proposal: x/exp/rand: give greater control over `PCGSource`

[DeedleFake]

The current implementation of PCGSource has a number of missing features, mostly related to control over the seeding process. The Seed() method even has a TODO in it:

// Seed uses the provided seed value to initialize the generator to a deterministic state.
func (pcg *PCGSource) Seed(seed uint64) {
	pcg.low = seed
	pcg.high = seed // TODO: What is right?
}

Similarly, there's no way to get access to the current seed data.

Bikeshed Color 1

I propose adding a new function and two methods:

func NewPCGSource(high, low uint64) *PCGSource
func (pcg *PCGSource) SeedPCG(high, low uint64)
func (pcg *PCGSource) PCG() (high, low uint64)

This would give much greater control over the source.

Bikeshed Color 2

Alternatively, I recently copied the existing implementation in order to remove the internal state from it completely for a somewhat specific use case. I replaced the entire system with single function, func Next(high, low uint64) (n, nhigh, nlow uint64), that gave me full control over the generation.

If such a function was added, the existing implementation could easily be changed to just call any such function instead:

func (pcg *PCGSource) Uint64() (n uint64) {
  n, pcg.high, pcg.low = NextPCG(pcg.high, pcg.low)
  return
}

Summary

I'm bikeshedding a bit, but my only point is that I'd like some greater control over the PCGSource implementation's seeding process.

[ianlancetaylor]

CC @robpike

[robpike]

The seeding methods seem reasonable. Moving to a Next model moves it further from the original math/rand than I'd like.

Send a CL? SeedPCG should probably be Seed64 so other sources could use it.

[kortschak]

Note that the PRNG state can be marshaled and unmarshaled which already provide these functionalities (though not directly and they require that the user knows the size of the seed fields — this could be noted in the documentation).

[gopherbot]

Change https://go.dev/cl/398354 mentions this issue: rand: add methods to allow greater control over PCGSource``

[seankhliao]

I believe this is done for math/rand/v2 https://pkg.go.dev/math/rand/v2#NewPCG