# 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.
  - `huf_compress` builds Huffman compression tables, writes table headers,
    and encodes one- and four-stream Huffman payloads.
  - `huf_decompress` builds Huffman decoding tables and decodes X1 and X2
    Huffman streams.
- Compression primitives
  - `hist` counts byte frequencies for FSE and Huffman compression.
  - `zstd_presplit` chooses split points for full compression blocks.
  - `zstd_compress_literals` emits raw, RLE, and Huffman literal sections
    while preserving the compressor's Huffman-table repeat state.
  - `zstd_compress_stats` converts stored sequences into symbol codes,
    selects each block's symbol encoding types, compresses a seqStore's
    literals and sequences into a compressed-block body, builds the block
    entropy statistics shared with the superblock writer and the block
    splitter, and exports collected sequences in the public `ZSTD_Sequence`
    format.  Its C shims extract the sequence store, the entropy-table
    leaves, and the two `ZSTD_CCtx_params` scalars these paths read.
  - `zstd_compress_frame` serializes frame headers, skippable frames, and the
    last empty block; it takes scalar frame parameters so the C-owned
    `ZSTD_CCtx_params` layout never crosses the language boundary.
  - `zstd_compress_params` owns the compression-level tables (formerly
    `clevels.h`), parameter bounds, clamping, validation, table selection,
    source/dictionary adjustment, and match-state/CDict size estimation.
    The C integration layer keeps the public `ZSTD_*` symbols and feeds the
    leaves configuration-owned scalars: the excluded-block-compressor
    strategy cascade, struct sizes, and sanitizer redzone policy.
  - `zstd_fast` and `zstd_double_fast` implement the single- and two-table
    fast block match finders, including attached and external dictionary paths.
  - `zstd_lazy` implements greedy, lazy, lazy2, and binary-tree matching,
    including row-based and dictionary search variants.
  - `zstd_opt_tree` maintains the binary-tree index used by optimal matching;
    the dynamic-programming optimal parser itself remains in C for now.
  - `zstd_ldm` implements long-distance-match parameter selection, table
    maintenance, sequence generation, and sequence consumption.
- Dictionary building
  - `divsufsort` constructs the suffix array that drives the legacy `ZDICT`
    trainer (`ZDICT_trainFromBuffer_legacy`).  The sample analysis and
    dictionary assembly in `zdict.c`, `cover.c`, and `fastcover.c` remain C.
- Runtime support
  - `threading` provides platform pthread wrappers required by zstd headers.
  - `pool` implements the bounded worker pool used by multithreaded compression.
- Dictionary support
  - `zstd_ddict` owns, loads, copies, and references decode dictionaries.
- Legacy decoding
  - `legacy` hosts one frozen module per historical format; `legacy::zstd_v01`
    ports the self-contained v0.1 decoder.  Versions v0.2 through v0.7 are
    still C.
- Block decompression
  - `zstd_decompress_block` decodes literal and sequence sections, maintains
    FSE/Huffman repeat state, and executes compressed-block sequences.
  - `zstd_decompress` owns the public decompression context, one-shot,
    dictionary, parameter, and streaming state machines.  Its C shim retains
    configuration-dependent context allocation plus legacy and trace leaves.
- Command-line frontend
  - `zstd_cli` owns the Rust parser, safety policy, and dispatch.  It is built
    by the separate `cli/` static-library package only for program archives,
    so library builds do not acquire program-only dependencies.  The C
    `fileio` backend still owns file opening, safe replacement, sparse writes,
    metadata, and streaming I/O.
  - `timefn` provides the monotonic nanosecond clock behind `UTIL_time_t`,
    and `benchfn` owns the benchmark run/timing loop (`BMK_benchFunction`,
    `BMK_benchTimedFn`) used by the CLI benchmark mode and by C test tools.
    Both live in the `cli/` package, but C test binaries (fullbench, fuzzer,
    zstreamtest, paramgrill, ...) link a helpers-only build of that archive,
    produced without the package's `cli` feature, because the parser layer
    requires the C `fileio` backend that tests do not compile.  Benchmark
    orchestration and reporting (`benchzstd.c`) remain C, reached from the
    Rust parser through the `ZSTD_NOBENCH`-gated bridge in `zstdcli.c`.

The optimal block matcher, high-level frame compression, dictionary-building
except suffix-array construction, the legacy v0.2-v0.7 decoders, benchmark
orchestration (`benchzstd`), and the CLI file-I/O backend 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.

## Legacy decoding

Each `lib/legacy/zstd_v0N.c` file is a frozen snapshot of the entropy coders
and frame logic of one historical release. The Rust ports in `src/legacy/`
keep that property: every version owns its own frozen FSE/Huff0 and frame
logic, ported line by line, and must never reuse the modern entropy modules
or share code with other legacy versions. Outputs and error codes must be
byte-identical to the original C files. Their only shared dependency is the
`errors` module, matching the C files' `error_private.h` include.

Cargo features `legacy-v01` .. `legacy-v07` gate the per-version modules and
are never default features. The build systems derive the feature list from
the C configuration:

- `lib/Makefile` and `programs/Makefile` map `ZSTD_LEGACY_SUPPORT=N` to the
  features for versions >= N (0 disables legacy), matching the
  `ZSTD_LEGACY_FILES` selection in `lib/libzstd.mk`.
- `tests/Makefile` always enables all seven features because the test
  objects compile every `lib/legacy/*.c` file regardless of dispatch level.
- `build/meson` maps `legacy_level` like the makefiles; `build/cmake`
  enables all seven whenever `ZSTD_LEGACY_SUPPORT` is on because it always
  compiles all seven C files.

Every build system also encodes the legacy selection in the Rust target
directory name (for example `c1-d1-default-legacy5`), for the same reason the
HUF mode is encoded there: a cached archive built for one configuration must
never be linked into a build that expects another.

A feature whose version has not been ported yet gates nothing; the original
C file still provides that decoder, so mixed C/Rust legacy levels link
cleanly. Porting a version means adding `src/legacy/zstd_v0N.rs`, registering
it in `src/legacy/mod.rs` behind its feature, and reducing
`lib/legacy/zstd_v0N.c` to a declaration-only shim. For v0.1 the streaming
`ZSTDv01_Dctx` state lives entirely in Rust: C code only ever holds an opaque
pointer, so the C-side struct definition is gone.

## 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 library, test, and program makefiles select an archive directory for the
active C configuration: enabled compression/decompression/dictionary-builder
modules, default or forced HUF X1/X2, and the matching Rust target for 32-bit
C binaries. The
native static archive flattens Rust object members rather than nesting a Rust
archive, while the native shared library retains all migrated Rust exports.
When the HUF mode changes, the test and program paths also rebuild cached C
outputs before linking. This prevents original C tests from using a stale or
configuration-incompatible implementation.

## Validation

Run focused Rust checks from this directory:

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

The program-only Rust archive has its own feature matrix and should be checked
from `rust/cli` as well:

```sh
cargo clippy --all-targets -- -D warnings
cargo test --all-targets
cargo test --no-default-features --features cli,compression --all-targets
cargo test --no-default-features --features cli,decompression --all-targets
cargo test --no-default-features --all-targets
```

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

```sh
make -C tests fuzzer
./tests/fuzzer -i1 --no-big-tests
make -C tests test-rust-lib-smoke
```

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