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JetBrains / markdown

Markdown parser written in kotlin
Apache License 2.0
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intellij-markdown official JetBrains project Maven Central IR

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:

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.:

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:

  1. Parse block structure without parsing inlines to provide lazy parsable blocks for IDE
  2. Quickly parse inlines of a given block to provide faster syntax highlighting update
  3. 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:

  1. Install Grammar Kit plugin. It should be suggested on the opening of any .flex file.
  2. Install jflexToKotlin plugin (you will need to build it and then install it manually, via settings).
  3. 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.
  4. A respective _<SomeName>Lexer.java will be generated somewhere. Move it near the existing _<SomeName>Lexer.kt.
  5. Delete the .kt lexer.
  6. Run Convert JFlex Lexer to Kotlin action while having the new .java file opened.
  7. 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:

  1. Splitting the document into blocks of logical structure (lists, blockquotes, paragraphs, etc.)
  2. 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:

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:

  1. The parsed ranges found in this text;
  2. 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 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, 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