feat(decoding): decode the v0.5, v0.6 and v0.7 frame formats by polaz · Pull Request #615 · structured-world/structured-zstd

polaz · 2026-10-20T18:54:23Z

Summary

Decodes the v0.5, v0.6 and v0.7 frame formats, as every libzstd 1.5.7 build does by default, through the same entry points as RFC 8878 frames: the Rust API, the C ABI and the command line.
legacy cargo feature, on by default; without it these frames are refused as an unknown prefix, as a libzstd built at legacy level 0 refuses them.
Replaces feat(decoding): decode the v0.5, v0.6 and v0.7 frame formats #614 (same change, squashed into one commit on current main).

Changes

decoding/legacy: one decoder per format over a shared entropy layer (FSE tables up to 4096 states, Huffman descriptions with the weight-run header, one and four streams), its tables and literal buffer reused from frame to frame. The declared content size is not enforced, since the v0.6.2 command line declares its first chunk's size; v0.5 and v0.6 refuse the RLE block type they reserve, as upstream's decoders do; a stored block (raw, or RLE in v0.7) runs to its 19-bit size field, as libzstd's one-shot decoders take it, while a compressed one stays within 128 KiB; a match is held to output and dictionary, as upstream holds it. A frame cut short reports the new ErrorCode::SrcSizeWrong (srcSize_wrong through the C ABI), as libzstd does.
The decoder's settings and accessors apply to these frames as to RFC 8878 ones, on the streaming and whole-buffer paths: the content checksum mode on v0.7's 22-bit checksum, the block limit, pinned dictionary IDs and window descriptors, per-block checksums, force_dict, dictionary, content_size, blocks_decoded, the checksum accessors and workspace_size. The window ceiling, a lowered one included mid-frame, binds them where the decoder holds the window (the streaming paths), as it binds RFC 8878 frames and as libzstd applies it.
A header field the formats refuse carries libzstd's code for the path: decoded whole, v0.6 and v0.7 report corruption, as their one-shot decoders do; through a stream, an unsupported parameter.
WorkspaceDecoder refuses these frames as Unsupported::LegacyFrame: their decoders allocate, as upstream's static context refuses them too.
Frame queries (read_frame_content_size, find_frame_compressed_size, find_decompressed_size, frame_decompressed_bound, frame_header_size, new is_frame) answer for these frames, a header measured by its own format's size fields, the decompressed bound counting a stored block by its own size; the size walk refuses a block the decoder refuses from its header, as the RFC 8878 walk does.
FrameDecoder::decode_all* decodes them into the caller's slice; a Read-driven reset that meets their magic number starts a stream, so decode_blocks, decode_from_to, collect, read, StreamingDecoder and the C stream decode them unchanged. The stream keeps its history in segments of the window and a block, a stored block past 128 KiB growing it by that block alone. The calls that take RFC 8878 blocks (decode_block, block_content_len, insert_block, decode_blocks_partial) refuse while one of these frames is current.
A v0.7 frame takes its dictionary from a parsed DictionaryHandle, by ID or as given; v0.5 and v0.6 read a dictionary as its content, and a serialized one whole, header included (a handle parsed from a serialized dictionary keeps that header, registered ones too, and max_heap_size counts it).
LegacyVersion names the formats; LegacyVersion::header_size measures a header arriving in pieces.
C ABI: ZSTD_isFrame, ZSTD_frameHeaderSize and ZSTD_decompressStream accept these frames; the queries and one-shot decoders go through the codec. No new exported symbols.
Command line: -d, -t, -c and -D decode them, -D keeping a serialized dictionary whole; -l refuses them, as the reference does.
scripts/legacy-fixtures.sh builds the v0.5.1, v0.6.2 and v0.7.5 command lines from digest-checked tags and writes zstd/legacy_frames; each frame decodes through libzstd 1.5.7 to its source.
decode_legacy fuzz target: whole-buffer and streaming decodes of the same input must agree; CI replays its committed crash inputs before a seeded run.

Testing

Gates: fmt, clippy in every CI feature set (no_std with and without legacy included), the full test suites of the codec, the C ABI crate and ffi-bench, doc tests and rustdoc. CI adds a no_std build with legacy, the decoding and command-line tests with the feature off, and a seeded fuzz run of decode_legacy.

