WIP: Introduce ReadOnlyVBuffer and use it all over.

[eerhardt]

This is a prototype of one direction we could take VBuffer - introduce a ReadOnlyVBuffer for methods that are guaranteed not to modify the buffer. That way a caller can safely be assured that the buffer will not be mutated during the method.

Working towards #608.

[eerhardt]

@Zruty0 @TomFinley - any initial thoughts to this direction? It's not fully complete. I was considering changing the Predictors ValueMapper to take in a ReadOnlyVBuffer<float> instead of a VBuffer<float> , since the Predictors shouldn't be modifying the input buffer.

Let me know what you think, if I should continue this direction or not.

[TomFinley]

Values => _values [](start = 31, length = 17)

I think this is a good beginning. I feel like though if we redesign VBuffer, my primary goal would be to get rid of all the difficulties that made my writing this document necessary.

That is, I think much of the stuff here should actually be in VBuffer itself.

In particular, I would move this to VBuffer and change this to Values => _values.AsSpan(0, count), and Indices => IsDense ? default : _indices.AsSpan(0, count).

We would need some separate object (I suspect internal) that is capable of mutating a VBuffer, to mutate a VBuffer.

In this world there would still I think be benefit to having a read-only version of VBuffer, since that thing could be a ref struct with direct fields of ReadOnlySpan.

Extenally, both VBuffer and ReadOnlyVuffer would both appear to be immutable objects, but VBuffer's buffers would be internally reusable. ROVB though would be more efficient, since in that.

[Zruty0]

Should we close this now @eerhardt ?

[eerhardt]

Oh yes, sorry. This fell off my radar. Closing as this is no longer the plan.