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====== RGG Code structure introduction ======

The RGG programming language repents the core of almost all GroIMP models. 
This language extends java and implements the XL-language specification (https://manual.grogra.de/XL/index.html), to enable rule based graph manipulation.

This leads to a programming language based on two paradigms, the object orientation of java and the rule based structure of XL. In order to separate these two rgg is using different brackets to define code blocks. 
The java blocks are framed by curly brackets ''{...}'' and the xl blocks by square brackets ''[...]''. These blocks can be embedded into each other recursively and share declared variables. 

===== Functions =====

Even so the function declaration follows the syntax of java, the body of the function can be of both code blocks. In difference to java rgg code allows to start directly with the declaration of functions without creating a class before. (Internally the rgg file is handled as one java class). 

If a function is defined by ''public void'' and needs no parameters, it is automatically added to the RGG toolbar after compilation (the list of buttons above the 3d view). This functions are the main way to interact with a running RGG simulation. 

==== init ====

Additionally there are several functions predefined that can be used to change the behavior of the simulation. At this point we will only talk about the initialization: ''protected void init ()''. this function is executed after the compilation or a model reset.

<code java>
protected void init ()
[
	{println("hallo");}
	Axiom ==> A(parameters.length);
]
</code>

The Axiom is the node that is added to the graph in the very beginning of the simulation and is used as an access point.
===== Java blocks =====

The java block allow java syntax up to java 1.6. (this is independent from the java version that is used to run GroIMP). Therefore it is possible to use features such as file reading and writing, mathematical calculations, abstraction/inheritance or library functions. If you are not experienced with java it might be useful for you to look into available online tutorials.(For example https://www.w3schools.com/java/default.asp)

Every new rgg file also imports by default a set of library functions that are used to interact with the graph or the GroIMP platform. This functions can be explored in the function explorer in the software. You can find it on the main menu under 'Help/Function Browser'. 

Moreover rgg comes with some additions to the java syntax, or more accurate to the syntax of java 6.  


==== Modules ====

A module in RGG is very similar to a class in java, it can extend other modules or implements java interfaces. It can also hold functions like 'getDiameter()' in the example below. One large difference is that a in a module the constructor is different. 
The parameters are unlink in a java class directly defined in the head of the module (see below 'float len') and the code executed on the initialization of a module is defined by just a second par of curly brackets (see below {setShader(GREEN);}).

Besides that the main difference is the use case. A module is always a GroIMP Node, meaning it can be added to the GroIMP simulation graph (ProjectGraph) similar to [[groimp-platform:turtle_commands|turtle commands]] or the base 3d Objects. 

<code java>
module A(float len) extends Sphere(0.1)
{
	{setShader(GREEN);}

	public float getDiameter(){
		return radius*2;
	}	
}
</code>

Its syntax is simple and provisional, changes or extensions are likely to occur.

The simplest form of a module declaration is

<code java>
module SimpleModule;
</code>

which declares a module of name SimpleModule with no fields. These simple modules can be useful for aggregations, multi scaling or just as place holders. 


The addition of fields is done as in

<code java>
module ModuleWithFields (int type, String name);
</code>

If o is an instance of ModuleWithFields, the usual Java syntax o.type can be used to access a field. This form of a module declaration provides a constructor with parameters corresponding to the specified module parameters.

A module is a part of the Java class hierarchy. It is possible for a module to be a subclass of another class or module, this is indicated by an extends-clause:

<code java>
module Sub (super.type, super.name, int extra) extends ModuleWithFields;
</code>

The module Sub is a subclass (submodule) of ModuleWithFields. The fields type and name are inherited from ModuleWithFields, extra is a new field. A constructor having three parameters (type, name, extra) is provided. It looks for a superclass constructor applicable to the inherited arguments type, name of type int, String (which is found in ModuleWithFields).

Sometimes, not all module parameters are to be inherited. In these cases, one has to specify the superclass constructor to be used by explicitly providing its arguments as in

<code java>
module Sub2 (super.name) extends ModuleWithFields (1, name);
</code>


==== Instantiation ====

While in theory every part/organ/etc... of the simulated object (normally a plant) is suppose to be represented in the graph, it is not allays suitable to add all geometrical objects that represent an object individually. Therefore the concept of instantiations was included. 
Using the syntax below the module Branch is one object in the graph and is during the creation of the object interpreted as an extension of M. Yet for the visualization and the ray tracing it is represented by the four F objects with the different shaders.     

<code java >

module Branch(float len) extends M(len)==> 
     F(len/4).(setShader(GREEN))
     F(len/4).(setShader(BLUE))
     F(len/4).(setShader(GREEN))
     F(len/4).(setShader(BLUE));
</code>

On the right side of the ''%%==>%%'' operator a syntax similar to the production part of a rule can be used.

It has to be noticed that even so instantiations create a smaller graph they tempt to slow down the visualization.

==== Queries ====

In GroIMP the project graph is considered to hold almost all information on the simulation. Therefore it can also be seen as a knowledge graph. To retrieve this knowledge rgg uses the [[tutorials:common_graph_queries|XL query system]]. To use this queries in a java block they have to be framed by ''(*...*)'' as shown in the examples below. In that way they return a collection object, which then can be like other java objects. Moreover GroIMP comes with a set of predefined  [[:groimp-platform:xl-builtin-methods|Analytical methods]] to analyze this collections.

As shown in the example below in ''%%count((*M*))%%'' this queries work with the concept of java objects. Therefore ''(*M*)'' is selecting all instances of Branch because Branch extends M. This also works with interfaces. 

