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PlantUML Text Encoding

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Introduction

PlantUML defines a standardized way to encode diagram text description to a simple string of characters that contains only digits, letters, underscore and minus character. The goal of such an encoding is to facilitate communication of diagrams through URL (see server). This encoding includes compression to keep encoded strings as short as possible.

The encoded metadata is stored in the generated PNG, so the diagram source can be extracted from the diagram itself! (see server#metadata).

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Compression

Deflate algorithm is used by default.

You can also use simple HEX encoding, see below. An initial ~h is added to indicate this encoding.

Principle

For example, the following uml text description:

@startuml
Alice -> Bob: Authentication Request
Bob --> Alice: Authentication Response
@enduml

is encoded as:

Syp9J4vLqBLJSCfFib9mB2t9ICqhoKnEBCdCprC8IYqiJIqkuGBAAUW2rO0LOr5LN92VLvpA1G00

To achieve such encoding, the text diagram is:

1. Encoded in UTF-8
2. Compressed using Deflate algorithm
3. Reencoded in ASCII using a transformation close to base64

Why not use Base64?

The main reason is historic: this format was not created to be public at first. Now, it's too late to change it. However, the only difference is in character order.

Where in base64 the mapping array for values 0-63 is:

ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/

For PlantUML, the mapping array for values 0-63 is:

0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-_

Compression comparison

The following diagram:

🎉 Copied!

@startuml skinparam backgroundColor #EEEBDC skinparam handwritten true skinparam sequenceArrowColor DeepSkyBlue skinparam sequenceActorBorderColor DeepSkyBlue skinparam sequenceLifeLineBorderColor blue skinparam sequenceLifeLineBackgroundColor #A9DCDF skinparam sequenceParticipantBorderColor DeepSkyBlue skinparam sequenceParticipantBackgroundColor DodgerBlue skinparam sequenceParticipantFontName Impact skinparam sequenceParticipantFontSize 17 skinparam sequenceParticipantFontColor #A9DCDF skinparam sequenceActorBackgroundColor aqua skinparam sequenceActorFontColor DeepSkyBlue skinparam sequenceActorFontSize 17 skinparam sequenceActorFontName Aapex actor User participant "First Class" as ParticipantA participant "Second Class" as ParticipantB participant "Last Class" as ParticipantC User -> ParticipantA: DoWork activate ParticipantA ParticipantA -> ParticipantB: Create Request activate ParticipantB ParticipantB -> ParticipantC: DoWork activate ParticipantC ParticipantC --> ParticipantB: WorkDone destroy ParticipantC ParticipantB --> ParticipantA: Request Created deactivate ParticipantB ParticipantA --> User: Done deactivate ParticipantA @enduml

is compressed to 428-char string length using Deflate

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Running

You can use -encodeurl or -decodeurl in the command line flags to encode or decode the text.

You will find here some implementation of this encoder:

• Code in PHP
• Code in Javascript
• Code in Python
• Code in Perl
• Code in Swift

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Simple HEX format

If you find Deflate and Brotli too complex, you can try the HEX format. In that case, you just have to encode each character in hexadecimal format.

For example :

@startuml
Alice->Bob : I am using hex
@enduml

will be turned into:

407374617274756d6c0a416c6963652d3e426f62203a204920616d207573696e67206865780a40656e64756d6c

To indicate the use of HEX format, you must add ~h at the start of the data sent to PlantUML server.

http://www.plantuml.com/plantuml/uml/~h4073...

Since there is no compression here, the URL will become very long as the diagram grows.