flexmark-java

flexmark-java is a Java implementation of CommonMark 0.27 spec parser using the blocks first, inlines after Markdown parsing architecture.

Its strengths are speed, flexibility, Markdown source element based AST with details of the source position down to individual characters of lexemes that make up the element and extensibility.

The API allows granular control of the parsing process and is optimized for parsing with a large number of installed extensions. The parser and extensions come with plenty of options for parser behavior and HTML rendering variations. The end goal is to have the parser and renderer be able to mimic other parsers with great degree of accuracy. This is now partially complete with the implementation of Markdown Processor Family Emulation

Motivation for this project was the need to replace pegdown parser in my Markdown Navigator plugin for JetBrains IDEs. pegdown has a great feature set but its speed in general is less than ideal and for pathological input either hangs or practically hangs during parsing.

Requirements

Java 7 or above
Android compatibility to be added
The project is on Maven: com.vladsch.flexmark
The core has no dependencies; for extensions, see below

The API is still evolving to accommodate new extensions and functionality.

Quick Start

You need to add flexmark-all as a dependency which includes core and all modules to the following sample:

<dependency>
    <groupId>com.vladsch.flexmark</groupId>
    <artifactId>flexmark-all</artifactId>
    <version>0.19.0</version>
</dependency>

Source: BasicSample.java

package com.vladsch.flexmark.samples;

import com.vladsch.flexmark.ast.Node;
import com.vladsch.flexmark.ext.gfm.strikethrough.StrikethroughExtension;
import com.vladsch.flexmark.ext.tables.TablesExtension;
import com.vladsch.flexmark.html.HtmlRenderer;
import com.vladsch.flexmark.parser.Parser;
import com.vladsch.flexmark.util.options.MutableDataSet;

import java.util.Arrays;

public class BasicSample {
    public static void main(String[] args) {
        MutableDataSet options = new MutableDataSet();

        // uncomment to set optional extensions
        //options.set(Parser.EXTENSIONS, Arrays.asList(TablesExtension.create(), StrikethroughExtension.create()));

        // uncomment to convert soft-breaks to hard breaks
        //options.set(HtmlRenderer.SOFT_BREAK, "<br />\n");

        Parser parser = Parser.builder(options).build();
        HtmlRenderer renderer = HtmlRenderer.builder(options).build();

        // You can re-use parser and renderer instances
        Node document = parser.parse("This is *Sparta*");
        String html = renderer.render(document);  // "<p>This is <em>Sparta</em></p>\n"
        System.out.println(html);
    }
}

More information can be found in the documentation:
Wiki Home
    Usage Examples
    Extension Details
    Writing Extensions

Pegdown Migration Helper

PegdownOptionsAdapter class converts pegdown Extensions.* flags to flexmark options and extensions list. Pegdown Extensions.java is included for convenience and new options not found in pegdown 1.6.0. These are located in flexmark-profile-pegdown module but you can grab the source from this repo: PegdownOptionsAdapter.java, Extensions.java and make your own version, modified to your project's needs.

You can pass your extension flags to static PegdownOptionsAdapter.flexmarkOptions(int) or you can instantiate PegdownOptionsAdapter and use convenience methods to set, add and remove extension flags. PegdownOptionsAdapter.getFlexmarkOptions() will return a fresh copy of DataHolder every time with the options reflecting pegdown extension flags.

import com.vladsch.flexmark.html.HtmlRenderer;
import com.vladsch.flexmark.parser.Parser;
import com.vladsch.flexmark.profiles.pegdown.Extensions;
import com.vladsch.flexmark.profiles.pegdown.PegdownOptionsAdapter;
import com.vladsch.flexmark.util.options.DataHolder;

public class PegdownOptions {
    static final DataHolder OPTIONS = PegdownOptionsAdapter.flexmarkOptions(
            Extensions.ALL
    );

    static final Parser PARSER = Parser.builder(OPTIONS).build();
    static final HtmlRenderer RENDERER = HtmlRenderer.builder(OPTIONS).build();

    // use the PARSER to parse and RENDERER to render with pegdown compatibility
}

A sample with a custom link resolver is also available.

ℹ️ flexmark-java has many more extensions and configuration options than pegdown in addition to extensions available in pegdown 1.6.0. Available Extensions via PegdownOptionsAdapter

Latest Additions

flexmark-all module that includes: core, all extensions, formatter, JIRA and YouTrack converters, pegdown profile module and HTML to Markdown conversion.
PDF converter module Usage: PDF Output using Open HTML To PDF
Typographic Extension Module implemented
XWiki Macros Extension Module
Jekyll tags Extension Module
HTML to Markdown Converter Module
Maven Markdown Page Generator Plugin
Markdown Formatter module to output AST as markdown with formatting options.

