deflate: configurable window (encoder max_distance + decoder window_size)

[MagicalTux]

Motivation

qemu/qcow2 decompresses compressed clusters with a 4 KiB deflate window (inflateInit2(-12)), so any back-reference farther than 4096 bytes makes qemu-img fail with Z_DATA_ERROR. compcol's deflate encoder was hardwired to the full 32 KiB window. This adds the two knobs needed to target small-window decoders — and to decode on memory-constrained systems.

Knobs

deflate::EncoderConfig::max_distance (default WINDOW_SIZE, clamped 1..=32768)
Caps the LZ77 back-reference distance the encoder emits — the match finder's bound becomes min(max_distance, WINDOW_SIZE, position). Set 4096 to produce a stream a 4 KiB-window inflater accepts.

deflate::DecoderConfig::window_size (default WINDOW_SIZE, clamped 1..=32768)
Sizes the decoder's circular history buffer — now a Box<[u8]> of win_cap instead of a fixed 32 KiB array, so a small window uses proportionally less memory — and is the maximum legal back-reference distance: distances beyond it are rejected with Error::InvalidDistance, mirroring zlib's small-window Z_DATA_ERROR. This doubles as a way to prove an encoded stream stays within a given window.

Compatibility

• Defaults are unchanged (full 32 KiB window), so existing behavior is identical.
• Internal deflate EncoderConfig constructions (zlib/gzip/factory) updated to ..Default::default(); zlib/gzip keep the full window. (Minor: deflate's public EncoderConfig/DecoderConfig gained a field — construct with ..Default::default() if you used a struct literal.)

Validation

New tests exercise the pairing end-to-end:

• max_distance: 4096 round-trips, and suppresses a far (~8 KiB) match (capped output is larger than the full-window encoding).
• A 4 KiB-window decoder reads the capped stream, rejects the full-window encoding of the same data with InvalidDistance, and the full-window decoder still reads it.
  All 38 deflate tests pass; zlib (32) and gzip (37) unaffected; full --all-features suite green; fmt / clippy -D warnings / docs clean.

🤖 Generated with Claude Code

[MagicalTux]

Follow-up: addressed the cargo-semver-checks constructible_struct_adds_field finding by marking deflate::EncoderConfig and deflate::DecoderConfig #[non_exhaustive] and adding with_* builders (EncoderConfig::default().with_level(9).with_max_distance(4096), DecoderConfig::default().with_window_size(4096)). One breaking bump now; future tuning knobs on these structs are non-breaking. Same-crate callers (zlib/gzip/factory) are unaffected; the external deflate tests use the builder form.