Solving the TTC 2011 Compiler Optimization Case with GReTL

GReTL (Graph Repository Transformation Language, [5]) is the operational transformation language of the TGraph technological space [2]. Models are represented as typed, directed, ordered, and attributed graphs. GReTL uses the GReQL (Graph Repository Query Language, [3]) for its matching part. The standard GReTL operations are currently targeted at out-place transformation which create the target metamodel and the target graph simultaneously, but this case requires changing the graph in-place. Luckily, GReTL is designed as an extensible language. Adding custom operations requires only specializing one framework class, overriding one method that implements the operation’s behavior, and implementing another factory method that is responsible for creating an instance of the new operation initialized with the given parameters. Consequently, some in-place operations have been added to the language. There is one operation MatchReplace which is similar to a rule in graph replacement systems. There is an operation Delete for deleting elements, an operation MergeVertices for merging a set of duplicate vertices into one canonical representative, and a higher-order operation Iteratively which receives another operation and applies it as long as possible. The implementation of the most complex one, MatchReplace, is about 380 lines of Java code including comments and empty lines, the other three operations are at most 80 lines or less. Thus, this case was solved using a mixed bottom-up and top-down approach: the language was extended with three new operations, and then these operations were used to write the transformations.