Table Extension for Markdown Formatter with column width and alignment of markdown tables:

Input Output

Input	Output
`day\|time\|spent :---\|:---:\|--: nov. 2. tue\|10:00\|4h 40m nov. 3. thu\|11:00\|4h nov. 7. mon\|10:20\|4h 20m total:\|\| 13h`	`\| day \| time \| spent \| \|:------------\|:-----:\|--------:\| \| nov. 2. tue \| 10:00 \| 4h 40m \| \| nov. 3. thu \| 11:00 \| 4h \| \| nov. 7. mon \| 10:20 \| 4h 20m \| \| total: \|\| 13h \|`

day|time|spent
:---|:---:|--:
nov. 2. tue|10:00|4h 40m
nov. 3. thu|11:00|4h
nov. 7. mon|10:20|4h 20m
total:|| 13h

| day         | time  |   spent |
|:------------|:-----:|--------:|
| nov. 2. tue | 10:00 |  4h 40m |
| nov. 3. thu | 11:00 |      4h |
| nov. 7. mon | 10:20 |  4h 20m |
| total:             ||     13h |

Releases, Bug Fixes, Enhancements and Support

I use flexmark-java as the parser for Markdown Navigator plugin for JetBrains IDEs. I tend to use the latest, unreleased version to fix bugs or get improvements. So if you find a bug that is a show stopper for your project or see a bug in github issues page marked fixed for next release that is affecting your project then please let me know and I may be able to promptly make a new release to address your issue. Otherwise, I will let bug fixes and enhancements accumulate thinking no one is affected by what is already fixed.

Extension points in the API are many and numerous

There are many extension options in the API with their intended use. If your extension lines up with the right API, the task is usually very short and sweet. If your extension uses the API in an unintended fashion or does not follow expected housekeeping protocols, you may find it an uphill battle with a rat's nest of if/else condition handling and fixing one bug only leading to creating another one.

Generally, if it takes more than a few dozen lines to add a simple extension, then either you are going about it wrong or the API is missing an extension point. If you look at all the implemented extensions you will see that most are a few lines of code other than boiler plate dictated by the API. That is the goal for this library: provide an extensible core that makes writing extensions a breeze.

The larger extensions are flexmark-ext-tables and flexmark-ext-spec-example, the meat of both is around 200 lines of code. You can use them as a guide post for size estimating your extension.

My own experience adding extensions shows that sometimes a new type of extension is best addressed with an API enhancement to make its implementation seamless, or a by fixing a bug that was not visible before the extension excersized the API in just the right way. Your intended extension may just be the one requiring such an approach.

Don't hesitate to open an issue if you can't find the answer

The takeaway is: if you want to implement an extension or a feature please don't hesitate to open an issue and I will give you pointers on the best way to go about it. It may save you a lot of time by letting me improve the API to address your extension's needs before you put a lot of fruitless effort into it.

I do ask that you realize that I am chief cook and bottle washer on this project, without an iota of Vulcan Mind Melding skills. I do ask that you describe what you want to implement because I can't read your mind and do some reconaissance background work around the source code and documentation because I cannot transfer what I know to you, without your willing effort.

Consulting is available

If you have a commercial application and don't want to write the extension(s) yourself or want to reduce the time and effort of implementing extensions and integrating flexmark-java, feel free to contact me. I am available on a consulting/contracting basis, All about me.

Markdown Processor Emulation

Despite its name, commonmark is neither a superset nor a subset of other markdown flavors. Rather, it proposes a standard, unambiguous syntax specification for the original, "core" Markdown, thus effectively introducing yet another flavor. While flexmark is by default commonmark compliant, its parser can be tweaked in various ways. The sets of tweaks required to emulate the most commonly used markdown parsers around are available in flexmark as ParserEmulationProfiles.

As the name ParserEmulationProfile implies, it's only the parser that is adjusted to the specific markdown flavor. Applying the profile does not add features beyond those available in commonmark. If you want to use flexmark to fully emulate another markdown processor's behavior, you have to adjust the parser and configure the flexmark extensions that provide the additional features available in the parser that you want to emulate.

Latest addition was a rewrite of the list parser to better control emulation of other markdown processors as per Markdown Processors Emulation. Addition of processor presets to emulate specific markdown processing behaviour of these parsers is on a short to do list.

