The calcviz project is an example of how to use the jaxb-visitor plugin. The schema defines simple binary and unary math operations and enables the construction of an expression graph. Sample visitors are provided to demonstrate a few different operations.
The image above shows a simple graph using the schema provided in the project. This is a toy example, but it allows us to consider a number of operations that we might want to perform on this graph. The most obvious operation is evaluating the graph in order to produce a result. This involves visiting each of the nodes and evaluating its child nodes so that we have the input for the node's operation. Another operation might be to serialize the graph to a string so it can be represented in a compact form. Other operations might be static analysis, simplification, partial evaluation, etc.
The table below lists the visitors that are implemented in the calcviz project:
| Visitor | Description |
|---|---|
| CalculatorVisitor | Evaluates the expression |
| ExpressionSerializerVisitor | Serializes the graph to a string. |
| PartialEvaluatorVisitor | Transforms the graph by evaluating any of the operations that contain literal arguments. |
The results of the visitor varies based on the operation it's performing. The CalculatorVisitor and ExpressionSerializerVisitor are designed to visit the graph once and produce a result. The PartialEvaluatorVisitor modifies the graph and is designed to run multiple times until the graph is in its simplest form. In a sense it performs a step by step evaluation which may help illustrate how the expression is evaluated.
| Visitor | Output |
|---|---|
| CalculatorVisitor | 13.0 |
| ExpressionSerializerVisitor | ((4.0 * 2.0) + (10.0 / (4.0 - 2.0))) |
| PartialEvaluatorVisitor | modified graph as shown below |
