The section graph

The section graph is a DAG. It behaves as a heterogeneous graph in many ways, though items are virtual. This was chosen instead of a node and leaf structure as then items and sections could be intermixed at the same level. Though this choice does complicate the graph implementation considerably, it does mesh perfectly with the need for sections and items to be unidentifiably different for Slms. It is also quite memory efficient.

Structural changes

Structural changes to the section graph are the removal and addition of sections. Simply removing all items in a subgraph will leave the section structure intact.

Consistency

To prevent traversing the graph too much, sections maintain a lot of context about their position within the graph. As such, structural operations on the graph, as well as many section property changes, trigger updates to earlier, or later, nodes; depending on the operation.

Layout operations

The entrance of layout, fill and trim ops are through the root node. It is primarily through each section's layout state these calls are passed appropriately down the graph structure.