbisAPI/commit/90d6731dbbbd1649a37525d7147121c11fd2b1d5

[vks]

The link to dsiutils is up again, here is the code:

	/* David Stafford's (http://zimbry.blogspot.com/2011/09/better-bit-mixing-improving-on.html)
         * "Mix4" variant of the 64-bit finalizer in Austin Appleby's MurmurHash3 algorithm. */
	private static int staffordMix4Upper32(long z) {
		z = (z ^ (z >>> 33)) * 0x62A9D9ED799705F5L;
		return (int)(((z ^ (z >>> 28)) * 0xCB24D0A5C88C35B3L) >>> 32);
	}

and

	@Override
	public int nextInt() {
		return staffordMix4Upper32(x += PHI);
	}

Do you know how much this unofficial splitmix64 variant was tested compared to the reference implementation in the paper?

[TheIronBorn]

I have no idea how much it was tested.

[vks]

I see, thanks!