7z: raw LZMA2 decoder + BCJ2 4-stream filter (#74)

CHANGELOG.md

@@ -9,6 +9,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- **Raw LZMA2 decoder** (`lzma2`): decodes the raw 7-Zip LZMA2 chunk stream
+  (codec id 21) — control-byte-framed chunks, self-terminating — distinct from
+  the `.xz` container. The 1-byte 7z dict-size coder property is passed via
+  `DecoderConfig::with_dict_prop`. Reuses the existing xz LZMA2 engine (the
+  shared codec was relocated to a crate-internal `lzma2_internal` module; `xz`
+  behavior unchanged). Decode-only.
+- **BCJ2 filter** (`bcj2`): the 7-Zip 4-stream x86 branch filter
+  (`0303011B`), encode + decode via a dedicated `compcol::bcj2::{encode,decode}`
+  function API (the 4-input shape doesn't fit the single-stream `Decoder`
+  trait). Public-domain LZMA SDK algorithm; round-trip validated.
 - **RLE90 codec** (`rle90`): the `0x90`/DLE run-length variant shared by ARC
   method 3 ("packed") and classic StuffIt method 1, encoder + decoder.
   Byte-compatible with the `arc_squeeze` internal RLE90 pre-pass.

Cargo.toml

@@ -29,7 +29,7 @@ default = ["alloc", "rle", "deflate", "zlib", "gzip", "factory"]
 all = [
     "alloc", "std", "tokio", "factory",
     "rle", "rle90", "deflate", "deflate64", "zlib", "gzip",
-    "lzma", "xz",
+    "lzma", "xz", "lzma2",
     "zstd", "brotli", "lz4", "snappy", "lzw", "lzss", "bzip2",
     "zstd", "brotli", "lz4", "lz5", "snappy", "lzw", "bzip2",
     "lzo", "lzx", "amiga_lzx", "quantum", "lzfse", "adc",
@@ -48,7 +48,7 @@ all = [
     "zip_shrink",
     "zip_reduce",
     "lha",
-    "bcj", "delta",
+    "bcj", "bcj2", "delta",
     "arc_crunch", "arc_squeeze", "arc_squash",
 ]
 # Enables `alloc`-backed conveniences (e.g. the `factory` module, the
@@ -82,6 +82,12 @@ lzma    = ["alloc"]
 # xz container (RFC-style stream/block headers + check codes; the inner
 # LZMA2 chunk codec is inlined inside `src/xz/`).
 xz      = ["lzma"]
+# Raw LZMA2 chunk stream (7-Zip coder id 21) — the dict-reset control bytes
+# + LZMA chunks, without the `.xz` container. Decode-only entry point that
+# reuses the same LZMA2 chunk codec the `xz` feature uses. The 7z coder
+# property is a 1-byte dictionary-size code. The encoder is an
+# `Error::Unsupported` stub (produce LZMA2 via the `xz` encoder).
+lzma2   = ["alloc", "lzma"]
 # Zstandard (RFC 8478).
 zstd    = ["alloc"]
 # Brotli (RFC 7932). Carries a 170 KiB built-in static dictionary when fully
@@ -210,6 +216,13 @@ lha     = ["alloc"]
 # compressors: forward rewrites relative branch operands to absolute form,
 # inverse restores them. Encoder + decoder both implemented.
 bcj     = ["alloc"]
+# BCJ2 — the 4-stream x86 branch filter from the public-domain LZMA SDK
+# (7-Zip filter id 0303011B), distinct from the single-stream BCJ above.
+# Decode recombines four input streams (main + call + jump + a range-coded
+# control stream) into the filtered output. Exposes a dedicated function
+# API (`compcol::bcj2::decode`) since the 4-input shape does not fit the
+# single-input `Decoder` trait. Encoder also provided for round-trip tests.
+bcj2    = ["alloc"]
 # Delta filter — byte-wise delta with a configurable distance (1..=256).
 # Reversible filter (xz/LZMA SDK lineage). Encoder + decoder both implemented.
 delta   = ["alloc"]

README.md

@@ -42,6 +42,7 @@ flag, and a `compcol` binary turns the library into a Unix-style filter.
 | LZW (`compress(1)` `.Z`) | `lzw` | `.lzw` | full | full | `compress(1)` / `uncompress(1)` |
 | LZMA (legacy `.lzma`) | `lzma` | `.lzma` | full | full | `python3 -m lzma` (FORMAT_ALONE) |
 | xz | `xz` | `.xz` | compressed-LZMA2 chunks + uncompressed fallback | full envelope + all reset variants | `xz(1)` both directions |
+| Raw LZMA2 (7z coder 21) | `lzma2` | `.lzma2` | `Unsupported` (decode-only) | full (raw LZMA2 chunk stream; reuses the xz LZMA2 engine) | round-trip vs the xz LZMA2 encoder |
 | Zstandard (RFC 8478) | `zstd` | `.zst` | LZ77 + Huffman literals + FSE_Compressed_Mode sequences + repeat offsets + RLE blocks | full Compressed_Block | `zstd(1)` both directions |
 | Brotli (RFC 7932) | `brotli` | `.br` | LZ77 + length-limited Huffman + 704-symbol IC alphabet + static-dictionary refs | full (with 122 KiB static dictionary) | `brotli(1)` both directions |
 | LZO (LZO1X-1) | `lzo` | `.lzo` | LZ77 hash matcher | full | `python3 -c "import lzo"` |
@@ -57,6 +58,7 @@ flag, and a `compcol` binary turns the library into a Unix-style filter.
 | LZNT1 (NTFS native compression) | `lznt1` | `.lznt1` | full | full (per [MS-XCA] §2.5; 4 KiB-chunked LZ77, no entropy coding) | hand-built fixtures |
 | LHA / LZH (`-lh1-`/`-lh4-`/`-lh5-`/`-lh6-`/`-lh7-`) | `lha` | `.lzh` | full (lh1 adaptive Huffman; lh4/5/6/7 static Huffman) | full (clean-room from Okumura LZHUF / ar002) | own round-trip (no reference fixture) |
 | BCJ branch filters (x86, ARM, ARMT, ARM64, PPC, SPARC, IA-64, RISC-V) | `bcj` | `bcj-<arch>` | full (reversible filter) | full | round-trip identity (public-domain LZMA SDK transform) |
+| BCJ2 (7z 4-stream x86 filter) | `bcj2` | — | `bcj2::encode` (fn API) | `bcj2::decode` (fn API) | round-trip identity (LZMA SDK algorithm) |
 | Delta filter (distance 1..=256) | `delta` | `delta` | full (reversible filter) | full | round-trip identity |
 | ARC Crunch (method 8) | `arc_crunch` | `.arc` | full (12-bit dynamic LZW) | full | own round-trip (no reference fixture) |
 | ARC Squeeze (method 4) | `arc_squeeze` | `.sqz` | full (RLE + static Huffman) | full | own round-trip (no reference fixture) |