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+--[[
+# PlantUML Pandoc filter
+PlantUML Pandoc filter to process code blocks with class "plantuml" containing PlantUML notation into images.
+
+* For textual output formats, use --extract-media=DIR
+* For HTML formats, you may alternatively use --self-contained
+
+## Example in markdown-file
+```plantuml
+@startuml
+Alice -> Bob: Authentication Request Bob --> Alice: Authentication Response
+Alice -> Bob: Another authentication Request Alice <-- Bob: another authentication Response @enduml
+```
+## Run pandoc
+```
+pandoc --self-contained --lua-filter=plantuml.lua readme.md -o output.htm
+```
+
+## Prerequisites
+* download PlantUML from http://plantuml.com (needs JAVA)
+* 3 ways to set the environment
+    1. plantuml.lua and plantuml.jar in the same folder and start pandoc always from this folder
+    2. set a Environment Variable PLANTUML with the path to plantuml.jar
+        * Windows - with powershell: Set-Item env:PLANTUML "c:\bin\plantuml.jar"
+    3. change path to plantuml.jar in plantuml.lua
+
+This script based on the example "Converting ABC code to music notation" from https://pandoc.org/lua-filters.html
+**This script was only tested with markdown to html on a windows environment!**
+]]
+
+-- Path to PlantUML.jar
+-- if you use opinion 3 change the path to plantuml.jar like this:
+-- local plantumlPath = os.getenv("PLANTUML") or "c:\\bin\\plantuml.jar"
+local plantumlPath = os.getenv("PLANTUML") or "plantuml.jar"
+
+-- SVG has a much better quality
+-- local filetype = "png"
+-- local mimetype = "image/png"
+local filetype = "svg"
+local mimetype = "image/svg+xml"
+
+-- call plantuml.jar wit some parameters (see plantuml help)
+local function plantuml(puml, filetype, plantumlPath)
+    local final = pandoc.pipe("java", {"-jar", plantumlPath, "-t" .. filetype, "-pipe", "-charset", "UTF8"}, puml)
+    return final
+end
+
+-- search for class "plantuml" and replace with image
+function CodeBlock(block)
+    if block.classes[1] == "plantuml" then
+        local img = plantuml(block.text, filetype, plantumlPath)
+        local fname = pandoc.sha1(img) .. "." .. filetype
+        pandoc.mediabag.insert(fname, mimetype, img)
+        return pandoc.Para{ pandoc.Image({pandoc.Str("PlantUML Diagramm")}, fname) }
+    end
+end