Files

T

feat(rust): port FSE compression primitives

Move FSE normalization, count-header writing, compression-table construction,
and stream encoding from C to Rust. The original C source remains as a
header-only shim, preserving the public header configuration while native
consumers receive the Rust archive exports.

This completes both codec directions for FSE in the migration crate and lets
the unchanged C compatibility tests exercise the Rust encoder implementation.

Test Plan:
- cargo clippy; cargo clippy --benches; cargo clippy --tests
- cargo +nightly fmt, then repeat the Clippy checks
- cargo test --all-targets
- cargo build --release
- Compile the C shim with strict warnings enabled
- 1,000-case pristine-C differential for tables, headers, and payloads
- ./tests/fuzzer -v -T10s
- ./tests/decodecorpus -t -T5

Refs: rust/README.md

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src

feat(rust): port FSE compression primitives

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Cargo.lock

feat(rust): add common primitive compatibility layer

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Cargo.toml

feat(rust): add common primitive compatibility layer

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README.md

feat(rust): port FSE compression primitives

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README.md

Rust rewrite

This directory contains the in-progress Rust replacement for the zstd library and command-line program. During the migration, the crate is built as a static library and linked into the original C test programs. Production C translation units become declaration-only shims as their implementations move to Rust; the original C tests remain unchanged and provide compatibility coverage.

Component map

The crate is organized from low-level representation helpers toward the public zstd ABI:

Common primitives
- mem, bits, bitstream, and cpu implement byte-order, bitstream, and target-feature operations used by the codecs.
- errors, debug, xxhash, and zstd_common provide common exported ABI functions and state.
- common contains shared frame constants and internal data types.
Entropy coding
- entropy_common reads FSE normalized counts and Huffman statistics.
- fse_decompress builds FSE decoding tables and decodes FSE streams.
- fse_compress normalizes counts, writes FSE headers, builds compression tables, and encodes FSE streams.
Compression primitives
- hist counts byte frequencies for FSE and Huffman compression.
Runtime support
- threading provides platform pthread wrappers required by zstd headers.
- pool implements the bounded worker pool used by multithreaded compression.

The remaining block compression, general decompression, dictionary, legacy, and CLI translation units are still C. They must move before the rewrite is complete. Keeping that boundary explicit prevents a passing hybrid build from being mistaken for the final all-Rust result.

Compatibility boundary

The public ABI continues to come from the existing headers under lib/. Exported Rust functions therefore use C layout and calling conventions. A C source file whose implementation has moved to Rust remains in the original makefile source list as a small shim so header configuration and platform preprocessor behavior stay available during the transition.

The test and program makefiles make target/release/libzstd_rs.a a normal link prerequisite. They rebuild the Rust archive and relink the executable when a Rust source changes, so original C tests do not accidentally use a stale implementation.

Validation

Run focused Rust checks from this directory:

cargo fmt --check
cargo clippy --all-targets -- -D warnings
cargo test --all-targets
cargo build --release

Then run original compatibility tests from the repository root, starting with the narrow target for the component being migrated. For example:

make -C tests fuzzer
./tests/fuzzer -i1 --no-big-tests

Broader tests/Makefile targets remain the authoritative integration gates as more of the library and CLI are rewritten.