Layering

AGG supports the possibility to set rule layers and thus to define layered graph grammar. The layers fix the order of how rules are applied. The interpretation process first has to apply all rules of layer 0 as long as possible, then all rules of layer 1, etc. Rule layers allow to specify a simple control flow graph transformation. Once the highest layer has been finished the transformation stops, unless option loop over layers is checked.

Moreover, rule layers allow some kind of optimization :

• During the critical pair analysis, critical pairs are searched for rules in the same layer only.
• During parsing, first all rules of one layer are applied as long as possible, before the rules of the next layer are applied.
• During termination checking, the termination criteria are based on assigning a layer to each rule, node and edge type.

The graphical user interface for setting rule layers is available by item Set Rule Layer in pop-up menu GraGra. The dialog is shown in Figure 11. Another possibility to set rule layers is to use item Set Layer in pop-up menu Rule.

Figure 11: Rule layer dialog

Using item Sort by Rule Layer in pop-up menu GraGra the rules of a grammar are sorted by layer.