Closes #131

Summary by CodeRabbit

New Features
- Added support for decoding Zstandard v0.5–v0.7 legacy frames through standard, streaming, C API, dictionary, and frame-inspection interfaces.
- Added legacy-frame recognition, metadata queries, checksums, dictionaries, concatenated frames, and detailed decoding errors.
- Legacy support is enabled by default and can be disabled when needed.
Bug Fixes
- Improved dictionary handling and corrected source-size error reporting.
- Legacy frames now receive format-specific validation and clearer errors when unsupported or malformed.

Decodes the frame formats zstd wrote before 1.0, as every libzstd 1.5.7 build does by default, through the same entry points as RFC 8878 frames: the Rust API, the C ABI and the command line. The `legacy` cargo feature is on by default; without it these frames are refused as an unknown prefix, as a libzstd built at legacy level 0 refuses them. - decoding/legacy: one decoder per format over a shared entropy layer (FSE tables up to 4096 states, Huffman descriptions with the weight-run header, one and four streams), its tables and literal buffer reused from frame to frame. v0.5 and v0.6 refuse the RLE block type they reserve; the declared content size is not enforced, since the v0.6.2 command line declares its first chunk's size; a match is held to output and dictionary, as upstream holds it. - The decoder's settings and accessors apply to these frames on the streaming and whole-buffer paths: the content checksum mode on v0.7's 22-bit checksum, the block limit, a window ceiling lowered mid-frame, pinned dictionary IDs and window descriptors, per-block checksums, force_dict, dictionary, content_size, blocks_decoded, the checksum accessors and workspace_size. - Frame queries (read_frame_content_size, find_frame_compressed_size, find_decompressed_size, frame_decompressed_bound, frame_header_size, new is_frame) answer for these frames, a header measured by its own format's size fields. - FrameDecoder::decode_all* decodes them into the caller's slice; a Read-driven reset that meets their magic number starts a stream, so decode_blocks, decode_from_to, collect, read, StreamingDecoder and the C stream decode them unchanged. The stream keeps its history in segments of the window and a block. The calls that take RFC 8878 blocks (decode_block, block_content_len, insert_block, decode_blocks_partial) refuse while one of these frames is current. - A v0.7 frame takes its dictionary from a parsed DictionaryHandle, by ID or as given; v0.5 and v0.6 read a dictionary as its content, a serialized one whole, header included: a handle parsed from serialized bytes, registered ones included, keeps them, and max_heap_size counts them. - WorkspaceDecoder refuses these frames as Unsupported::LegacyFrame: their decoders allocate, as upstream's static context refuses them. - LegacyVersion names the formats; LegacyVersion::header_size measures a header arriving in pieces. - C ABI: ZSTD_isFrame, ZSTD_frameHeaderSize and ZSTD_decompressStream accept these frames; the queries and one-shot decoders go through the codec. No new exported symbols. - Command line: -d, -t, -c and -D decode them, -D keeping a serialized dictionary whole; -l refuses them, as the reference does. - scripts/legacy-fixtures.sh builds the v0.5.1, v0.6.2 and v0.7.5 command lines from digest-checked tags and writes zstd/legacy_frames; each frame decodes through libzstd 1.5.7 to its source. - decode_legacy fuzz target: whole-buffer and streaming decodes of the same input must agree; CI replays its committed crash inputs before a seeded run. Closes #131

chatgpt-codex-connector · 2026-10-20T18:54:39Z

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📝 Code Review	✅ Completed 2026-10-20T22:30:52.336091Z	`8a50120`	New commits

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📒 Files selected for processing: 8

zstd/Cargo.toml
zstd/examples/legacy_decode_loop.rs
zstd/src/decoding/frame_decoder.rs
zstd/src/decoding/legacy/mod.rs
zstd/src/decoding/legacy/tests.rs
zstd/src/decoding/legacy/v06.rs
zstd/src/decoding/legacy/v07.rs
zstd/src/decoding/mod.rs

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📝 Walkthrough

This change adds default-enabled decoding for Zstandard v0.5–v0.7 frames. It integrates legacy frames with Rust, the C API, the CLI, inspection, streaming, dictionaries, fixtures, fuzzing, and CI feature coverage.

