Re: rand48 replacement

On Sat, 3 Jul 2021 at 08:06, Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr> wrote:
>
> Here is a v4, which:
>
> - moves the stuff to common and fully removes random/srandom (Tom)
> - includes a range generation function based on the bitmask method (Dean)
> but iterates with splitmix so that the state always advances once (Me)

At the risk of repeating myself: do *not* invent your own scheme.

The problem with iterating using splitmix is that splitmix is a simple
shuffling function that takes a single input and returns a mutated
output depending only on that input. So let's say for simplicity that
you're generating numbers in the range [0,N) with N=2^64-n and n<2^63.
Each of the n values in [N,2^64) that lie outside the range wanted are
just mapped in a deterministic way back onto (at most) n values in the
range [0,N), making those n values twice as likely to be chosen as the
other N-n values. So what you've just invented is an algorithm with
the complexity of the unbiased bitmask method, but basically the same
bias as the biased integer multiply method.

I don't understand why you object to advancing the state more than
once. Doing so doesn't make the resulting sequence of numbers any less
deterministic.

In fact, I'm pretty sure you *have to* advance the state more than
once in *any* unbiased scheme. That's a common characteristic of all
the unbiased methods I've seen, and intuitively, I think it has to be
so.

Otherwise, I'm happy with the use of the bitmask method, as long as
it's implemented correctly.

Regards,
Dean