vendor/pulldown-cmark/README.md

   1 # pulldown-cmark
   2
   3 [![Build Status](https://dev.azure.com/raphlinus/pulldown-cmark/_apis/build/status/pulldown-cmark-CI?branchName=master)](https://dev.azure.com/raphlinus/pulldown-cmark/_build/latest?definitionId=2&branchName=master)
   4 [![Docs](https://docs.rs/pulldown-cmark/badge.svg)](https://docs.rs/pulldown-cmark)
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   6
   7 [Documentation](https://docs.rs/pulldown-cmark/)
   8
   9 This library is a pull parser for [CommonMark](http://commonmark.org/), written
  10 in [Rust](http://www.rust-lang.org/). It comes with a simple command-line tool,
  11 useful for rendering to HTML, and is also designed to be easy to use from as
  12 a library.
  13
  14 It is designed to be:
  15
  16 * Fast; a bare minimum of allocation and copying
  17 * Safe; written in pure Rust with no unsafe blocks
  18 * Versatile; in particular source-maps are supported
  19 * Correct; the goal is 100% compliance with the [CommonMark spec](http://spec.commonmark.org/)
  20
  21 Further, it optionally supports parsing footnotes,
  22 [Github flavored tables](https://github.github.com/gfm/#tables-extension-),
  23 [Github flavored task lists](https://github.github.com/gfm/#task-list-items-extension-) and
  24 [strikethrough](https://github.github.com/gfm/#strikethrough-extension-).
  25
  26 ## Why a pull parser?
  27
  28 There are many parsers for Markdown and its variants, but to my knowledge none
  29 use pull parsing. Pull parsing has become popular for XML, especially for
  30 memory-conscious applications, because it uses dramatically less memory than
  31 constructing a document tree, but is much easier to use than push parsers. Push
  32 parsers are notoriously difficult to use, and also often error-prone because of
  33 the need for user to delicately juggle state in a series of callbacks.
  34
  35 In a clean design, the parsing and rendering stages are neatly separated, but
  36 this is often sacrificed in the name of performance and expedience. Many Markdown
  37 implementations mix parsing and rendering together, and even designs that try
  38 to separate them (such as the popular [hoedown](https://github.com/hoedown/hoedown)),
  39 make the assumption that the rendering process can be fully represented as a
  40 serialized string.
  41
  42 Pull parsing is in some sense the most versatile architecture. It's possible to
  43 drive a push interface, also with minimal memory, and quite straightforward to
  44 construct an AST. Another advantage is that source-map information (the mapping
  45 between parsed blocks and offsets within the source text) is readily available;
  46 you can call `into_offset_iter()` to create an iterator that yields `(Event, Range)`
  47 pairs, where the second element is the event's corresponding range in the source
  48 document.
  49
  50 While manipulating ASTs is the most flexible way to transform documents,
  51 operating on iterators is surprisingly easy, and quite efficient. Here, for
  52 example, is the code to transform soft line breaks into hard breaks:
  53
  54 ```rust
  55 let parser = parser.map(|event| match event {
  56         Event::SoftBreak => Event::HardBreak,
  57         _ => event
  58 });
  59 ```
  60
  61 Or expanding an abbreviation in text:
  62
  63 ```rust
  64 let parser = parser.map(|event| match event {
  65         Event::Text(text) => Event::Text(text.replace("abbr", "abbreviation").into()),
  66         _ => event
  67 });
  68 ```
  69
  70 Another simple example is code to determine the max nesting level:
  71
  72 ```rust
  73 let mut max_nesting = 0;
  74 let mut level = 0;
  75 for event in parser {
  76         match event {
  77                 Event::Start(_) => {
  78                         level += 1;
  79                         max_nesting = std::cmp::max(max_nesting, level);
  80                 }
  81                 Event::End(_) => level -= 1,
  82                 _ => ()
  83         }
  84 }
  85 ```
  86
  87 There are some basic but fully functional examples of the usage of the crate in the
  88 `examples` directory of this repository.
  89
  90 ## Using Rust idiomatically
  91
  92 A lot of the internal scanning code is written at a pretty low level (it
  93 pretty much scans byte patterns for the bits of syntax), but the external
  94 interface is designed to be idiomatic Rust.
  95
  96 Pull parsers are at heart an iterator of events (start and end tags, text,
  97 and other bits and pieces). The parser data structure implements the
  98 Rust Iterator trait directly, and Event is an enum. Thus, you can use the
  99 full power and expressivity of Rust's iterator infrastructure, including
 100 for loops and `map` (as in the examples above), collecting the events into
 101 a vector (for recording, playback, and manipulation), and more.
 102
 103 Further, the `Text` event (representing text) is a small copy-on-write string.
 104 The vast majority of text fragments are just
 105 slices of the source document. For these, copy-on-write gives a convenient
 106 representation that requires no allocation or copying, but allocated
 107 strings are available when they're needed. Thus, when rendering text to
 108 HTML, most text is copied just once, from the source document to the
 109 HTML buffer.
 110
 111 When using the pulldown-cmark's own HTML renderer, make sure to write to a buffered
 112 target like a `Vec<u8>` or `String`. Since it performs many (very) small writes, writing
 113 directly to stdout, files, or sockets is detrimental to performance. Such writers can
 114 be wrapped in a [`BufWriter`](https://doc.rust-lang.org/std/io/struct.BufWriter.html).
 115
 116 ## Build options
 117
 118 By default, the binary is built as well. If you don't want/need it, then build like this:
 119
 120 ```bash
 121 > cargo build --no-default-features
 122 ```
 123
 124 Or put in your `Cargo.toml` file:
 125
 126 ```toml
 127 pulldown-cmark = { version = "0.5", default-features = false }
 128 ```
 129
 130 SIMD accelerated scanners are available for the x64 platform from version 0.5 onwards. To
 131 enable them, build with simd feature:
 132
 133 ```bash
 134 > cargo build --release --features simd
 135 ```
 136
 137 Or add the feature to your project's `Cargo.toml`:
 138
 139 ```toml
 140 pulldown-cmark = { version = "0.5", default-features = false, features = ["simd"] }
 141 ```
 142
 143 ## Authors
 144
 145 The main author is Raph Levien. The implementation of the new design (v0.3+) was completed by Marcus Klaas de Vries.
 146
 147 ## Contributions
 148
 149 We gladly accept contributions via GitHub pull requests. Please see
 150 [CONTRIBUTING.md](CONTRIBUTING.md) for more details.