Changes: Legacy frame decoding

Layer / File(s)	Summary
Feature and format contracts `zstd/Cargo.toml`, `zstd/src/decoding/errors.rs`, `zstd/src/decoding/legacy/mod.rs`, `README.md`, `zstd/src/lib.rs`	Adds the default-enabled `legacy` feature, `LegacyVersion`, legacy error reporting, inspection behavior, dictionary handling, and documentation.
Entropy decoding and frame engine `zstd/src/decoding/legacy/*`, `zstd/src/decoding/dictionary.rs`, `zstd/src/huff0/huff0_decoder.rs`	Adds shared FSE and Huffman decoding plus v0.5, v0.6, and v0.7 frame decoders. Dictionary handles retain serialized headers for legacy decoding.
Rust decoder and frame inspection `zstd/src/decoding/frame_decoder.rs`, `zstd/src/decoding/mod.rs`, `zstd/src/decoding/streaming_decoder.rs`, `zstd/src/decoding/workspace.rs`	Routes whole-buffer, incremental, streaming, dictionary, checksum, output, bulk, and inspection paths through legacy decoding. Workspace decoding reports legacy frames as unsupported.
C API and CLI integration `c-api/src/`, `zstd/src/bin/structured-zstd/`	Recognizes legacy frames through the C API and CLI, buffers incomplete streaming headers, maps source-size errors, and supports serialized dictionaries.
Fixtures, tests, fuzzing, and CI `scripts/legacy-fixtures.sh`, `zstd/src/decoding/legacy/tests.rs`, `c-api/src/tests/legacy.rs`, `zstd/fuzz/*`, `.github/workflows/ci.yml`	Generates pinned upstream fixtures and adds coverage for decoding, dictionaries, streaming, mixed frames, malformed input, fuzzing, and feature configurations.

Priority: ➖ Normal

Estimated code review effort: 5 (Critical) | ~120 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant FrameDecoder
  participant LegacyStream
  participant VersionDecoder
  participant Output
  FrameDecoder->>LegacyStream: Detect legacy magic and start frame
  LegacyStream->>VersionDecoder: Parse header and decode blocks
  VersionDecoder->>LegacyStream: Return decoded block data
  LegacyStream->>Output: Deliver decoded bytes

Possibly related PRs

structured-world/structured-zstd#480: Adds the CLI and dictionary APIs extended here for legacy decoding.
structured-world/structured-zstd#558: Adds the streaming and concatenated-frame paths extended here for legacy frames.

Merge Risk: ⚪ Minimal · up to 8a501

This change adds default-enabled decoding of v0.5–v0.7 Zstandard frames across the Rust API, C API, and CLI. The earlier concerns about stale legacy-stream state and excess memory growth on large stored blocks have been addressed, and no outstanding defect remains. The change appears ready to merge after normal checks.

Pre-merge checks: 5

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: adding decoding support for Zstandard v0.5, v0.6, and v0.7 frame formats.
Linked Issues check		The pull request meets the coding requirements in [`#131`]. It adds v0.5, v0.6, and v0.7 decoders behind the default-on `legacy` feature. Current-head tests cover byte-exact fixtures, frame inspection, …
Out of Scope Changes check		The changes stay within [`#131`]. Decoder internals, dictionary handling, frame queries, streaming integration, C ABI and CLI dispatch, fixtures, fuzz coverage, and CI changes directly support legacy fr…
Docstring Coverage		Docstring coverage is 82.15% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 381 functions across 31 files. (1 skipped: …

Full details: Linked Issues check

Explanation

The pull request meets the coding requirements in [#131]. It adds v0.5, v0.6, and v0.7 decoders behind the default-on legacy feature. Current-head tests cover byte-exact fixtures, frame inspection, mixed RFC 8878 and legacy streams, streaming and whole-buffer Rust APIs, C ABI paths, CLI decoding, malformed and truncated input, and feature-off rejection. CI adds legacy and non-legacy builds and tests, plus the decode_legacy fuzz target. The implementation does not add v0.1-v0.4 decoding or legacy encoding.

