Solr FunctionQueries allow you to modify the ranking of a search query in Solr by applying functions to the results.

There are a list of out-of-box FunctionQueries available here: http://wiki.apache.org/solr/FunctionQuery

In order to write a custom Solr FunctionQuery, you'll need to do 2 things:

1. Subclass org.apache.solr.search.ValueSourceParser. Here's a stub ValueSourceParser.

public class MyValueSourceParser extends ValueSourceParser {
  public void init(NamedList namedList) {
  }

  public ValueSource parse(FunctionQParser fqp) throws ParseException {
    return new MyValueSource();
  }
}

2. In solrconfig.xml, register your new ValueSourceParser directly under the <config> tag

<valueSourceParser name="myfunc" class="com.mycompany.MyValueSourceParser" />

3. Subclass org.apache.solr.search.ValueSource and instantiate it in your ValueSourceParser.parse() method.

Lets take a look at 2 ValueSource implementations to see what they do, starting with the simplest:

org.apache.solr.search.function.ConstValueSource

Example SolrQuerySyntax: _val_:1.5

It simply returns a float value.

public class ConstValueSource extends ValueSource {
  final float constant;

  public ConstValueSource(float constant) {
    this.constant = constant;
  }

  public DocValues getValues(Map context, IndexReader reader) throws IOException {
    return new DocValues() {
      public float floatVal(int doc) {
        return constant;
      }
      public int intVal(int doc) {
        return (int)floatVal(doc);
      }
      public long longVal(int doc) {
        return (long)floatVal(doc);
      }
      public double doubleVal(int doc) {
        return (double)floatVal(doc);
      }
      public String strVal(int doc) {
        return Float.toString(floatVal(doc));
      }
      public String toString(int doc) {
        return description();
      }
    };
  }
// commented out some boilerplate stuff
}

As you can see, the important method is DocValues getValues(Map context, IndexReader reader). The gist of the method is return a DocValues object which returns a value given a document id.

org.apache.solr.search.function.OrdFieldSource

ord(myfield) returns the ordinal of the indexed field value within the indexed list of terms for that field in lucene index order (lexicographically ordered by unicode value), starting at 1. In other words, for a given field, all values are ordered lexicographically; this function then returns the offset of a particular value in that ordering.

Example SolrQuerySyntax: _val_:"ord(myIndexedField)"

public class OrdFieldSource extends ValueSource {
  protected String field;

  public OrdFieldSource(String field) {
    this.field = field;
  }
  public DocValues getValues(Map context, IndexReader reader) throws IOException {
    return new StringIndexDocValues(this, reader, field) {
      protected String toTerm(String readableValue) {
        return readableValue;
      }
     
      public float floatVal(int doc) {
        return (float)order[doc];
      }

      public int intVal(int doc) {
        return order[doc];
      }

      public long longVal(int doc) {
        return (long)order[doc];
      }

      public double doubleVal(int doc) {
        return (double)order[doc];
      }

      public String strVal(int doc) {
        // the string value of the ordinal, not the string itself
        return Integer.toString(order[doc]);
      }

      public String toString(int doc) {
        return description() + '=' + intVal(doc);
      }
    };
  }
}

OrdFieldSource is almost identical to ConstValueSource, the main differences being the returning of the order rather than a const value, and the use of StringIndexDocValues which is for obtaining the order of values.

Our own ValueSource

We now have a pretty good idea what a ValueSource subclass has to do:

return some value for a given doc id.

This can be based on the value of a field in the index (like OrdFieldSource), or nothing to do with the index at all (like ConstValueSource).

Here's one that performs the opposite of MaxFloatFunction/max() - MinFloatFunction/min():

public class MinFloatFunction extends ValueSource {
  protected final ValueSource source;
  protected final float fval;

  public MinFloatFunction(ValueSource source, float fval) {
    this.source = source;
    this.fval = fval;
  }
 
  public DocValues getValues(Map context, IndexReader reader) throws IOException {
    final DocValues vals =  source.getValues(context, reader);
    return new DocValues() {
      public float floatVal(int doc) {
    float v = vals.floatVal(doc);
        return v > fval ? fval : v;
      }
      public int intVal(int doc) {
        return (int)floatVal(doc);
      }
      public long longVal(int doc) {
        return (long)floatVal(doc);
      }
      public double doubleVal(int doc) {
        return (double)floatVal(doc);
      }
      public String strVal(int doc) {
        return Float.toString(floatVal(doc));
      }
      public String toString(int doc) {
    return "max(" + vals.toString(doc) + "," + fval + ")";
      }
    };
  }

  @Override
  public void createWeight(Map context, Searcher searcher) throws IOException {
    source.createWeight(context, searcher);
  }

// boilerplate methods omitted
}

And the corresponding ValueSourceParser:

public class MinValueSourceParser extends ValueSourceParser {
  public void init(NamedList namedList) {
  }

  public ValueSource parse(FunctionQParser fqp) throws ParseException {
        ValueSource source = fp.parseValueSource();
        float val = fp.parseFloat();
        return new MinFloatFunction(source,val);
  }
}