[snissn]

Summary

Avoid constructing a bestFastEncoder in BuildDict when callers provide an explicit BuildDictOptions.Level.

The previous code allocated the default best-compression encoder before checking the requested level. For explicit levels, that encoder was immediately overwritten by eOpts.encoder(), so its large tables were allocated and discarded without being used.

This delays the best encoder construction until the default Level == 0 path, where it is still used. The PR also adds:

• TestBuildDictLevelPathsRoundTrip for default and explicit-level dictionary builds.
• BenchmarkBuildDictLevelPaths to report the allocation difference between those paths.

Evidence

Correctness validation:

go test ./zstd

Allocation benchmark included in this PR:

go test ./zstd -run '^$' -bench '^BenchmarkBuildDictLevelPaths$' -benchmem -benchtime=5x -count=5

Environment: Go 1.25.5, darwin/arm64, Apple M3.

For the baseline, the benchmark/test additions were run on upstream/master without the zstd/dict.go implementation change.

Explicit SpeedFastest dictionary build:

Branch	B/op	allocs/op
upstream/master	~37,137,232	82
this PR	~1,477,456	81

That is about 35.66 MB/op less allocation and one fewer allocation for explicit-level dictionary builds on this fixture. The default Level == 0 path remains around 72.26 MB/op, as expected, since it still constructs the best-compression encoder.

Summary by CodeRabbit

• Tests
  • Added comprehensive test coverage for dictionary building with different encoder level configurations and round-trip validation.

[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: 1082fe27-3570-4f61-834a-5502fb682272

📥 Commits

Reviewing files that changed from the base of the PR and between 28ae503 and d2a08cf.

📒 Files selected for processing (2)

• zstd/dict.go
• zstd/dict_test.go

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📝 Walkthrough

Walkthrough

The BuildDict function in zstd/dict.go refactors its encoder initialization by separating the enc variable declaration from its initial assignment. New test coverage validates round-tripping behavior with default and explicit SpeedFastest encoder levels.

Changes

BuildDict Encoder Initialization Refactoring

Layer / File(s)	Summary
Implementation Refactoring zstd/dict.go	BuildDict refactors encoder initialization: introduces var enc encoder declaration and assigns bestFastEncoder to it, with the same assignment repeated in the o.Level == 0 branch when the conditional path is not taken.
Test Coverage zstd/dict_test.go	New fixture helper, round-trip test, and benchmark validate BuildDict behavior across default and explicit SpeedFastest encoder levels, ensuring dictionary correctness during encoding and decoding operations.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~15 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: avoiding unused encoder allocation in BuildDict when explicit levels are provided, matching the core optimization in the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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 unit tests (beta)

• Create PR with unit tests

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[klauspost]

lgtm