Railroad Diagrams in Redmine Wiki

I recently needed to show the grammar of our XText based UML guard and action language in the Azmun Wiki. I decided to use railroad diagrams for that purpose, since I remembered that the XText documentation contains such cool diagrams in the MWE2 sub-chapter. For example, here is the railroad diagram for Module definitions:



I don't know how the XText documentation is generated, but I know that the rail package for LaTeX is able to generate such diagarams. So I wanted to integrate such diagrams in our wiki using rail.

We host all the projects of our research training group METRIK in a Redmine instance. One of the Redmine Plug-Ins is the Wiki External Filter, which allows defining macros that process macro argument using external filter program and render its result in Redmine wiki. The Plug-In is shipped with support for PlantUML to draw UML diagrams, Graphviz for abritrary diagrams, ritex for MathML, and ffmpeg to embed videos.

In order to add a new filter, I extended the redmine/config/wiki_external_filter.yml file with the following entry:

rail:
    description: "Constructs railroad diagrams for (E)BNF grammars, see http://notendur.hi.is/snorri/091263/rail/rail.html"
    template: image
    outputs:
      - command: "SOME_PATH/rail.sh"
        content_type: "image/png"
        prolog: "\documentclass{article} \n \usepackage{rail} \n \pagestyle{empty} \n \\begin{document} \n \\begin{figure} \n"
        epilog: "\n \\end{figure} \n \\end{document} \n"

Our filter is named rail and calls the shell script rail.sh, which I will show in a second. It defines the needed prolog and epilog LaTeX commands including the usage of the rail package and the definition of a figure, so that the user only needs to specify the rail commands.

Here now the code of the rail.sh shell script:

#!/bin/sh

# pipe STDIN to file
cat - > input.tex

# run first time with latex
latex input.tex 1> /dev/null 2> /dev/null

# run rail
rail input 1> /dev/null 2> /dev/null

# run second time with latex
latex input.tex 1> /dev/null 2> /dev/null

# convert DVI file to PNG
dvipng -q -Ttight -M -pp1 --noghostscript -D150 -o out.png input.dvi 1> /dev/null 2> /dev/null

# remove temporary files
rm input.*

# pipe contents of PNG to STDOUT
cat out.png -

This script has following prerequesites:

• LaTeX installation
• compiled version of the rail binary
• dvipng Installation

(Note that there are precompiled Debian and RPM packages for LaTeX and dvipng available.)

Having all the pieces together, we now can use rail scripts to create nice railroad diagarams in Redmine. Here is an example taken from Azmun:

{{rail(
\railalias{IMPLIES}{->}
\railalias{EQUIVALENCE}{<>}
\railalias{OR}{||}
\railalias{XOR}{\textasciicircum}
\railalias{AND}{\&\&}
\railalias{EQ}{==}
\railalias{NEQ}{!=}
\railalias{LT}{<}
\railalias{GT}{>}
\railalias{LTE}{<=}
\railalias{GTE}{>=}
\railalias{SHIFTLEFT}{<<}
\railalias{SHIFTRIGHT}{>>}
\railalias{MUL}{*}
\railalias{DIV}{/}
\railalias{MOD}{\%}
\railalias{PLUS}{+}
\railalias{MINUS}{-}
\railalias{NOT}{!}
\railalias{PO}{(}
\railalias{PC}{)}
\railalias{DOT}{.}
\railalias{FALSE}{false}
\railalias{TRUE}{true}
\railalias{INT}{0..9}

\railterm{IMPLIES,EQUIVALENCE,OR,XOR,AND,EQ,NEQ,LT,GT,LTE,GTE,SHIFTLEFT,SHIFTRIGHT,MUL,DIV,MOD,PLUS,MINUS,NOT,PO,PC,DOT,FALSE,TRUE,INT}

\begin{rail}  
  BasicExpression :
    [constants] ( BooleanConstant  
        | IntegerConstant )
    | [attribute reference] AttributeReference
    | PO BasicExpression PC
    | [logical NOT] NOT BasicExpression
    | ( [integer multiplication] BasicExpression MUL BasicExpression
        | [integer division] BasicExpression DIV BasicExpression 
        | [integer remainder] BasicExpression MOD BasicExpression )
    | ( [integer addition] BasicExpression PLUS BasicExpression
        | [integer substraction] BasicExpression MINUS BasicExpression )
    | ( [bit shift left] BasicExpression SHIFTLEFT BasicExpression
        | [bit shift right] BasicExpression SHIFTRIGHT BasicExpression )
    | ( [equality] BasicExpression EQ BasicExpression
        | [inequality] BasicExpression NEQ BasicExpression
        | [less than] BasicExpression LT BasicExpression
        | [greater than] BasicExpression GT BasicExpression
        | [less than or equal] BasicExpression LTE BasicExpression
        | [greater than or equal] BasicExpression GTE BasicExpression )
    | [logical AND] BasicExpression AND BasicExpression
    | ( [logical OR] BasicExpression OR BasicExpression
        | [logical exclusive OR] BasicExpression XOR BasicExpression )
    | [logical equivalence] BasicExpression EQUIVALENCE BasicExpression
    | [logical implication] BasicExpression IMPLIES BasicExpression
  ;

  AttributeReference : UMLPropertyReference ( DOT AttributeReference )?;
  BooleanConstant : FALSE | TRUE ;
  IntegerConstant : ( MINUS | PLUS )? (INT+) ;   
\end{rail}  

)}}

This script results in the following diagram:



So, what is missing? Yes, an automatic conversion of Xtext grammars to rail scripts.
I am also looking forward to have the new Xtext Syntax Graph View.

Happy grammar hacking!