A special feature of the graph management is the manipulation of extents. For each used node class in the graph, there exists an Extent which collects all nodes of this class and also has a link to the extent of its superclass and to the extents of its subclasses. This feature is currently used within the RGG plug-in for a fast processing of queries which specify node type patterns like in the rule F(x) == > F(2*x);. Instead of having to scan the whole graph for nodes of class F, the corresponding extent is used together with its sub-extents to directly iterate over all nodes of class F.

Extents are double linked lists. They could be used for any type of java classes. But, in GroIMP they are only created for Nodes in the graph. Thus, the elements of the extents will be referred as Node (and their subclasses). To be part of the project graph an object needs to extends Node. Thus, the highest level of Extent is the one wrapping the type Node. Then, for each new type of Node added in the graph, an extent is created and linked to its direct super class. The type of Node is described by its NType. The NType is an internal class defined by each Node. In java it is usually created with the code enhancer to ensure that all the required attributes for the persistence management are created. In RGG, the NType is automatically created for all modules. If a Node is added to the graph and it extends several level of Nodes, an extent for each intermediate NTypes is created.

E.g. in the following: (the init method is simplified, and we assume that there are not other nodes in the graph)

module A;
module B extends A;
module C extends B;
 
init()[ Axiom ==> C; ]

the graph only contains the root: Node, the RGGRoot, and one C node. GroIMP will create the following extents:

• one for Node of size 1
• one for RGGRoot of size 1
• one for A of size 0
• one for B of size 0
• one for C of size 1

Extent implements the following methods:

• getType(): return the type of the node of this extent.
• getSubExtents(): return the list of extents whose type is a sub type.
• getFirstNode(): return the first element added to this extent.
• getNextNode(): return the next element in the chain.
• getTotalSize(): return the size of the extent including the size of its sub extent.

An extent collects its nodes not by a single list, but by a number of (doubly linked) lists. Currently, eight lists are used, but this can easily be changed. This feature is used to partition the extents.

The extent indexes are defined from 0 to 7. By default most nodes added to the graph are added with the index 0. There are two exceptions: meta objects, and GUI inserted objects.

The most common example of extent index is the meta-graph. Each project in GroIMP includes a meta-graph with at least one node per compiled file in the project. Yet, these Nodes are not returned by default in XL queries. The reason is that, they were added to an invisible (by default) extent index.

Note: before GroIMP version 2.1.5, all objects added through the GUI>objects>insert> … where also added on the extent index 7. Thus, not queriable by default. E.g. If you inserted a Plane in the scene, it was not reachable by default queries. In GroIMP version 2.1.5 their extent index have been changed to 6.

For instance the following graph, where RGGRoot extends Node, and Model.A extends Node:

GroIMP creates three extents, one for each type: Node, RGGRoot, and Model.A.

Extents work as linked list.