Randomizing top-n results in Solr

RandomValueSource.java

import java.io.IOException;
import java.util.Map;
import java.util.Random;

import org.apache.lucene.index.AtomicReaderContext;
import org.apache.lucene.queries.function.FunctionValues;
import org.apache.lucene.queries.function.ValueSource;
import org.apache.lucene.queries.function.docvalues.DoubleDocValues;

public class RandomValueSource extends ValueSource {
	final static ThreadLocal<Random> RANDOMIZERS = new ThreadLocal<Random>() {
		protected Random initialValue() {
			return new Random();
		}
	};

	@Override
	public String description() {
		return "rnd()";
	}

	@Override
	@SuppressWarnings("rawtypes")
	public FunctionValues getValues(Map context, AtomicReaderContext readerContext) throws IOException {
		return new DoubleDocValues(this) {
			@Override
			public float floatVal(int doc) {
				return RANDOMIZERS.get().nextFloat();
			}

			@Override
			public int intVal(int doc) {
				return RANDOMIZERS.get().nextInt();
			}

			@Override
			public long longVal(int doc) {
				return RANDOMIZERS.get().nextLong();
			}

			@Override
			public double doubleVal(int doc) {
				return RANDOMIZERS.get().nextDouble();
			}

			@Override
			public String strVal(int doc) {
				return Double.toString(RANDOMIZERS.get().nextDouble());
			}

			@Override
			public Object objectVal(int doc) {
				return RANDOMIZERS.get().nextDouble();
			}

			@Override
			public String toString(int doc) {
				return description();
			}
		};
	}

	@Override
	public boolean equals(final Object o) {
		return o == this;
	}

	@Override
	public int hashCode() {
		return RANDOMIZERS.get().hashCode();
	}

RandomValueSourceParser.java

import org.apache.lucene.queries.function.ValueSource;
import org.apache.solr.search.FunctionQParser;
import org.apache.solr.search.SyntaxError;
import org.apache.solr.search.ValueSourceParser;

public class RandomValueSourceParser extends ValueSourceParser {

	@Override
	public ValueSource parse(final FunctionQParser fp) throws SyntaxError {
		return new RandomValueSource();
	}
}