Full details: Out of Scope Changes check

Explanation

The changes stay within [#131]. Decoder internals, dictionary handling, frame queries, streaming integration, C ABI and CLI dispatch, fixtures, fuzz coverage, and CI changes directly support legacy frame decoding and its required validation. No unrelated product feature or excluded legacy-format support is demonstrated.

Full details: Docstring Coverage

Explanation

Docstring coverage is 82.15% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 381 functions across 31 files. (1 skipped: 1 unsupported.)

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Inline comments:
Review comments at @zstd/src/decoding/frame_decoder.rs:
- Line 2361: Move the self.end_legacy_frame() call before
frame::read_frame_header_from_slice in both reset_from_slice and
reset_from_slice_with_dict_handles, so failed header parsing also clears any
active legacy stream. Preserve the existing successful-header behavior and
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📥 Commits

Reviewing files that changed from the base of the PR and between a620b6d and 181949e.

⛔ Files ignored due to path filters: 58

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📒 Files selected for processing: 36

.github/workflows/ci.yml
README.md
c-api/src/attach.rs
c-api/src/frame.rs
c-api/src/streaming.rs
c-api/src/tests.rs
c-api/src/tests/legacy.rs
scripts/legacy-fixtures.sh
zstd/Cargo.toml
zstd/fuzz/Cargo.toml
zstd/fuzz/fuzz_targets/decode_legacy.rs
zstd/legacy_frames/dictionary.v05
zstd/legacy_frames/dictionary.v06
zstd/legacy_frames/dictionary.v07
zstd/src/bin/structured-zstd/main.rs
zstd/src/bin/structured-zstd/tests.rs
zstd/src/decoding/dictionary.rs
zstd/src/decoding/dictionary/tests.rs
zstd/src/decoding/errors.rs
zstd/src/decoding/frame.rs
zstd/src/decoding/frame_decoder.rs
zstd/src/decoding/frame_inspection_tests.rs
zstd/src/decoding/legacy/entropy.rs
zstd/src/decoding/legacy/entropy/tests.rs
zstd/src/decoding/legacy/mod.rs
zstd/src/decoding/legacy/tests.rs
zstd/src/decoding/legacy/v05.rs
zstd/src/decoding/legacy/v06.rs
zstd/src/decoding/legacy/v07.rs
zstd/src/decoding/mod.rs
zstd/src/decoding/simd_copy/tests.rs
zstd/src/decoding/streaming_decoder.rs
zstd/src/decoding/workspace.rs
zstd/src/decoding/workspace/tests.rs
zstd/src/huff0/huff0_decoder.rs
zstd/src/lib.rs

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codecov · 2026-10-20T19:12:09Z

Codecov Report

❌ Patch coverage is 99.85272% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
zstd/src/decoding/legacy/mod.rs	99.77%	2 Missing ⚠️
zstd/src/decoding/errors.rs	96.87%	1 Missing ⚠️
zstd/src/decoding/legacy/v07.rs	99.71%	1 Missing ⚠️

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- reset_from_slice and its dictionary form end a v0.5-v0.7 frame a Read-driven reset started before the header is parsed, so a skippable frame or a damaged header no longer leaves the old stream current - a frame cut short reports the new ErrorCode::SrcSizeWrong, the C ABI's srcSize_wrong, as libzstd reports it, not corruption - a stored block (raw, or RLE in v0.7) runs to its 19-bit size field, as libzstd's one-shot decoders take it; a compressed one stays within 128 KiB, and a block limit the caller lowered binds every type; the frame bound counts a stored block by its own size - the offset-0 checks in the match copies become debug assertions: every format builds offsets of at least 1 - the FSE weight decoder alternates its two states in one loop, without a second limit check its odd limit never reached - ZSTD_frameHeaderSize says why it measures these headers by their own format rather than as RFC 8878 ones - tests for the decoder's refusals of malformed tables, sequence headers, literal sections and dictionaries, the literal-only and degenerate literal blocks, v0.5 run tables and dumps, and the step interface Each fix carries a regression test that failed before it. Part of #131

