[klauspost]

Add interface to build tables from the histograms themselves.

Adds direct method to export the current table and helpers for size reuse (also via histograms).

Summary by CodeRabbit

• New Features

  • Build Huffman compression tables from symbol frequency data
  • Estimate compressed size and validate table compatibility with input histograms
  • Serialize and append compression tables for reuse and transfer between encoder/decoder

• Tests

  • Comprehensive tests covering table building, error cases, estimation/eligibility, serialization roundtrips, and end-to-end compression/decompression workflows

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 3853e93b-44a3-4c67-925f-9f3648608d70

📥 Commits

Reviewing files that changed from the base of the PR and between 7e29a67 and 6382a30.

📒 Files selected for processing (2)

• huff0/build_table.go
• huff0/build_table_test.go

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📝 Walkthrough

Walkthrough

This PR adds table-building capabilities to the huff0 Huffman compression codec by introducing four methods to Scratch: BuildCTable constructs tables from histograms with validation, EstimateSize and CanUseTable probe table viability against histograms, and AppendTable serializes tables to self-delimiting headers. Comprehensive tests validate basic cases, error handling, table reuse in compression workflows, serialization roundtrips, and payload consistency.

Changes

Huffman Table Building and Serialization

Layer / File(s)	Summary
Core table-building API and basic validation huff0/build_table.go (lines 17–136), huff0/build_table_test.go (lines 1–78)	BuildCTable validates histogram input and constructs Huffman tables, rejecting single-symbol and uniform cases via ErrUseRLE; EstimateSize and CanUseTable provide table-based probing returning -1 and false when encoding is impossible; AppendTable serializes prevTable to a self-delimiting header with lazy FSE state initialization. Tests validate basic success, empty/nil histogram error handling, and single-symbol ErrUseRLE.
Table integration and roundtrip tests huff0/build_table_test.go (lines 80–259)	EstimateSize and CanUseTable reject foreign histograms; end-to-end roundtrip builds table, transfers to encoder with ReusePolicyMust, compresses with verified table reuse (no emitted table bytes), then deserializes and decompresses using ReadTable and Decompress4X to recover original input; AppendTable header roundtrips preserving non-zero symbols; payload-consistency test ensures reused-compression payload matches reference non-reuse payload.

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately and concisely describes the main change: adding functionality to build Huffman tables from histograms, which is the core feature introduced in both the new build_table.go file and its comprehensive test suite.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch huff0-buildtables

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[coderabbitai]

Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@huff0/build_table.go`:
- Around line 66-73: The EstimateSize(hist *[256]uint32) int method (and the
companion probe API that also accepts hist *[256]uint32 around lines 86-92)
currently dereference and iterate hist without a nil guard; add an early nil
check in both functions and return the appropriate sentinel (for EstimateSize
return -1; for the probe API return false) before any iteration or access to
hist to avoid panics when callers pass nil.
- Around line 17-27: BuildCTable currently dereferences the pointer parameter
count (s.count = *count) without checking for nil; add a nil guard at the start
of BuildCTable to return a clear error (e.g., errors.New("huff0: nil count
passed to BuildCTable")) when count == nil before any dereference or use. Locate
the function BuildCTable and insert the check before s.count = *count (and
before any other use of count) so callers passing nil receive a proper error
instead of a panic.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: c8610f78-59b3-4b57-9f35-3011db506c49

📥 Commits

Reviewing files that changed from the base of the PR and between 38efa45 and 7e29a67.

📒 Files selected for processing (2)

• huff0/build_table.go
• huff0/build_table_test.go