feat(theta): introduce intersection theta set operation

[ZENOTME]

This PR:

• introduce ThetaSketchView trait and enhance ThetaHashTable used for set operation
• introduce intersection theta set operation

[ZENOTME]

cc @tisonkun @PsiACE @notfilippo

[Copilot]

Pull request overview

This PR refactors the ThetaHashTable and introduces the ThetaSketchView trait to prepare for implementing set operations (union, intersection, difference) on theta sketches. The changes improve the API design by separating hash computation from insertion logic and clarifying the semantics of emptiness.

Changes:

• Introduced ThetaSketchView trait providing a unified read-only interface for both mutable ThetaSketch and immutable CompactThetaSketch
• Refactored ThetaHashTable to separate hash computation (hash) from insertion with theta screening (try_insert_hash)
• Changed is_empty semantics to track logical emptiness (whether updates were attempted) rather than physical emptiness (whether entries exist)
• Renamed num_entries to num_retained for clarity
• Added new_with_state constructor and from_parts factory method for creating tables/sketches with explicit state

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
datasketches/src/theta/sketch.rs	Introduced ThetaSketchView trait, implemented it for ThetaSketch and CompactThetaSketch, added seed_hash() method, refactored update() to use try_insert(), added from_parts() constructor
datasketches/src/theta/mod.rs	Exported ThetaSketchView trait
datasketches/src/theta/hash_table.rs	Split hash_and_screen into hash() and try_insert_hash(), added is_empty field for logical emptiness, renamed num_entries to num_retained, added new_with_state() constructor, made REBUILD_THRESHOLD pub(crate), updated all tests

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[Copilot]

The refactoring to use from_parts is good, but note that the empty parameter passed to from_parts is computed from entries.is_empty() (line 239), which checks if there are retained hashes. However, with the new is_empty semantics in ThetaHashTable that tracks logical emptiness (whether any updates were attempted), there's now a potential inconsistency: if a ThetaSketch has been updated but all values were screened out, self.table.is_empty() would return false, but the CompactThetaSketch created here would have empty = true. This may be intentional for CompactThetaSketch, but it creates a semantic difference between sketch.is_empty() and sketch.compact(false).is_empty().

[tisonkun]

I suggest you directly implement all the requirements (set ops).

These abstractions look intermediate - I can't judge whether they are correct isolated.

[ZENOTME]

I suggest you directly implement all the requirements (set ops).

These abstractions look intermediate - I can't judge whether they are correct isolated.

Have push commit for intersection op

[tisonkun]

Thanks for your contribution @ZENOTME! LGTM.

Add commits to seal the ThetaSketchView trait.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 7 changed files in this pull request and generated no new comments.

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