How to execute functions using CARML processor to convert JSON to RDF format and perform arithmetic operations?

Question

I am currently working on a project where I need to convert a JSON file into RDF format using the CARML processor. Additionally, I need to perform arithmetic operations on the data during the conversion process. Specifically, I want to subtract two numbers present in the JSON file: first and second. Here is a sample JSON file:

{
  "first": "2",
  "second": "1"
}

I would like to know how I can achieve this using the CARML processor. Are there any specific functions or transformations available in CARML that can help me subtract these numbers? It would be helpful if someone could provide an example or guide me on how to accomplish this task.

According to the documentation, a function can be created in any Java class and needs to be annotated with @FnoFunction, providing the function IRI as a string value, and the function parameters with @FnoParam, providing the parameter IRIs as string values. The class containing the annotated functions should then be registered on the mapper using the addFunctions method.

Here is an example of a function provided in the documentation:

public class RmlFunctions {

  @FnoFunction("http://example.org/sumFunction")
  public int sumFunction(
      @FnoParam("http://example.org/intParameterA") int intA,
      @FnoParam("http://example.org/intParameterB") int intB
  ) {
    return intA + intB;
  }

}

To add the functions, you can use the following code snippet:

RdfRmlMapper mapper = RdfRmlMapper.builder()
    .triplesMaps(mapping)
    .setLogicalSourceResolver(Rdf.Ql.JsonPath, JsonPathResolver::getInstance)
    .addFunctions(new RmlFunctions())
    .build();

Model result = mapper.mapToModel();

However, I am uncertain about where exactly I should place the function code and how to integrate it into my existing codebase.

Could someone provide guidance on where to write the function and how to add it to the CARML mapper in my specific scenario? Any code examples or step-by-step instructions would be greatly appreciated.

Thank you in advance for your assistance!