---
id: c8829454-03db-4c42-bb71-3c47adfadbfe
title: 'highlight: O(n²) per-char suffix allocation makes builds take minutes'
state: closed
labels:
- kind:build
assignees: []
milestone: null
created: 2026-10-27T17:24:52.311886997Z
updated: 2026-10-27T17:45:41.596372078Z
---
## What

`src/highlight.rs:252` rebuilds the rest of the line for **every character**:

```rust
let rest: String = chars[i..].iter().collect();
```

That is O(n²) allocation/copy per line, and `highlight_lines` runs for every text file up to `MAX_HIGHLIGHT_BYTES = 512 KiB`.

## Reproduced

Repo with one 64 KB single-line JS file (`'var x=1;' * 8000`):

```
hu13 build /tmp/hlprobe -o /tmp/hlout
real 0m27.5s
```

Quadratic scaling means a 512 KiB minified file can take ~30 minutes; several such files stall a build.

## Impact

Build-time DoS / unacceptable builds on ordinary minified assets. `MAX_HIGHLIGHT_BYTES` does not protect because the cost is per line.

## Fix

- Scan on byte/char indices of `&str` (or one `Vec<char>` per line) without building suffix `String`s.
- Consider capping a single line (e.g. 8 KiB) to plain text; keep whole-file cap.
- Add a timing/scaling unit test with a 64 KB single-line fixture (coarse bound, not wall-clock tight).