test(bench): expand benchmark parity matrix

[polaz]

Summary

• benchmark decompression on both rust_stream and c_stream inputs for symmetric Rust vs C parity checks
• document benchmark matrix behavior and issue test: comprehensive benchmark suite against C zstd #24 acceptance mapping in BENCHMARKS.md
• ignore accidental root artifact file = in .gitignore

Validation

• cargo fmt -- --check
• cargo bench -p structured-zstd --bench compare_ffi --no-run
• cargo clippy -p structured-zstd --benches --features hash,std,dict_builder -- -D warnings
• cargo build --workspace
• cargo nextest run --workspace

Closes #24

Summary by CodeRabbit

• Documentation
  • Expanded decompression benchmark docs to describe symmetric Rust-vs-C decoder comparisons, acceptance checklist across levels/scenarios, inclusion of small-data scenarios, flamegraph usage, and publication of benchmark reports.
• Chores
  • Added a repo ignore pattern to exclude a new root-level entry and updated benchmark testing setup to run per-source comparisons and emit memory reports.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between dbc4a30 and 43e184f.

📒 Files selected for processing (2)

• BENCHMARKS.md
• zstd/benches/compare_ffi.rs

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📝 Walkthrough

Walkthrough

Precomputes Rust- and C-compressed inputs and refactors decompression benchmarks to run per-source via a new helper; updates BENCHMARKS.md to document symmetric Rust-vs-C decompression runs and adds a single .gitignore entry.

Changes

Cohort / File(s)	Summary
Ignore Patterns \.gitignore	Added a repository-root ignore pattern matching / =.
Benchmark Documentation BENCHMARKS.md	Clarified decompression benchmarks run against two frame sources (rust_stream, c_stream), added Issue #24 acceptance mapping and adjusted example flamegraph command paths.
Decompression Benchmark Refactor zstd/benches/compare_ffi.rs	Refactored bench_decompress to precompute both Rust- and C-compressed buffers and introduced bench_decompress_source + assert_decompress_matches_reference. Benchmark groups and stage labels now include the source; emit_memory_report's stage param changed from &'static str to &str.

Sequence Diagram(s)

sequenceDiagram
  participant Bench as Criterion
  participant Harness as bench_decompress
  participant Helper as bench_decompress_source
  participant RustEnc as structured_zstd (rust compressor)
  participant CEnc as zstd (C FFI compressor)
  participant RustDec as pure_rust decoder
  participant CDec as c_ffi decoder

  Bench->>Harness: run bench_decompress(scenario, level)
  Harness->>RustEnc: compress_to_vec(level.rust_level)
  Harness->>CEnc: zstd::encode_all(level.ffi_level)
  Harness->>Helper: bench_decompress_source(..., "rust_stream", rust_compressed, ...)
  Harness->>Helper: bench_decompress_source(..., "c_stream", c_compressed, ...)
  Helper->>RustDec: assert_decompress_matches_reference(rust_compressed)
  Helper->>CDec: assert_decompress_matches_reference(rust_compressed) 
  Helper->>Bench: create group "decompress/{level}/{scenario}/{source}/matrix"
  Bench->>RustDec: run pure_rust benchmark on provided compressed bytes
  Bench->>CDec: run c_ffi benchmark on provided compressed bytes
  Note over Bench,Helper: emit_memory_report called with stage "decompress-{source}"

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Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Possibly related PRs

• test(bench): expand zstd benchmark suite #38 — Overlapping benchmark refactor that changes zstd/benches/compare_ffi.rs, BENCHMARKS.md, and related benchmark artifact ignores.

Poem

🐰 I hopped through bytes both Rust and C,
Two compressed paths for all to see,
I precompute, I check, then run the race,
Benchmarks bloom in measured pace,
A tiny carrot for every case 🥕

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% 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 clearly describes the main change: expanding the benchmark parity matrix to include symmetric Rust vs C decompression comparisons, which is the primary focus of the changeset.
Linked Issues check	✅ Passed	The PR addresses key benchmarking objectives from issue #24: implements decompression benchmarks for both Rust and C sources [#24], documents the benchmark matrix in BENCHMARKS.md [#24], and validates decoded outputs [#24].
Out of Scope Changes check	✅ Passed	All changes are within scope: benchmark parity expansion (decompression source comparison), documentation updates, .gitignore entry for build artifacts, and test infrastructure improvements directly support issue #24 objectives.

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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 test/#24-benchmark-suite

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull request overview

Expands the Criterion parity benchmarks to measure decompression performance symmetrically on frames produced by both the pure-Rust encoder and the C zstd encoder, and documents the resulting benchmark matrix.

Changes:

• Extend decompression benchmarks to run on two compressed-frame sources (rust_stream and c_stream) per scenario/level.
• Update BENCHMARKS.md to describe the new decompression matrix and map Issue #24 acceptance criteria.
• Ignore an accidental root-level artifact named = via .gitignore.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 2 comments.

File	Description
zstd/benches/compare_ffi.rs	Refactors decompression benchmarks into a helper and runs both Rust/C decoders against both Rust/C-produced frames.
BENCHMARKS.md	Documents the expanded decompression matrix and adds an Issue #24 acceptance mapping section.
.gitignore	Ignores an accidental root artifact file named =.

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@zstd/benches/compare_ffi.rs`:
- Around line 135-155: The decompression benchmarks ("pure_rust" using
FrameDecoder::new() and decode_all and "c_ffi" using
zstd::bulk::Decompressor::new() and decompress_to_buffer) should wrap
inputs/outputs with std::hint::black_box to prevent unwanted compiler
optimization and match the compression benches: inside each b.iter closure call
black_box on the compressed input before passing it to
decode_all/decompress_to_buffer and black_box the result (written or
output/target) before the assert_eq! checks so the benchmark work cannot be
optimized away.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

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

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro

Run ID: cf315879-b2f2-4b33-bdb5-baf3284b7d61

📥 Commits

Reviewing files that changed from the base of the PR and between 4857f1e and dbc4a30.

📒 Files selected for processing (3)

• .gitignore
• BENCHMARKS.md
• zstd/benches/compare_ffi.rs

[sw-release-bot]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'structured-zstd vs C FFI'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.15.

Benchmark suite	Current: 43e184f	Previous: 4857f1e	Ratio
compress/default/high-entropy-1m/matrix/c_ffi	0.384 ms	0.298 ms	1.29
compress/default/low-entropy-1m/matrix/pure_rust	12.005 ms	10.329 ms	1.16

This comment was automatically generated by workflow using github-action-benchmark.

CC: @polaz

[Copilot]

Pull request overview

Copilot reviewed 2 out of 3 changed files in this pull request and generated no new comments.