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Review comments at @zstd/src/decoding/legacy/mod.rs:
- Line 1452: Update the streaming segment initialization in start to set
capacity to the window plus the maximum of STORED_BLOCK_MAX and BLOCK_SIZE_MAX.
In step, replace the fixed BLOCK_SIZE_MAX rollover check with
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📒 Files selected for processing: 12

c-api/src/error.rs
c-api/src/frame.rs
c-api/src/tests.rs
c-api/src/tests/legacy.rs
zstd/src/decoding/dictionary/tests.rs
zstd/src/decoding/errors.rs
zstd/src/decoding/frame_decoder.rs
zstd/src/decoding/legacy/entropy.rs
zstd/src/decoding/legacy/entropy/tests.rs
zstd/src/decoding/legacy/mod.rs
zstd/src/decoding/legacy/tests.rs
zstd/src/decoding/mod.rs

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…cannot use - a stored block past 128 KiB that arrives before the segment holds the window grows the stream's buffer by that block, not by doubling it; a segment wraps by the block's real bound and only once it holds the window, so matches into the segment before stay in reach - the size walk behind find_frame_compressed_size and frame_decompressed_bound refuses a compressed block past 128 KiB (OversizedBlock) and an RLE block in v0.5 or v0.6 (ReservedBlock), as the RFC 8878 walk refuses the blocks its decoder refuses - a v0.6 or v0.7 header field refused while a frame is decoded whole reports corruption, as libzstd's one-shot decoders do; through a stream it stays an unsupported parameter, as their buffered decoders report it - say why a frame decoded whole holds no window ceiling - an example that decodes a v0.5-v0.7 frame in a loop, for timing Each fix carries a regression test that failed before it. Part of #131

A match longer than its offset was copied one byte at a time in all three outputs (caller's slice, growing vector, stream segments). It is now copied in pieces that double as the written bytes become source, one path for overlapping and plain matches alike. Measured with examples/legacy_decode_loop, bench profile, prebuilt binaries interleaved three times (Apple M1): - 4 MiB of zeros, v0.7.5 -1 (offset-1 matches): 8.5-13.3 ms -> 0.42-0.61 ms per decode, whole; 8.8-10.8 ms -> 0.56-1.48 ms through the stream - z000033, v0.7.5 -1 (mixed): 4.06-4.66 ms vs 3.83-4.66 ms, within noise Part of #131

