perf(lzss,huffman): hash-chain match finder + table Huffman decode

[MagicalTux]

Reviewed the whole codec suite for optimization headroom (benchmarked encode+decode throughput across every algorithm) and kept the changes that clear the ≥10% bar. Two clear algorithmic wins:

lzss encode — O(N·n) brute force → hash chain

The encoder compared every position against all 4096 ring-buffer slots, i.e. O(N·n) regardless of content, so incompressible input collapsed to ~0.3 MB/s (~110k instructions/byte). Replaced it with a hash-chain finder over the raw input, translating a match source at input position cand to the ring index the decoder expects, (cand + N - F) & (N - 1).

• Output size is unchanged: it depends only on match lengths, which the fully-walked chain reproduces; only the tie-broken source position can differ.
• ~9× faster on text, ~700× on random (at 1 MiB); zeros neutral. Compressed sizes within 0.01% across text/binary/zeros/source.
• u32 chain array keeps the fixed allocation small.

huffman decode — bit-by-bit → table lookup

The standalone canonical-Huffman decoder walked each code one bit at a time (a BitReader call per bit). It now builds a single peek-and-lookup table indexed by the next max_length bits (≤15 ⇒ ≤64 KiB) and decodes one symbol per lookup.

• ~1.9–2.1× fewer decode instructions (deterministic callgrind) on both text and high-entropy input.
• Output identical; corrupt/truncated streams still rejected (Corrupt/UnexpectedEnd) without panicking.

Notes

Other slow paths were checked and left alone as already-optimal or inherently costly: the range coder (8-bit bit-tree, ~37 instr/bit), h2-huffman (already a byte-wide FSA), mtf (already single-pass; random is a non-goal), bwt (SA-IS). The multi-MB, highly-repetitive lzss case is ~30% slower than the old early-breaking brute force but stays >270 MB/s — an acceptable trade for fixing the 700× worst case.

Verification

Full suite (61 binaries), clippy, fmt clean. lzss ratio preserved + round-trips; 60-case huffman fuzz + 30 corrupt inputs round-trip through our decoder without panic.

🤖 Generated with Claude Code