Module zip_shrink 

Available on crate feature zip_shrink only.

ZIP method 1 — Shrink: dynamic LZW with a partial-clear marker.

Shrink is the original PKZIP 1.x LZW variant. Codes are emitted at a variable width starting at 9 bits and growing up to 13 bits as the dictionary fills. Codes 0..=255 are literals. Code 256 is a control escape; the next code at the current width is interpreted as a sub-command:

• 1 — increase the code width by one (capped at 13).
• 2 — partial clear: walk the dictionary and free every entry that has no successors (a leaf), keeping the prefix nodes intact.

Bit packing is LSB-first within each byte, matching every other PKZIP LZW variant. The dictionary is sized to 8192 entries (HSIZE).

Wire format on our side

ZIP method-1 payloads have no self-describing length. To make the codec usable as a standalone streaming format, we wrap the raw payload in a minimal four-byte header that carries the uncompressed length:

+-----------+================+
| u32 LE n  | shrink payload |
+-----------+================+

n is the number of decompressed bytes the decoder will emit. This is analogous to legacy .lzma’s “alone” framing (which uses an 8-byte length). If you are extracting a method-1 entry from a real ZIP, splice the length you already have from the central directory in front of the raw payload and feed the result to this decoder.

Scope

Decoder only. The encoder is shipped as Error::Unsupported from every method — implementing the partial-clear heuristic plus the leaf-aware dictionary maintenance is well beyond the decoder, and producing byte-identical streams to PKZIP 1.x’s reference encoder isn’t required by any consumer we care about. Compress new data with a more modern method (deflate, deflate64, lzma, zstd) instead.

References

• PKWARE APPNOTE.TXT — the historical “Shrinking” section (now mostly removed; see archived 1.x copies).
• Info-ZIP unshrink.c (BSD-style; reference for the partial-clear leaf-marking algorithm).

Structs

Decoder

Streaming Shrink decoder.

Encoder

Encoder stub. ZIP Shrink encoding is out of scope for this build; every method here returns Error::Unsupported.

ZipShrink

Zero-sized marker type implementing Algorithm for ZIP Shrink.