Some emulation families do a better better job of emulating their target than others. Most of the effort was directed at emulating how these processors parse standard Markdown and list related parsing specifically. For processors that extend original Markdown, you will need to add those extensions that are already implemented in flexmark-java to the Parser/Renderer builder options.

Extensions will be modified to include their own presets for specific processor emulation, if that processor has an equivalent extension implemented.

If you find a discrepancy please open an issue so it can be addressed.

Major processor families are implemented and some family members also:

CommonMark (spec 0.27)
- League/CommonMark
- GitHub Comments
Kramdown
- Jekyll
Markdown.pl
- Php Markdown Extra
- GitHub Docs
FixedIndent
- MultiMarkdown
- Pegdown

ℹ️ profiles to encapsulate configuration details for variants within the family were added in 0.11.0:

CommonMark (default for family): ParserEmulationProfile.COMMONMARK
FixedIndent (default for family): ParserEmulationProfile.FIXED_INDENT
GitHub Comments (just CommonMark): ParserEmulationProfile.COMMONMARK
GitHub Docs: ParserEmulationProfile.GITHUB_DOC
Kramdown (default for family): ParserEmulationProfile.KRAMDOWN
Markdown.pl (default for family): ParserEmulationProfile.MARKDOWN
MultiMarkdown: ParserEmulationProfile.MULTI_MARKDOWN
Pegdown, with pegdown extensions use PegdownOptionsAdapter in flexmark-profile-pegdown
Pegdown, without pegdown extensions ParserEmulationProfile.PEGDOWN

History and Motivation

flexmark-java is a fork of commonmark-java project, modified to generate an AST which reflects all the elements in the original source, full source position tracking for all elements in the AST and easier JetBrains Open API PsiTree generation.

The API was changed to allow more granular control of the parsing process and optimized for parsing with a large number of installed extensions. The parser and extensions come with many tweaking options for parser behavior and HTML rendering variations. The end goal is to have the parser and renderer be able to mimic other parsers with great degree of accuracy.

Motivation for this was the need to replace pegdown parser in Markdown Navigator plugin. pegdown has a great feature set but its speed in general is less than ideal and for pathological input either hangs or practically hangs during parsing.

commonmark-java has an excellent parsing architecture that is easy to understand and extend. The goal was to ensure that adding source position tracking in the AST would not change the ease of parsing and generating the AST more than absolutely necessary.

Reasons for choosing commonmark-java as the parser are: speed, ease of understanding, ease of extending and speed. More detailed description in Pegdown - Achilles heel of the Markdown Navigator plugin. Now that I have reworked the core and added a few extensions I am extremely satisfied with my choice.

Another goal was to improve the ability of extensions to modify parser behavior so that any dialect of markdown could be implemented through the extension mechanism. An extensible options API was added to allow setting of all options in one place. Parser, renderer and extensions use these options for configuration, including disabling some core block parsers.

This is a work in progress with many API changes. No attempt is made to keep backward API compatibility to the original project and until the feature set is mostly complete, not even to earlier versions of this project.

Feature Comparison

Feature	flexmark-java	commmonmark-java	pegdown
Relative parse time (less is better)	✔️ 1x (1)	✔️ 0.6x to 0.7x (2)	❌ 25x average, 20,000x to ∞ for pathological input (3)
All source elements in the AST	✔️	❌	✔️
AST elements with source position	✔️	✔️	✔️ with some errors and idiosyncrasies
AST can be easily manipulated	✔️ AST post processing is an extension mechanism	✔️ AST post processing is an extension mechanism	❌ not an option. No node's parent information, children as List<>.
AST elements have detailed source position for all parts	✔️	❌	❌ only node start/end
Can disable core parsing features	✔️	❌	❌
Core parser implemented via the extension API	✔️	❌ `instanceOf` tests for specific block parser and node classes	❌ core exposes few extension points
Easy to understand and modify parser implementation	✔️	✔️	❌ one PEG parser with complex interactions (3)
Parsing of block elements is independent from each other	✔️	✔️	❌ everything in one PEG grammar
Uniform configuration across: parser, renderer and all extensions	✔️	❌ none beyond extension list	❌ `int` bit flags for core, none for extensions
Parsing performance optimized for use with extensions	✔️	❌ parsing performance for core, extensions do what they can	❌ performance is not a feature
Feature rich with many configuration options and extensions out of the box	✔️	❌ limited extensions, no options	✔️
Dependency definitions for processors to guarantee the right order of processing	✔️	❌ order specified by extension list ordering, error prone	❌ not applicable, core defines where extension processing is added

