Table of Contents
Extent
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.
Structure
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.
Index
An extent collects its nodes not by a single list, but by a number of (doubly linked) lists. Currently, 16 lists are used, but this can easily be changed. This feature is used to partition the extents. The 16 lists are called the extent indexes.
Extents are used by the query graph to access the nodes of the graph. In this process, nodes can be “hidden” to queries by being on a extent index non visible by the query scope (see RGG's extent index visibility). Non visible nodes are not visited and “ignored” by the queries, they are not visited at all (thus, efficient to speed up queries). For the RGG model graph, queries use only the seven first indexes by default. The default index usage is:
- 0 to 6: Default visibility - used by default in queries. Only these indexes are visible by default.
- 7: Meta data index.
- 8: Type graph data - used for the multi-scales.
- 9 to 15: Unused indexes - non visible by default.
Note: the index visibility is not used for the display. Thus, nodes on a “non” visible index are still displayed in the 3d view.