# html5gum

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`html5gum` is a WHATWG-compliant HTML tokenizer.

```rust
use std::fmt::Write;
use html5gum::{Tokenizer, Token};

let html = "<title   >hello world</title>";
let mut new_html = String::new();

for Ok(token) in Tokenizer::new(html) {
    match token {
        Token::StartTag(tag) => {
            write!(new_html, "<{}>", String::from_utf8_lossy(&tag.name)).unwrap();
        }
        Token::String(hello_world) => {
            write!(new_html, "{}", String::from_utf8_lossy(&hello_world)).unwrap();
        }
        Token::EndTag(tag) => {
            write!(new_html, "</{}>", String::from_utf8_lossy(&tag.name)).unwrap();
        }
        _ => panic!("unexpected input"),
    }
}

assert_eq!(new_html, "<title>hello world</title>");
```

`html5gum` provides multiple kinds of APIs:

* Iterating over tokens as shown above.
* Implementing your own `Emitter` for maximum performance, see [the `custom_emitter.rs` example][examples/custom_emitter.rs].
* A callbacks-based API for a middleground between convenience and performance, see [the `callback_emitter.rs` example][examples/callback_emitter.rs].
* With the `tree-builder` feature, html5gum can be integrated with `html5ever` and `scraper`. See [the `scraper.rs` example][examples/scraper.rs].

## What a tokenizer does and what it does not do

`html5gum` fully implements [13.2.5 of the WHATWG HTML
spec](https://html.spec.whatwg.org/#tokenization), i.e. is able to tokenize HTML documents and passes [html5lib's tokenizer
test suite](https://github.com/html5lib/html5lib-tests/tree/master/tokenizer). Since it is just a tokenizer, this means:

* `html5gum` **does not** [implement charset
  detection.](https://html.spec.whatwg.org/#determining-the-character-encoding)
  This implementation takes and returns bytes, but assumes UTF-8. It recovers
  gracefully from invalid UTF-8.
* `html5gum` **does not** [correct mis-nested
  tags.](https://html.spec.whatwg.org/#an-introduction-to-error-handling-and-strange-cases-in-the-parser)
* `html5gum` doesn't implement the DOM, and unfortunately in the HTML spec,
  constructing the DOM ("tree construction") influences how tokenization is
  done. For an example of which problems this causes see [this example
  code][examples/tokenize_with_state_switches.rs].
* `html5gum` **does not** generally qualify as a browser-grade HTML *parser* as
  per the WHATWG spec. This can change in the future, see [issue
  21](https://github.com/untitaker/html5gum/issues/21).

With those caveats in mind, `html5gum` can pretty much ~parse~ _tokenize_
anything that browsers can. However, using the experimental `tree-builder`
feature, html5gum can be integrated with `html5ever` and `scraper`. See [the
`scraper.rs` example][examples/scraper.rs].

## Other features

* No unsafe Rust
* Only dependency is `jetscii`, and can be disabled via crate features (see `Cargo.toml`)

## Alternative HTML parsers

`html5gum` was created out of a need to parse HTML tag soup efficiently. Previous options were to:

* use [quick-xml](https://github.com/tafia/quick-xml/) or
  [xmlparser](https://github.com/RazrFalcon/xmlparser) with some hacks to make
  either one not choke on bad HTML. For some (rather large) set of HTML input
  this works well (particularly `quick-xml` can be configured to be very
  lenient about parsing errors) and parsing speed is stellar. But neither can
  parse all HTML.

  For my own usecase `html5gum` is about 2x slower than `quick-xml`.

* use [html5ever's own
  tokenizer](https://docs.rs/html5ever/0.25.1/html5ever/tokenizer/index.html)
  to avoid as much tree-building overhead as possible. This was functional but
  had poor performance for my own usecase (10-15x slower than `quick-xml`).

* use [lol-html](https://github.com/cloudflare/lol-html), which would probably
  perform at least as well as `html5gum`, but comes with a closure-based API
  that I didn't manage to get working for my usecase.

## Etymology

Why is this library called `html5gum`?

* G.U.M: **G**iant **U**nreadable **M**atch-statement

* \<insert "how it feels to <s>chew 5 gum</s> _parse HTML_" meme here\>

## License

Licensed under the MIT license, see [`./LICENSE`][LICENSE].


<!-- These link destinations are defined like this so that src/lib.rs can override them. -->
[LICENSE]: ./LICENSE
[examples/tokenize_with_state_switches.rs]: ./examples/tokenize_with_state_switches.rs
[examples/custom_emitter.rs]: ./examples/custom_emitter.rs
[examples/callback_emitter.rs]: ./examples/callback_emitter.rs
[examples/scraper.rs]: ./examples/scraper.rs