Complete ReservoirItemsUnion implementation

[Fengzdadi]

I am very sorry, but I realized this problem while reviewing the algorithm these past two days. The current ReservoirItemsUnion implementation is missing some features compared to the Java reference implementation, and there are also some algorithmic simplification issues. Even though it passed the tests at the time, the tests seem to be incomplete.

I want to keep this issue open until I am certain that the ReservoirItemsUnion functionality is correctly implemented.

Again, I apologize for the inconvenience.

Remaining Work

Serialization

• ToSlice(serde) → Java: toByteArray(ArrayOfItemsSerDe)
• NewReservoirItemsUnionFromSlice(data, serde) → Java: heapify(MemorySegment, ArrayOfItemsSerDe)

Update from bytes

• UpdateFromSlice(data, serde) → Java: update(MemorySegment, ArrayOfItemsSerDe)

Utility

• String() → Java: toString()

Already Fixed

• Core merge logic → Java: twoWayMergeInternal
• Weighted sampling → Java: twoWayMergeInternalWeighted
• Gadget initialization → Java: createNewGadget
• Downsampling → Java: downsampledCopy

[Fengzdadi]

I noticed that Java's ReservoirCrossLanguageTest.java only has tests for ReservoirItemsSketch and ReservoirLongsSketch, but no tests for Union types.

Should we add cross-language compatibility tests for ReservoirItemsUnion?

• Generate serialized Union files from Java
• Deserialize and verify in Go (and other languages)

This would ensure binary compatibility across languages for the Union type.

[proost]

@Fengzdadi

Thank you for sharing!
I tell you what i understand, could you please check that I understand correctly?

you mean two issues exist in the reservoir sampling sketch:

1. missing features compared to java.
2. missimplementation in current code

1 -> Nobody expects all features released at once. So it's ok. I will add cross language test cases for union case.
2 -> It should be handled ASAP. can you send PRs for fix?

[Fengzdadi]

@proost Thank you for your kindness!

Both of your understandings are correct:

1. Missing features - Yes, some features were missing compared to Java
2. Misimplementation - Yes, there were issues in the merge logic

I have already completed the fixes for both issues:

1. Fixed twoWayMergeInternal logic
2. Added DownsampledCopy, Copy, and other missing methods
3. Implemented serialization (ToSlice / NewReservoirItemsUnionFromSlice)
4. Added comprehensive tests (17 test cases covering all scenarios). Here, the Go implementation only supports Serialization Version 2 (the current format).

I will submit the PR shortly after double-checking the code logic.

I will add cross language test cases for union case.

Thank you! I think that adding cross-language compatibility tests for ReservoirItemsUnion would be very helpful for ensuring binary compatibility across languages.

Thank you again for your patient guidance!