markdown

Project Url: JetBrains/markdown

Introduction: Markdown parser written in kotlin

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A multiplatform Markdown processor written in Kotlin.

Introduction

intellij-markdown is an extensible Markdown processor written in Kotlin. It aims to suit the following needs:

Use one code base for both client and server-side processing
Produce consistent output on different platforms
Support different Markdown flavours
Be easily extensible

The processor is written in pure Kotlin (with a little flex), so it can be compiled not only for the JVM target, but for JS and Native. This allows for the processor to be used everywhere.

Usage

Adding `intellij-markdown` as a dependency

The library is hosted in the Maven Central Repository, so to be able to use it, you need to configure the central repository:

repositories {
  mavenCentral()
}

If you have Gradle >= 5.4, you can just add the main artifact as a dependency:

dependencies {
  implementation("org.jetbrains:markdown:<version>")
}

Gradle should resolve your target platform and decide which artifact (JVM or JS) to download.

For the multiplatform projects you can the single dependency to the commonMain class path:

commonMain {
  dependencies {
    implementation("org.jetbrains:markdown:<version>")
  }
}

If you are using Maven or older Gradle, you need to specify the correct artifact for your platform, e.g.:

org.jetbrains:markdown-jvm:<version> for the JVM version
org.jetbrains:markdown-js:<version> for the JS version

Using `intellij-markdown` for parsing and generating HTML

One of the goals of this project is to provide flexibility in terms of the tasks being solved. Markdown Plugin for JetBrains IDEs is an example of a usage when Markdown processing is done in several stages:

Parse block structure without parsing inlines to provide lazy parsable blocks for IDE
Quickly parse inlines of a given block to provide faster syntax highlighting update
Generate HTML for preview

These tasks may be completed independently according to the current needs.

Simple html generation (Kotlin)

val src = "Some *Markdown*"
val flavour = CommonMarkFlavourDescriptor()
val parsedTree = MarkdownParser(flavour).buildMarkdownTreeFromString(src)
val html = HtmlGenerator(src, parsedTree, flavour).generateHtml()

Simple html generation (Java)

final String src = "Some *Markdown*";
final MarkdownFlavourDescriptor flavour = new GFMFlavourDescriptor();
final ASTNode parsedTree = new MarkdownParser(flavour).buildMarkdownTreeFromString(text);
final String html = new HtmlGenerator(src, parsedTree, flavour, false).generateHtml();

Development gotchas

The only non-Kotlin files are .flex lexer definitions. They are used for generating lexers, which are the first stage of inline elements parsing. Unfortunately, due to bugs, native java->kt conversion crashes for these files. Because of that, conversion from .flex to respective Kotlin files requires some manual steps:

Install Grammar Kit plugin. It should be suggested on the opening of any .flex file.
Install jflexToKotlin plugin (you will need to build it and then install it manually, via settings).
Run Run JFlex Generator action while having .flex file opened.
- On the first run, a dialog will open, suggesting to place to download JFlex - select the project root, then delete excessively downloaded .skeleton file.
A respective _<SomeName>Lexer.java will be generated somewhere. Move it near the existing _<SomeName>Lexer.kt.
Delete the .kt lexer.
Run Convert JFlex Lexer to Kotlin action while having the new .java file opened.
Fix the small problems such as imports in the generated .kt file. There should be no major issues. Please try to minimize the number of changes to the generated files. This is needed for keeping a clean Git history.

Parsing algorithm

The parsing process is held in two logical parts:

Splitting the document into blocks of logical structure (lists, blockquotes, paragraphs, etc.)
Parsing the inline structure of the resulting blocks

This is the same way as the one being proposed by the Commonmark spec.

Building the logical structure

Each (future) node (list, list item, blockquote, etc.) is associated with the so-called MarkerBlock. The rollback-free parsing algorithm is processing every token in the file, one by one. Tokens are passed to the opened marker blocks, and each block chooses whether to either:

do nothing
drop itself
complete itself

The MarkerProcessor stores the blocks, executes the actions chosen by the blocks, and, possibly, adds some new ones.

