SPARQL trigonometric functions

Question

I am using Sesame server for storing and querying sets of triples. In my repository I have sets of triples that represent locations on Earth defined by longitude and latitude.

Example:

PREFIX ont:<http://example.org/myontology.owl#>

<http://foo.com/location-a> a ont:Location.
<http://foo.com/location-a> ont:hasLatitude "25.91239"^^xsd:double.
<http://foo.com/location-a> ont:hasLongitude "30.3911"^^xsd:double.

<http://foo.com/location-b> a ont:Location.
<http://foo.com/location-b> ont:hasLatitude "15.7778"^^xsd:double.
<http://foo.com/location-b> ont:hasLongitude "13.6755"^^xsd:double.

...

I want to write a query that gives me back all the locations that are on the surface of a circle defined by it's center point (Latidude_Center, Longitude_Center) and a radius (in km or degrees).

To achieve this thing I have to use some math formulas that imply trigonometric functions. As far as I know SPARQL does not support trigonometric functions.

Is there another way for creating this functionality?

Answer 1

You can add custom functions to Sesame's SPARQL engine yourself, programmaticaly. I wrote a tutorial on how to do this some time ago. The gist of it is:

1. create a class for your function that implements the org.openrdf.query.algebra.evaluation.function.Function interface.

For example:

public class SinusFunction implements Function {

     /** return the name of the function for use in SPARQL */
     public String getURI() {
          return "http://example.org/function/sin";
     }

     public Value evaluate(ValueFactory valueFactory, Value... args)
         throws ValueExprEvaluationException
     {
           // TODO implement computing the function return value
           // based on the input args 
     }
}

2. create a jar with a Service Provider Interface (SPI) registry configuration for your custom function(s).

This involves having a file called org.openrdf.query.algebra.evaluation.function.Function in the META-INF/services directory inside your jar file. The contents of this file should be the fully-qualified names of each of your function implementations, one per line.

3. Drop your jar on Sesame Server's runtime classpath somewhere, and restart.