Single-stream gzip as an expanding comparator

Date: 2026-06-01 Status: Implemented

Context

Some datasets ship one logical file inside a single-stream compression wrapper, for example data.csv.gz or census.txt.gz. Unlike zip or tar, gzip has no member list to fan out. The useful comparison target is the decompressed byte stream with the wrapper suffix stripped so existing comparators can dispatch on the inner name.

The controller must remain type-ignorant, and transformers operate only on the completed IR with no raw data access. The input loader should not acquire format knowledge that would bypass the plugin dispatch model.

Decision

Implement single-stream gzip in binoc-stdlib as an expanding comparator, binoc.gzip, ordered after binoc.tar and before ordinary file comparators. It claims .gz, streams the decompressed bytes into a DataAccess workspace with a bounded maximum output size, strips only the final .gz suffix from the logical path, and returns one child ItemPair. The controller then re-dispatches that child normally, so data.csv.gz is compared as data.csv.

Tar remains earlier in the default order so .tar.gz continues to use the tar fan-out path. Gzip does not sniff or reinterpret decompressed content; for example, data.csv.gz redispatches as data.csv, while notes.txt.gz redispatches as notes.txt.

Alternatives Considered

Input-loader decompression. This would make gzip invisible to the comparator pipeline, but it would put format knowledge in the host boundary and make stdlib behavior special relative to third-party plugins. Rejected because it violates the type-ignorant controller/input boundary.

Pre-pass transformer. Transformers see IR after comparison and have no raw data access by design. Decompression is acquisition/parsing work, not an IR optimization pass. Rejected.

Treat gzip like zip. Zip expansion enumerates members and produces a directory-shaped subtree. Gzip has exactly one stream and no stable member path, so the correct child identity is the stripped inner filename, not a synthetic archive member list.

Sniff delimited .txt content. Some motivating .txt.gz files contain pipe-delimited tables, but identifying those tables is format-specific parsing, not compression acquisition. Rejected for this change; generic text table detection remains a separate follow-up.