<code java>
public void calc(){
	long countA = count((*RU(30) RH(90) A*));
	long countM = count((*M*));
	if(countM>0){
		println((float)countA/(float)countM);
	}
	println(sum((*Branch*).len));
}
</code>

in the last line of the example above: ''%%println(sum((*Branch*).len));%%'' one way to access attributes (or functions) of modules is shown. The other way is shown below in: ''%% (*A*)[radius]+=0.1; %%''. 
The first way is more in the style of java while the second way is better adapted to GroIMP. Therefore the second way is not only changing the attribute of the object but also notifies the change to the platform.
Moreover it manages the access of the attributes differently, (for instance would ''%%(*A*).radius%%'' not be allowed because radius is not accessible in a java way. 

<code java>
public void growRadius(){
	(*A*)[radius]+=0.1;
}
</code>


==== Lambda expressions ====

Since java 1.6 did not include lamda expression, an own implementation was added to rgg. 
The syntax an the explanation can be found [[groimp-platform:xl-lambda|here]] 


===== XL Blocks - Rules =====

While using XL queries mainly three rules are used: 
 
  * Graph rules (''%%==>>%%'')
  * String replacement rules (''%%==>%%'')
  * Update or Execution rules (''%%::>%%'')

All rules have in common that we have a left and a right hand side, both are separated by one of the arrows.


=== Graph rules ===

Graph rules are indicated by the rule arrow ''%%==>>%%''. 

The right hand side is a semicolon-terminated list of graph statements building the replacing graph for the matched graph. While the left hand side represents a fixed graph pattern, the graph statements can build the replacing graph dynamically, including loops and conditional execution. Thus it is not always possible to find a graphical representation of the right hand side.

The graph statements use a stack s of nodes and two state variables n – the last created node – and e – the next edge to create – during execution. 

After the execution of all graph statements, a final step is executed (if the execution has not been terminated using break): A matching non-context node instance of the left hand side, which has not been the value of an evaluated node expression of the right hand side, is deleted. Similarly, an edge matching a non-context edge pattern of the left hand side is deleted if it is not specified on the right hand side.


=== String rules ===

The concept of an replacement rule in XL follows the 'Lindenmayer-form' where basically all parts of the project string that are similar to the left part (before ''%%==>%%'') are replaced by the right part ( behind ''%%==>%%''). 
The left part can thereby be any [[tutorials:common_graph_queries|xl-query]].
The right part (the production) is a collection of Nodes ( instances of either turtle commands, 3d objects or modules) which are linked by different edges. In the example below only successor- and branch-edges are used. A node that is separated to its predecessor by only a white space is added as a sucessor to the predecessor. A Node or a set of nodes framed by square brackets in that is added to the predecessor with a branch-edge. 

The most noticeable difference between these edges is that a Node should always only have one successor child. Therefore branches are needed to grow into different directions based on the way nodes interpreted by the visualization. In very simple terms this interpretation means that the transformation (rotation, location ...) of a Node is the sum of all its predecessors. 

In our example the first A on the right side would be moved a distance of ''x'' in the current rotation axis (based on the ''F(x)'') and be rotated  on the up/x axis by 30 (''RU(30)'') and on the head/z axis 90 (''RH(90)'') degrees.   


<code java>
public void run ()[
	A(x) ==> F(x) [RU(30) RH(90) A(x*0.8)] [RU(-30) RH(90) A(x*0.8)];
]
</code>

As shown above it is also possible to forward parameters from the left side to the right (similar to a parametric rewriting rule in formal systems). Moreover it is possible to use ''a:A''  instead of ''A(x)'' and then on the right side ''a.len'' instead of ''x''. 

=== Execution rule ===

The left side of an execution rule is similar to the left side of the production rule. Yet the main difference is that the nodes (or pattern of nodes) found by the query are not replaced. Instead the (Java) code on the right side is applied on them.

<code java>
public void getDiameter()[
    	a:A::>{println(a.getDiameter());}
]
</code>

This works very similar to looping over the result of the query. This also works with manipulation several nodes at the same time:

<code java>
public void change()[

	l:RU h:RH ::> {
		float a = h[angle];
		h[angle]=l[angle];
		l[angle]=a;
	}
]
</code>

There are no additional actions executed, so update rules don’t change the topological properties of the
host graph per se.


==== Working with several Files ====

{{ :tutorials:file_explorer.png?260|}}

In GroIMP it is possible to split your code into several files and folders to improve the structure and keep an overview.
<color #ed1c24>It is highly recommended to only have public void functions in one file. Otherwise your project will have several rest buttons that do different things... </color>


To use function from another file this file needs to be imported similar to a java class or package.
Yet in the case of GroIMP all files are imported by their name without the directories, therefore to import all modules, and classes in ''param/parameters.rgg'' you just write:
<code java>
import parameters.YourModule;
</code> 


To get this a bit clearer lets assume our parameters.rgg file contains the following code:
<code java>
public static int length = 1;
public static float diameter = 0.1;
module Leaf() ==> leaf3d(1);
module Shoot() extends F(1);
</code>

To import one specific object once, you can simply do ''parameters.length'' as for example in ''A(parameters.length'' in the new RGG project. Or to use module:
<code java>
protected void init ()
[
Axiom ==> parameters.Leaf();
]
</code>

If you want to use your module several times you can also import it one in the beginning and then reuse it:

<code java>
import parameters.Leaf;
protected void init ()
[
Axiom ==> Leaf()[RL(180) Leaf()];
]
</code>

Finally you can import all modules with the * operator:
<code java>
import parameters.*;
protected void init ()
[
Axiom ==> Shoot() Leaf()[RL(180) Leaf()];
]
</code>

This can also be done with the static variables using static import:
<code java>

import static parameters.*;
protected void init ()
[
	Axiom ==> F(length,diameter);
]
</code>