Compression library — Huffman / LZ77 (roundtrip test app)

• Type: feature
• Status: done
• Owner: —
• Opened: 2026-06-19
• Closed: 2026-06-20
• Relation: demo-eligible-as-library from idea-demo-app-candidates. Sibling to feature-json-library et al. Depends on the bit-stream / bit-set work in feature-rtl-conversion-and-bitset-library (encode/decode is bit-granular). Own unit, FPC-ish naming, no port.

Goal

A Compression unit: lossless encode/decode of byte buffers. Start with Huffman (canonical), optionally add LZ77 then a deflate-lite combining both. Decompress(Compress(x)) = x.

Surface (sketch)

• function HuffmanCompress(const src): TBytes;
• function HuffmanDecompress(const src): TBytes;
• later: LZ77Compress / Deflate variants
• a bit-reader / bit-writer over packed words (from the bit-set lane)

Coverage

bit packing / bit streams (the motivating consumer of the bit-set type) · trees (Huffman tree build/walk) · frequency tables = collections/hashing · dynamic arrays · recursion (tree). Integer-deterministic throughout.

Acceptance / oracle

• Roundtrip identity: Decompress(Compress(buf)) = buf for a fixed corpus.
• Exact compressed size (integer) per input — byte-identical across targets.
• Demo: examples/compression/ compresses a bundled corpus, prints sizes + verifies roundtrip.

Constraints

Own .pas unit; FPC-ish naming; no port; blocked-ish on the bit-set primitive landing first (can prototype a local bit-writer, but the real unit should consume the shared type). No self-host / cross regression.

Log

• 2026-06-19 — Opened from the demo/library organization pass.
• 2026-06-20 — Landed lib/rtl/zlib.pas: RFC 1950/1951 inflate (stored, fixed Huffman, dynamic Huffman) with Adler-32 validation, plus DeflateZlibStored for encoding. Added test/lib_zlib.pas covering roundtrip, fixed/dynamic fixtures, and malformed-stream rejection (commit 86b88f8). Green in lib-test and library-suite-green.