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fix(encoding): commit a dictionary's short last chunk into the history - #586
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| Check name | Status | Explanation | Resolution |
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| Out of Scope Changes check |  |
Issue [#585] does not require the entropy-table commit redesign, 64-slot sequence histograms, or lazy-search hash-width monomorphisation. These changes affect compressed.rs, fastest.rs, and `row/m… |
Remove these independent optimizations from this pull request, or move them to a separate pull request with a directly linked coding requirement. |
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Check name Status Explanation Description Check Check skipped - CodeRabbit’s high-level summary is enabled. Title check The title clearly and concisely describes the primary change: committing a dictionary's short final chunk into matcher history during encoding. Linked Issues check Issue [ #585] requires complete dictionary priming and interoperability coverage.prime_with_dictionary_implnow includes every hashable dictionary byte and leaves dictionaries shorter than the backe…Docstring Coverage Docstring coverage is 80.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 60 functions across 10 files. (1 skipped: 1… Full details: Linked Issues check
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Explanation
Issue [
#585] requires complete dictionary priming and interoperability coverage.prime_with_dictionary_implnow includes every hashable dictionary byte and leaves dictionaries shorter than the backend hash read out. The new matrix covers the five frame paths, the required raw and trained dictionaries, 0–4 byte tails, boundary and mixed inputs, one level per strategy, both project decoders, and libzstd. The unchanged chunk path preserves output when no short final piece exists.
Full details: Docstring Coverage
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Explanation
Docstring coverage is 80.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 60 functions across 10 files. (1 skipped: 1 unsupported.)
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Full details: Out of Scope Changes check
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Explanation
Issue [
#585] does not require the entropy-table commit redesign, 64-slot sequence histograms, or lazy-search hash-width monomorphisation. These changes affectcompressed.rs,fastest.rs, androw/mod.rsand are independent of dictionary priming and interoperability coverage.Resolution
Remove these independent optimizations from this pull request, or move them to a separate pull request with a directly linked coding requirement.
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Inline comments:
Review comments at @zstd/src/encoding/blocks/compressed.rs:
- Around line 1845-1856: Update the SAFETY comment in fill_and_count to identify
the LL_EXTRA_BITS and ML_EXTRA_BITS lookups as the checks that prevent
out-of-range codes before the unchecked histogram writes; do not add a separate
assertion.
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📒 Files selected for processing: 13
ffi-bench/Cargo.tomlffi-bench/tests/dictionary_interop.rszstd/src/encoding/blocks/compressed.rszstd/src/encoding/blocks/compressed/tests.rszstd/src/encoding/frame_compressor.rszstd/src/encoding/levels/fastest.rszstd/src/encoding/levels/fastest/tests.rszstd/src/encoding/match_generator/dict_prime.rszstd/src/encoding/match_generator/tests.rszstd/src/encoding/row/mod.rszstd/src/encoding/streaming_encoder.rszstd/src/encoding/streaming_encoder/tests.rszstd/tests/cross_validation.rs
💤 Files with no reviewable changes: 2
- zstd/src/encoding/streaming_encoder.rs
- zstd/src/encoding/levels/fastest/tests.rs
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Priming fed the dictionary into the matcher history in slice-sized chunks and dropped a last chunk shorter than the hash read. Those bytes were missing from the history while the decoder places the whole dictionary before the frame, so every offset into the dictionary landed one to three bytes off and the frame decoded to wrong data (the content checksum caught it). A pledged small source shrinks the slice to the window, which is how `zstd -D dict file` hit it; every level was affected. The piece before a short last one now ends early enough that the last keeps a full hash read, or runs to the end when that would leave it short; a dictionary too short to hash at all stays out whole, as no match can reach it. Output is unchanged for dictionaries that leave no short last piece. Regression test: cross_dictionary_with_a_short_last_piece_roundtrips_at_every_level (through the C decoder, tails 0-3, levels 1-22). The two driver tests that pinned the dropped tail now pin the whole dictionary committed. Closes #585
The dictionary interop tests compressed through the one-shot FrameCompressor alone, with dictionaries of round lengths, so the streamed path with a pledged size and a dictionary one to three bytes past a whole number of priming pieces was never decoded by libzstd. The matrix crosses five ways a frame is made (one-shot, streamed, pledged, hinted, a context reused for three frames) with raw dictionaries at 4, 16, 64, 110 and 128 KiB plus 0-4 bytes and a dictionary libzstd trained, inputs reaching the dictionary's start, its end and a multi-block mix, and a level of every strategy. Each frame is decoded by libzstd, FrameDecoder and StreamingDecoder; libzstd's own frames for the same dictionaries go through both of ours. One test per dictionary length so the matrix runs in parallel. Part of #585
Experiment: the buffer change alone, to isolate its instruction count.
