fix: correct stride bug in BitVector::unmarshal_bytes

[meguruli]

• Fix 8-byte stride issue when reading multiple words from serialized bytes
• Add test case to verify multi-word marshal/unmarshal roundtrip works correctly
• The bug was using 1-byte stride (12+i:20+i) instead of 8-byte stride (12+i8:20+i8)

Fixes serialization/deserialization for BitVectors larger than 64 bits.

[gemini-code-assist]

Summary of Changes

Hello @meguruli, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug in the BitVector deserialization logic, specifically within the unmarshal_bytes function. The core issue stemmed from an incorrect byte stride calculation when processing multiple 64-bit words, which led to data corruption for BitVectors exceeding 64 bits. By adjusting the stride to correctly account for 8-byte words, the PR ensures accurate reconstruction of larger bit vectors from their serialized byte representation. A new comprehensive test case has been introduced to validate this fix and prevent future regressions.

Highlights

• Bug Fix: Corrected an 8-byte stride issue in the BitVector::unmarshal_bytes function, which previously used a 1-byte stride, leading to incorrect deserialization of multi-word bit vectors.
• Impact: This fix resolves serialization/deserialization problems for BitVectors larger than 64 bits, ensuring data integrity for larger bit vector operations.
• New Test Case: Added a new test case, marshal_unmarshal_bytes_roundtrip_multiword, to specifically verify the correct roundtrip functionality for multi-word BitVectors after serialization and deserialization.

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[gemini-code-assist]

Code Review

This pull request correctly addresses a stride bug in the BitVector::unmarshal_bytes function, ensuring that multi-word bit vectors are deserialized correctly. The addition of a specific test case for a 128-bit vector is excellent for verifying the fix and preventing future regressions. The changes are logical and well-implemented. I have one suggestion to refactor the deserialization loop for improved readability and a minor performance gain.

[gemini-code-assist]

While the fix is correct, this loop can be made more readable and slightly more efficient by avoiding the multiplication on each iteration. Using an offset variable that is incremented by the word size makes the logic clearer.

For example:

  let mut offset = 12
  for i in 0..<words_needed {
    data[i] = bytes[offset:offset + 8].to_uint64_le()
    offset += 8
  }

As a follow-up, you might consider defining constants for the magic numbers used in serialization (e.g., header size 12, word size 8) to improve maintainability.