refactor: encapsulate read-only Theta sketch views

[ZENOTME]

Summary

• keep RawThetaSketchView crate-private while preserving the raw algorithm abstraction
• add a transparent ThetaSketchView that can only be created by supported sketches
• expose the six read-only view methods and accept views in Theta union/intersection
• add reference forwarding plus From conversions for mutable and compact Theta sketches

Validation

• cargo test -p datasketches --features theta
• cargo clippy --workspace --all-features --all-targets -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo doc -p datasketches --features theta --no-deps

[tisonkun]

This would make the document of update cannot linked to RawThetaSketchView and remain a dead link on docs.rs.

Let me think of how to expose the symbols here.

[ZENOTME]

To avoid trait here, we can use enum to dispatch.🤔

[ZENOTME]

Another way is to export using new trait like TupleSketchView. We don't need to test it, and it can be sealed::Sealed. And thetacommon can internal and not export to user.

pub trait TupleSketchView<S>: sealed::Sealed {
    /// Return the 16-bit seed hash.
    fn seed_hash(&self) -> u16;

    /// Return theta as a `u64` threshold.
    fn theta(&self) -> u64;

    /// Return whether this sketch has not received any updates.
    fn is_empty(&self) -> bool;

    /// Return whether retained entries are ordered by ascending hash.
    fn is_ordered(&self) -> bool;

    /// Return an iterator over retained entries.
    fn iter(&self) -> impl Iterator<Item = TupleEntry<S>> + '_;

    /// Return the number of retained entries.
    fn num_retained(&self) -> usize;
}

impl<S, T> RawThetaSketchView<TupleEntry<S>> for T
where
    T: TupleSketchView<S> + ?Sized,
{
    fn seed_hash(&self) -> u16 {
        TupleSketchView::seed_hash(self)
    }

    fn theta(&self) -> u64 {
        TupleSketchView::theta(self)
    }

    fn is_empty(&self) -> bool {
        TupleSketchView::is_empty(self)
    }

    fn is_ordered(&self) -> bool {
        TupleSketchView::is_ordered(self)
    }

    fn iter(&self) -> impl Iterator<Item = TupleEntry<S>> + '_ {
        TupleSketchView::iter(self)
    }

    fn num_retained(&self) -> usize {
        TupleSketchView::num_retained(self)
    }
}

[tisonkun]

Looks like an incorrect abstraction.

I think we can review and merge the tuple sketches code and later try to factor out the abstraction.

So far, I don't see a concrete reason for all these abstratcions. We can inline the ThetaSketchView if this is the only impl.