MEP functions can be used to perform arbitrary things with your data (e.g., mathematic operations, string operations, etc.). These functions can be used in different operators like Map, Select, or Join. To implement a MEP function, ones has to extend the AbstractFunction class. To implement your own MEP function you have to implement 4 methods, namely the getSymbol that returns the unique name of the function, the getArity, getAcceptedTypes, and getReturnType that describe the parameters and result of you function, and the getValue method which includes the processing of your function.
Remark: The MEP optimizer tries to determine if an expression is a constant and should not be evaluated each time. For this, the getValue method is called. This behaviour can be changed by implementing the method optimizeConstantParameter and returning false.
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public class MyFunction extends AbstractFunction<Double> {
public static final SDFDatatype[][] accTypes = new SDFDatatype[][] {{ SDFDatatype.DOUBLE },
{ SDFDatatype.DOUBLE }};
@Override
public String getSymbol() {
return "myFunction";
}
@Override
public Double getValue() {
double a = (double) this.getInputValue(0);
double b = this.getNumericalInputValue(1);
return a +b;
}
@Override
public SDFDatatype getReturnType() {
return SDFDatatype.DOUBLE;
}
@Override
public int getArity() {
return 2;
}
@Override
public SDFDatatype[] getAcceptedTypes(int argPos) {
if (argPos < 0) {
throw new IllegalArgumentException(
"negative argument index not allowed");
}
if (argPos > this.getArity()) {
throw new IllegalArgumentException(this.getSymbol() + " has only " + this.getArity() + " arguments: Two double values.");
}
return accTypes[argPos];
}
} |
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