# pulldown-cmark

This library is a pull parser for [CommonMark](http://commonmark.org/), written
in [Rust](http://www.rust-lang.org/). It comes with a simple command-line tool,
useful for rendering to HTML, and is also designed to be easy to use from as
a library.

It is designed to be:

* Fast; a bare minimum of allocation and copying
* Safe; written in pure Rust with no unsafe blocks
* Versatile; in particular source-maps are supported
* Correct; the goal is 100% compliance with the [CommonMark spec](http://spec.commonmark.org/)

## Why a pull parser?

There are many parsers for Markdown and its variants, but to my knowledge none
use pull parsing. Pull parsing has become popular for XML, especially for
memory-conscious applications, because it uses dramatically less memory than
construcing a document tree, but is much easier to use than push parsers. Push
parsers are notoriously difficult to use, and also often error-prone because of
the need for user to delicately juggle state in a series of callbacks.

In a clean design, the parsing and rendering stages are neatly separated, but
this is often sacrificed in the name of performance and expedience. Many Markdown
implementations mix parsing and rendering together, and even designs that try
to separate them (such as the popular [hoedown](https://github.com/hoedown/hoedown)),
make the assumption that the rendering process can be fully represented as a
serialized string.

Pull parsing is in some sense the most versatile architecture. It's possible to
drive a push interface, also with minimal memory, and quite straightforward to
construct an AST. Another advantage is that source-map information (the mapping
between parsed blocks and offsets within the source text) is readily available;
you basically just call `get_offset()` as you consume events.

While manipulating AST's is the most flexible way to transform documents,
operating on iterators is surprisingly easy, and quite efficient. Here, for
example, is the code to transform soft line breaks into hard breaks:

```rust
let parser = parser.map(|event| match event {
	Event::SoftBreak => Event::HardBreak,
	_ => event
});
```

Or expanding an abbreviation in text:

```rust
let parser = parser.map(|event| match event {
	Event::Str(text) => Event::Str(text.replace("abbr", "abbreviation")),
	_ => event
});
```

Another simple example is code to determine the max nesting level:

```rust
let mut max_nesting = 0;
let mut level = 0;
for event in parser {
	match event {
		Event::Start(_) => {
			level += 1;
			max_nesting = std::cmp::max(max_nesting, level);
		}
		Event::End(_) => level -= 1,
		_ => ()
	}
}
```

## Using Rust idiomatically

A lot of the internal scanning code is written at a pretty low level (it
pretty much scans byte patterns for the bits of syntax), but the external
interface is designed to be idiomatic Rust.

Pull parsers are at heart an iterator of events (start and end tags, text,
and other bits and pieces). The parser data structure implements the
Rust Iterator trait directly, and Event is an enum. Thus, you can use the
full power and expressivity of Rust's iterator infrastructure, including
for loops and `map` (as in the examples above), collecting the events into
a vector (for recording, playback, and manipulation), and more.

Further, the Str event (representing text) is a copy-on-write string (note:
this isn't quite true yet). The vast majority of text fragments are just
slices of the source document. For these, copy-on-write gives a convenient
representation that requires no allocation or copying, but allocated
strings are available when they're needed. Thus, when rendering text to
HTML, most text is copied just once, from the source document to the
HTML buffer.

## Authors

The main author is Raph Levien.

## Contributions

We gladly accept contributions via GitHub pull requests, as long as the author
has signed the Google Contributor License. Please see CONTRIBUTIONS.md for
more details.

### Disclaimer

This is not an official Google product (experimental or otherwise), it
is just code that happens to be owned by Google.