Parsing inlines

For the sake of speed and parsing convenience, the text is passed to the MarkdownLexer first. Then the resulting set of tokens is processed in a special way.

Some inline constructs in Markdown have priorities, i.e., if two different ones overlap, the parsing result depends on their types, not their positions - e.g. *code, `not* emph` and `code, *not` emph* are both code spans + literal asterisks. This means that normal recursive parsing is inapplicable.

Still, the parsing of inline elements is quite straightforward. For each inline construct, there is a particular SequentialParser which accepts some input text and returns:

The parsed ranges found in this text;
The sub-text(s), which are to be passed to the subsequent inline parsers.

Building AST

After building the logical structure and parsing inline elements, a set of ranges corresponding to some markdown entities (i.e. nodes) is given. In order to work with the results effectively, it ought to be converted to the [AST].

As a result, a root ASTNode corresponding to the parsed Markdown document is returned. Each AST node has its own type which is called IElementType as in the IntelliJ Platform.

Generating HTML

For a given AST root, a special [visitor][visitor-pattern] to generate the resulting HTML is created. Using a given mapping from IElementType to the GeneratingProvider it processes the parsed tree in [Depth-First order][DFS], generating HTML pieces for on each node visit.

Extending the parser

Many routines in the above process can be extended or redefined by creating a different Markdown flavour. The minimal default flavour is CommonMark which is implemented in this project.

GitHub Flavoured Markdown is an example of extending CommonMark flavour implementation. It can be used as a reference for implementing your own Markdown features.

API

MarkdownFlavourDescriptor is a base class for extending the Markdown parser.
- markerProcessorFactory is responsible for block structure customization.
  - stateInfo value allows to use a state during document parsing procedure.
    
    updateStateInfo(pos: LookaheadText.Position) is called at the beginning of each position processing
  - populateConstraintsTokens is called to create nodes for block structure markers at the beginning of the lines (for example, > characters constituting blockquotes)
  - getMarkerBlockProviders is a place to (re)define types of block structures
- sequentialParserManager
  
  getParserSequence defines inlines parsing procedure. The method must return a list of SequentialParsers where the earliest parsers have the biggest operation precedence. For example, to parse code spans and emphasis elements with the correct priority, the list should be [CodeSpanParser, EmphParser] but not the opposite.
  
  SequentialParser has only one method:
  
  parse(tokens: TokensCache, rangesToGlue: List<IntRange>): ParsingResult
  - tokens is a special holder for the tokens returned by lexer
  - rangesToGlue is a list of ranges in the document which are to be searched for the structures in question.
    
    Considering the input: A * emph `code * span` b * c for the emph parser ranges [A * emph, b * c] mean that emph must be searched in the input A * emph | b * c.
    
    The method must essentially return the parsing result (nodes for the found structures) and the parts of the text to be given to the next parsers.
    
    Considering the same input for the code span parser the result would be `code * span` of the type "code span" and the delegate pieces would be [A * emph, b * c].
- createInlinesLexer should return the lexer to split the text to the tokens before inline parsing procedure run.
- createHtmlGeneratingProviders(linkMap: LinkMap, baseURI: URI?) is the place where generated HTML is customized. This method should return a map which defines how to handle the particular kinds of the nodes in the resulting tree.
  
  linkMap here is precalculated information about the links defined in the document with the means of link definition. baseURI is the URI to be considered the base path for the relative links resolving. For example, given baseUri='/user/repo-name/blob/master' the link foo/bar.png should be transformed to the /user/repo-name/blob/master/foo/bar.png.
  
  Each returned provider must implement processNode(visitor: HtmlGenerator.HtmlGeneratingVisitor, text: String, node: ASTNode) where
  - text is the whole document being processed,
  - node is the node being given to the provider,
  - visitor is a special object responsible for the HTML generation. See GeneratingProviders.kt for the samples.