Part of #131

## Summary - Decodes the v0.5, v0.6 and v0.7 frame formats, as every libzstd 1.5.7 build does by default, through the same entry points as RFC 8878 frames: the Rust API, the C ABI and the command line. - `legacy` cargo feature, on by default; without it these frames are refused as an unknown prefix, as a libzstd built at legacy level 0 refuses them. - Replaces #615 (same change, squashed into one commit on current `main`). ## Changes - `decoding/legacy`: one decoder per format over a shared entropy layer (FSE tables up to 4096 states, Huffman descriptions with the weight-run header, one and four streams), its tables and literal buffer reused from frame to frame. The declared content size is not enforced, since the v0.6.2 command line declares its first chunk's size; v0.5 and v0.6 refuse the RLE block type they reserve, as upstream's decoders do; a stored block (raw, or RLE in v0.7) runs to its 19-bit size field, as libzstd's one-shot decoders take it, while a compressed one stays within 128 KiB; a match is held to output and dictionary, as upstream holds it, and one longer than its offset is copied in doubling pieces. A frame cut short reports the new `ErrorCode::SrcSizeWrong` (`srcSize_wrong` through the C ABI), as libzstd does. - The decoder's settings and accessors apply to these frames as to RFC 8878 ones, on the streaming and whole-buffer paths: the content checksum mode on v0.7's 22-bit checksum, the block limit, pinned dictionary IDs and window descriptors, per-block checksums, `force_dict`, `dictionary`, `content_size`, `blocks_decoded`, the checksum accessors and `workspace_size`. The window ceiling, a lowered one included mid-frame, binds them where the decoder holds the window (the streaming paths), as it binds RFC 8878 frames and as libzstd applies it. - A header field the formats refuse carries libzstd's code for the path: decoded whole, v0.6 and v0.7 report corruption, as their one-shot decoders do; through a stream, an unsupported parameter. - `WorkspaceDecoder` refuses these frames as `Unsupported::LegacyFrame`: their decoders allocate, as upstream's static context refuses them too. - Frame queries (`read_frame_content_size`, `find_frame_compressed_size`, `find_decompressed_size`, `frame_decompressed_bound`, `frame_header_size`, new `is_frame`) answer for these frames, a header measured by its own format's size fields, the decompressed bound counting a stored block by its own size; the size walk refuses a block the decoder refuses from its header, as the RFC 8878 walk does. - `FrameDecoder::decode_all*` decodes them into the caller's slice; a `Read`-driven reset that meets their magic number starts a stream, so `decode_blocks`, `decode_from_to`, `collect`, `read`, `StreamingDecoder` and the C stream decode them unchanged. The stream keeps its history in segments of the window and a block, a stored block past 128 KiB growing it by that block alone. The calls that take RFC 8878 blocks (`decode_block`, `block_content_len`, `insert_block`, `decode_blocks_partial`) refuse while one of these frames is current. - A v0.7 frame takes its dictionary from a parsed `DictionaryHandle`, by ID or as given; v0.5 and v0.6 read a dictionary as its content, and a serialized one whole, header included (a handle parsed from a serialized dictionary keeps that header, registered ones too, and `max_heap_size` counts it). The byte-taking entry points (`decode_all_with_dict_bytes`, `new_with_dictionary_bytes`) also take a dictionary in the v0.5 or v0.6 layout, whole as content, as `ZSTD_decompress_usingDict` does. - New `FrameDecoder::dictionaries` lists every dictionary handle the decoder holds, those kept for reuse across frames of the other kind included; `ZSTD_sizeof_DCtx` counts them. - `LegacyVersion` names the formats; `LegacyVersion::header_size` measures a header arriving in pieces. - C ABI: `ZSTD_isFrame`, `ZSTD_frameHeaderSize` and `ZSTD_decompressStream` accept these frames; the queries and one-shot decoders go through the codec. No new exported symbols. - Command line: `-d`, `-t`, `-c` and `-D` decode them, `-D` keeping a serialized dictionary whole; `-l` refuses them, as the reference does. - `scripts/legacy-fixtures.sh` builds the v0.5.1, v0.6.2 and v0.7.5 command lines from digest-checked tags and writes `zstd/legacy_frames`; each frame decodes through libzstd 1.5.7 to its source. - `decode_legacy` fuzz target: whole-buffer and streaming decodes of the same input must agree; CI replays its committed crash inputs before a seeded run. - `examples/legacy_decode_loop` decodes one frame N times, whole or through a stream, for timing. ## Testing - Gates: fmt, clippy in every CI feature set (no_std with and without `legacy` included), the full test suites of the codec, the C ABI crate and ffi-bench, doc tests and rustdoc. CI adds a no_std build with `legacy`, the decoding and command-line tests with the feature off, and a seeded fuzz run of `decode_legacy`. Closes #131   ## Summary by CodeRabbit - **New Features** - Added decoding support for legacy Zstandard v0.5–v0.7 frames through one-shot, streaming, command-line, and C APIs. - Added legacy frame detection, size queries, dictionary handling, checksums, and concatenated-frame support. - Legacy decoding is enabled by default. - **Bug Fixes** - Improved dictionary preservation and memory-size reporting. - Corrected source-size error reporting and legacy frame-header recognition. - **Documentation** - Documented legacy format support and feature behavior. - **Tests** - Added extensive coverage for legacy frames, dictionaries, streaming, malformed data, and mixed-format content.

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polaz added 3 commits October 21, 2026 01:10

docs(decoding): say which code a refused v0.6 or v0.7 header carries

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Part of #131

polaz closed this Oct 20, 2026

polaz mentioned this pull request Oct 20, 2026

feat(decoding): decode the v0.5, v0.6 and v0.7 frame formats #616

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feat(decoding): decode the v0.5, v0.6 and v0.7 frame formats - #615

feat(decoding): decode the v0.5, v0.6 and v0.7 frame formats#615
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