Module lzah 

Available on crate feature lzah only.

StuffIt classic compression method 5 (“LZAH”): LZSS sliding window over a pre-seeded 4 KiB dictionary, with a single adaptive (sibling- property) Huffman tree for literal/length tokens and a static canonical prefix code for the high bits of each match offset.

This is the raw method-5 fork payload — there is no StuffIt container header here, exactly like the other archive-method codecs in this crate (rar*, sit13, lha). A caller that walks a SIT! archive reads the per- fork compressed bytes and the out-of-band uncompressed length from the 112-byte entry header, then hands the payload to this decoder.

Method 5 vs method 13

The display name “LZAH” is used by some tools for two incompatible classic StuffIt methods. This module implements method low-nibble 5 only (4 KiB window, MSB-first bits, one adaptive Huffman tree, fixed window pre-seed). Method 13 is a different codec (64 KiB window, LSB- first, transmitted prefix codes, in-band end-of-stream) handled by the separate sit13 module. Always disambiguate on the numeric method byte.

Framing — length is out of band

Method 5 has no in-band end-of-stream symbol; decoding stops exactly when the declared uncompressed length has been produced. That length lives in the archive entry header, not in the bitstream, so it is supplied through DecoderConfig::with_len. A non-empty stream decoded with no length (expected_len == None) cannot be terminated and is rejected with Error::Unsupported.

No encoder

No StuffIt method-5 encoder exists (the historical tooling is decode-only and there is no modern need to write the format), so the Encoder permanently returns Error::Unsupported.

Licensing

Clean-room from a functional format description: the adaptive-Huffman mechanics, the window pre-seed pattern, the offset prefix-code length rule, and the container field offsets were implemented from a behavioural specification, not from any reference source code or lookup tables.

Structs

Decoder

Streaming method-5 decoder.

DecoderConfig

Decoder configuration: the out-of-band uncompressed length.

Encoder

Method-5 encoder stub: permanently returns Error::Unsupported.

Lzah

StuffIt classic method 5 (“LZAH”): LZSS + adaptive Huffman, decode-only.