[davecromberge]

Replaces the unrolled per-width bit-packing module with word-based packing and unpacking for compressed (serial version 4) Theta sketches. The serialized bytes are unchanged.

• Compressed deserialize 2.2–4.2× and serialize 1.1–2.2× faster across sketch sizes.
• Removes bit_pack.rs (5,401 lines) and adds a round-trip test for every entry width.
• A clean build of the crate with only the theta feature takes 0.45 s instead of 1.35 s in release mode, and 0.38 s instead of 0.78 s in debug mode.
• The compiled .rlib (theta feature only) shrinks from 1.66 MB to 0.86 MB.

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

No active review issues remain for the compressed Theta serialization changes.

No open issues 🎉

What changed in this PR

This PR optimizes compressed Theta sketch serialization and deserialization with word-based bit packing while preserving the serial version 4 byte format.

Changes:

• Replaces block-based packing and unpacking with word-based operations.
• Adds round-trip and expected-payload checks for all entry widths from 1 to 63 bits.
• Removes the obsolete module declaration and documents the performance improvement.

File	Description
datasketches/​src/​thetafamily/​theta/​sketch.rs	Implements word-based packing and unpacking and adds entry-width coverage.
datasketches/​src/​thetafamily/​theta/​mod.rs	Removes the bit_pack module declaration.
CHANGELOG.md	Records faster compressed Theta serialization and deserialization.

_{Review effort: Balanced}

[tisonkun]

Thanks!

[davecromberge]

@tisonkun thanks for your review and for adding the extra safeguard for invalid hashes