The three sequence-code histograms were byte-wide arrays, zeroed on every block (and on every split probe) though the alphabets hold 36, 53 and 32 codes. They are now 64 slots, as upstream's HIST_countFast_wksp counts only up to the last code: 1.5 KiB cleared a block instead of 6 KiB. Output is unchanged.
Every compressed block copied the Huffman table it started from into a rollback slot, and cloned the three FSE handles, in case the block lost to a raw one; it then applied its decisions and, on fallback, put the copies back. Upstream builds into nextCBlock and confirms it only when the block is kept. encode_block_parts now returns the block's table decisions, Huffman included, and the caller commits them once the size test keeps the block or discards them when it writes the block raw. Nothing the block started from is copied, the rollback slots and the FSE rollback go, and a Huffman table built by a discarded block is kept as a buffer. Output is unchanged.
A byte index into a 256-slot histogram needed no bounds check; into the 64-slot one it got three a sequence. The code functions keep every code inside its alphabet (at most 35, 52 and 31), so the increments are unchecked under a debug_assert of that bound.
- The lazy monolith takes the hash key width (4, 5 or 6) as a const parameter, chosen once per block next to the finder and row log, as upstream instantiates its searches per (rowLog, mls). - The row, chain and tree hashes and the dictionary probes read the width from the scan context, so the per-position key hash carries no runtime branch on it. Other callers keep the runtime width. - Frames are byte-identical across levels on corpus, random and dictionary inputs.
The release-mode bound on the literal- and match-length codes comes from the bounds-checked extra-bits lookups in the code functions; the safety comment now says so, so those lookups are not made unchecked unnoticed.
- A block written raw keeps the tables it started from and parks the Huffman table it built as a buffer instead of carrying it. - The splitter's retained workspace is counted in the context size. - The row hash salt only spreads bucket indices; its doc says it guards no secret, so a constant salt is not a cryptographic value.
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Actionable comments posted: 1
- 🪄 Fix comments: Fix CodeRabbit comments on this PR
🤖 Prompt to fix review comments
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Inline comments:
Review comments at @zstd/src/encoding/blocks/compressed/tests.rs:
- Line 595: Update the test using FseTables::new() to seed at least one
*_previous slot with an existing FSE table before calling discard_block_tables,
then assert that the seeded table remains unchanged afterward.
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zstd/src/encoding/blocks/compressed.rszstd/src/encoding/blocks/compressed/tests.rszstd/src/encoding/row/mod.rs
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The discarded-block test started from empty previous slots, so a discard that replaced an existing FSE table went unnoticed. Each slot now holds a custom table, and the test checks the same handle is still there.
Monomorphising the lazy search on the key width triples its instances per SIMD tier. The crate's test build is serial-bound and Windows builds it three times, so the job ran past the 15-minute limit while every test that had run passed.
The codec's hash salts are fixed mixing constants for the match finder, not keys, and the crate has no cryptographic code, so every report of this query is a false positive.
## 🤖 New release * `structured-zstd`: 0.2.0 -> 0.2.1 <details><summary><i><b>Changelog</b></i></summary><p> <blockquote> ## [0.2.1](v0.2.0...v0.2.1) - 2026-10-16 ### Added - *(decoding)* add caller-owned streaming workspace decoder ([#583](#583)) ### Fixed - *(encoding)* commit a dictionary's short last chunk into the history ([#586](#586)) </blockquote> </p></details> --- This PR was generated with [release-plz](https://github.com/release-plz/release-plz/). Co-authored-by: sw-release-bot[bot] <255865126+sw-release-bot[bot]@users.noreply.github.com>
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Summary
zstd -D dict filecompresses). The dictionary is now committed whole.Changes
prime_with_dictionary_impl: the piece before a short last one ends early enough that the last keeps a full hash read, or runs to the end when that would leave it short; a dictionary too short to hash stays out whole.ffi-bench/tests/dictionary_interop.rs: one-shot, streamed, pledged, hinted and reused-context frames; raw dictionaries at 4, 16, 64, 110 and 128 KiB plus 0-4 bytes and a trained one; inputs reaching the dictionary's start, its end and a multi-block mix; a level of every strategy; decoded by libzstd and both of our decoders, and libzstd's frames decoded by ours.nextCBlock; the per-block Huffman copy, the rollback slots and the FSE rollback are gone.(rowLog, mls); the row, chain and tree hashes and the dictionary probes read it from the scan context.Testing
ffi-bench), formatting and the no-std build pass; frames are byte-identical tomainacross levels on corpus, random and dictionary inputs.main, this branch and libzstd interleaved in one run, three rounds ofperf stat -r 3 -e task-clock, median ms. Compressed sizes ofmainand this PR are equal on every row.mainmainin both directions, so its delta is code layout, not cost. Level 19 overlaps between rounds.Closes #585
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Bug Fixes
Tests