(1)

flexmark-java pathological input of 100,000 [ parses in 68ms, 100,000 ] in 57ms, 100,000 nested [ ] parse in 55ms

(2)

commonmark-java pathological input of 100,000 [ parses in 30ms, 100,000 ] in 30ms, 100,000 nested [ ] parse in 43ms

(3)

pegdown pathological input of 17 [ parses in 650ms, 18 [ in 1300ms

Progress

Parser options, items marked as a task item are to be implemented the rest are complete:
- Typographic
  - Quotes
  - Smarts
- GitHub Extensions
  - Fenced code blocks
  - Anchor links for headers with auto id generation
  - Table Spans option to be implemented for tables extension
  - Wiki Links with GitHub and Creole syntax
  - Emoji Shortcuts with use GitHub emoji URL option
- GitHub Syntax
  - Strikethrough
  - Task Lists
  - No Atx Header Space
  - No Header indents
  - Hard Wraps (achieved with SOFT_BREAK option changed to "<br />")
  - Relaxed HR Rules Option
  - Wiki links
- Publishing
  - Abbreviations
  - Footnotes
  - Definitions
  - Table of Contents
- Suppress
  - inline HTML: all, non-comments, comments
  - HTML blocks: all, non-comments, comments
- Processor Extensions
  - Jekyll front matter
  - Jekyll tag elements, with support for {% include file %}, Include Markdown and HTML File Content
  - GitBook link URL encoding. Not applicable
  - HTML comment nodes: Block and Inline
  - Multi-line Image URLs
  - Spec Example Element
- Commonmark Syntax suppression
  - Manual loose lists
  - Numbered lists always start with 1.
  - Fixed list item indent, items must be indented by at least 4 spaces
  - Relaxed list start option, allow lists to start when not preceded by a blank line.

I am very pleased with the decision to switch to commonmark-java based parser for my own projects. Even though I had to do major surgery on its innards to get full source position tracking and AST that matches source elements, it is a pleasure to work with and is now a pleasure to extend. If you don't need source level element AST or the rest of what flexmark-java added and CommonMark is your target markdown parser then I encourage you to use commonmark-java as it is an excellent choice for your needs and its performance does not suffer for the overhead of features that you will not use.

Benchmarks

Latest, Jan 28, 2017 flexmark-java 0.13.1, intellij-markdown from CE EAP 2017, commonmark-java 0.8.0:

File	commonmark-java	flexmark-java	intellij-markdown	pegdown
README-SLOW	0.420ms	0.812ms	2.027ms	15.483ms
VERSION	0.743ms	1.425ms	4.057ms	42.936ms
commonMarkSpec	31.025ms	44.465ms	600.654ms	575.131ms
markdown_example	8.490ms	10.502ms	223.593ms	983.640ms
spec	4.719ms	6.249ms	35.883ms	307.176ms
table	0.229ms	0.623ms	0.800ms	3.642ms
table-format	1.385ms	2.881ms	4.150ms	23.592ms
wrap	3.804ms	4.589ms	16.609ms	86.383ms

Ratios of above:

File	commonmark-java	flexmark-java	intellij-markdown	pegdown
README-SLOW	1.00	1.93	4.83	36.88
VERSION	1.00	1.92	5.46	57.78
commonMarkSpec	1.00	1.43	19.36	18.54
markdown_example	1.00	1.24	26.34	115.86
spec	1.00	1.32	7.60	65.09
table	1.00	2.72	3.49	15.90
table-format	1.00	2.08	3.00	17.03
wrap	1.00	1.21	4.37	22.71
overall	1.00	1.41	17.47	40.11

File	commonmark-java	flexmark-java	intellij-markdown	pegdown
README-SLOW	0.52	1.00	2.50	19.07
VERSION	0.52	1.00	2.85	30.12
commonMarkSpec	0.70	1.00	13.51	12.93
markdown_example	0.81	1.00	21.29	93.66
spec	0.76	1.00	5.74	49.15
table	0.37	1.00	1.28	5.85
table-format	0.48	1.00	1.44	8.19
wrap	0.83	1.00	3.62	18.83
overall	0.71	1.00	12.41	28.48

Because these two files represent the pathological input for pegdown, I no longer run them as part of the benchmark to prevent skewing of the results. The results are here for posterity.

File	commonmark-java	flexmark-java	intellij-markdown	pegdown
hang-pegdown	0.082ms	0.326ms	0.342ms	659.138ms
hang-pegdown2	0.048ms	0.235ms	0.198ms	1312.944ms