Theta sketch output doesn't match between Java and CPP

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Needs clarification
Activity status
Active
Tech stack
cpp, java
Domain
data

Research direction

Start by reproducing the Java Union and C++ updateThetaSketch examples shown in the issue, then compare their serialized byte layouts around the highlighted bytes. Trace the serialization and deserialization entry points used by both implementations; done means the differing bytes are explained and compatible output or documented format behavior is established.

Written by the indexing model from the issue text.

Description

I have run a simple theta sketch computation as below in Java and CPP

Union union = Union.builder().buildUnion();
       union.update(1);
       union.update(2);
       CompactSketch compactSketch = union.getResult();
byte[] bytes = union.getResult().toByteArray()

Java output:

02 03 03 00 00 1a cc 93 02 00 00 00 00 00 80 3f
15 f9 7d cb bd 86 a1 05 c3 97 fc 12 81 70 9d 1e

CPP :

updateThetaSketch update_sketch = updateThetaSketch::builder().build();

  update_sketch.update(1);
  update_sketch.update(2);
  auto bytes = update_sketch.compact().serialize();

CPP output:

02 03 03 00 00 1a cc 93 02 00 00 00 00 00 00 00
15 f9 7d cb bd 86 a1 05 c3 97 fc 12 81 70 9d 1e

The output seems to not match, as we see in bold at the end of first line, 4 bytes "80 3f" is missing in CPP.

Can anyone share why this is so?

Dominant language
C++
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Merged PRs (30d)
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