Rule72 – A Git Commit Formatter

4 months ago 2

Smart command-line formatter that rewraps Git commit messages while preserving structure (headline, paragraphs, nested lists, tables, code blocks, footers, emoji bullets, etc.). It reads from stdin and writes the reformatted message to stdout so it plugs into editors, Git hooks, pipes, or batch jobs.

Performance: ~2ms per commit message on a laptop ⚡.
Run just profile for detailed benchmarks across the test corpus.

Enforces 50-char headline and 72-char body width (configurable).
Understands Markdown-style bullets (*, -, numbered, emoji).
Keeps indentation, continuation alignment, fenced code, URLs, tables.
Chunk-aware – headline, body blocks, footers detected automatically.
Written in safe, fast Rust.

# Rewrap the current COMMIT_EDITMSG from a Git hook cat "$1" | rule72 > "$1.tmp" && mv "$1.tmp" "$1" # Ad-hoc from shell printf '%s\n' "fix: extremely long headline ..." | rule72 # Batch-reformat repository message corpus (Justfile target) just reflow-data # Creates data.out/ mirrored directory just compare-data # Diff original vs reflowed with colordiff/less

CLI flags:

-w, --width <N> set body wrap width (default 72) --headline-width <N> advisory headline width (default 50) --no-ansi strip colour codes before measuring width

Test-Catalogue: data/ vs data.out/

The repo ships with a large set of real-world commit messages under data/.
Running just reflow-data pipes every *.txt file through rule72, writing the result to identical relative paths under data.out/.
just compare-data opens a unified color diff so you can inspect:

Correct wrapping of long paragraphs
List continuation alignment and nested bullets
Emoji bullets retained as list markers
Code/table blocks untouched

This serves as an integration regression suite on top of unit tests.

Algorithm (bounding rectangles)

Computer-vision inspired, but works on text.

Line scan → indent matrix: count leading spaces/tabs per line.
Marching-squares-style segmentation: find rectangular regions of equal min-indent – yields a nesting tree (outer → inner).
Heuristic classification of each rectangle:
- Prose / List / Code / Table / URL.
Pretty-print per node type: greedy wrap for prose & list items, verbatim for others, mandatory blank lines enforced.
Reassemble chunks: headline + body blocks + footers.

The rectangle tree allows arbitrarily nested lists or code blocks without complex parsing.

src/ ├─ main.rs → arg-parse + stdin/stdout glue └─ lib.rs → parser, classifier, wrapper

Key crates: clap, regex, unicode-segmentation, unicode-width, anyhow.

Build tooling via Nix + Just (shell.nix, Justfile).

commitmsgfmt – Vim filter that inspired many rules; rule72 generalises with rectangle parser & Rust CLI.
fmt(1), par(1) – generic text wrappers (no commit-specific semantics).

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