mapsize/RUST-PORT-NOTES.md

mapsize — Rust port

A port of the Go implementation (repo root) to Rust, with the same CLI, TUI, keys, themes and snapshot format. .msz snapshots are byte-compatible in both directions. The code is Linux/Unix only; the Windows code paths were not ported.

cd rust
cargo build --release          # target/release/mapsize
cargo test --release           # 69 tests (ported from the Go suites, fuzz tests excepted)

Module layout mirrors internal/: inventory, scan, platform, treemap, filter, snapshot (+ compare), duplicate, export, config, textutil, tui, app. The tree is shared as Arc<RwLock<Tree>> with the same single-writer invariants as ARCHITECTURE.md; cancel::Cancel replaces context.Context; crossterm replaces Bubble Tea as the event loop (the canvas renderer is ported as-is, plus line-diffed frame output).

Known differences from Go

• --log/--log-level are accepted but there is no debug log yet.
• A scanner thread panic restores the terminal and aborts the process instead of returning an "internal error" message.
• The colour profile is detected from NO_COLOR/TERM/COLORTERM only (no terminal query); --color still overrides.
• JSON export does not HTML-escape <>&; started is written in UTC.
• The filter lexer no longer hangs on queries containing bytes Go treats as white space but never skips (e.g. voilà, whose UTF-8 contains 0xA0).
• Huge sizes saturate rather than wrap in filters and snapshot comparison.

Benchmarks

Machine: Intel Core i7-7700 (4C/8T), 32 GiB, ext4 on md RAID over 7200 rpm HDDs. Go 1.27.1, Rust 1.91.1 (lto, codegen-units = 1). The host is shared, and its load average was 5–9 on 8 threads during the runs. Go's absolute times are close to those in PERFORMANCE.md. Go and Rust runs were interleaved; the tables report best-of (micro) or median and best (end-to-end). To re-run:

go test -c -o /tmp/tui.test ./internal/tui && go test -c -o /tmp/treemap.test ./internal/treemap
(cd rust && cargo build --release --example bench)
rust/bench/micro.py /tmp/tui.test /tmp/treemap.test rust/target/release/examples/bench 5
rust/bench/compare.py bin/mapsize rust/target/release/mapsize ~/go/pkg/mod 7

In-process (Go go test -bench suite vs examples/bench.rs)

The tree is 1,008,201 synthetic nodes, rendered at 200×60.

benchmark	Go	Rust	speed-up
treemap/Squarify(300)	84.3 µs	8.4 µs	10.07×
treemap/Neighbour(300)	2.1 µs	1.1 µs	1.94×
tui/BigColdFrame	36.01 ms	11.34 ms	3.18×
tui/BigResize	2.02 ms	1.50 ms	1.35×
tui/BigFlatDirCold	32.00 ms	9.37 ms	3.41×
tui/BigNavigate	1.50 ms	1.31 ms	1.14×
tui/BigFilter/size > 1MB	29.44 ms	26.75 ms	1.10×
tui/BigFilter/*.dat AND size > 500k	121.40 ms	67.84 ms	1.79×
tui/BigFilter/path contains sub3	101.86 ms	63.80 ms	1.60×

End to end (~/go/pkg/mod: 388k files + 65k dirs, warm cache)

scenario	Go median	Rust median	Go best	Rust best	Go RSS	Rust RSS	speed-up
scan, 1 workers	1.89 s	1.52 s	1.72 s	1.44 s	112 MB	75 MB	1.24×
scan, 8 workers	0.63 s	0.47 s	0.58 s	0.43 s	117 MB	76 MB	1.34×
scan, 32 workers	0.60 s	0.46 s	0.48 s	0.43 s	118 MB	80 MB	1.30×
scan + save snapshot	1.22 s	0.75 s	1.13 s	0.71 s	119 MB	77 MB	1.63×
load own snapshot	0.34 s	0.23 s	0.30 s	0.23 s	85 MB	78 MB	1.48×
load Go snapshot	0.36 s	0.26 s	0.30 s	0.23 s	85 MB	78 MB	1.38×
scan + JSON export	1.27 s	0.86 s	1.04 s	0.68 s	153 MB	76 MB	1.48×
scan + top 50 largest files	0.70 s	0.60 s	0.64 s	0.47 s	114 MB	76 MB	1.17×

Reading the numbers:

• Scanning is 1.2–1.3× faster at every worker count. It is syscall-bound (statx/getdents), so the gain comes from lower per-entry overhead in the controller (no GC, no per-chunk allocation churn), not from faster I/O.
• Memory: peak RSS is about a third lower (76 MB vs 117 MB while scanning 458k nodes). The JSON export halves memory (76 MB vs 153 MB) because it streams without per-node allocations.
• Snapshots: scan + save is 1.6× faster and load is 1.4–1.5× faster. The two binaries load each other's files.
• UI: cold frames are 3.2–3.4× faster, and squarified layout is about 10× faster. Navigation and resize gain less (1.1–1.35×) because they mostly reuse cached layout.
• Filters: 1.1–1.8× faster. This required caching lower-cased directory paths in a Vec with reused buffers, and a *.ext fast path in the filepath.Match port.