// BSD License (http://www.galagosearch.org/license)

 package org.galagosearch.core.scoring;

 import java.io.IOException;

 import org.galagosearch.core.retrieval.structured.CountIterator;

 import org.galagosearch.core.retrieval.structured.RequiredStatistics;

 import org.galagosearch.core.retrieval.structured.ScoringFunctionIterator;

 import org.galagosearch.tupleflow.Parameters;

 /**

  *

  * @author trevor

  */

 @RequiredStatistics(statistics = {"collectionLength"})

 public class DirichletScorer extends ScoringFunctionIterator {

     double background;

     double mu;

     public DirichletScorer(Parameters parameters, CountIterator iterator) throws IOException {

         super(iterator);

         mu = parameters.get("mu", 1500);

         if (parameters.containsKey("collectionProbability")) {

             background = parameters.get("collectionProbability", 0.0001);

         } else {

             long collectionLength = parameters.get("collectionLength", (long)0);

             long count = 0;

             while (!iterator.isDone()) {

                 count += iterator.count();

                 iterator.nextDocument();

             }

             background = (double)count / (double)collectionLength;

             iterator.reset();

         }

     }

     public double scoreCount(int count, int length) {

         double numerator = count + mu * background;

         double denominator = length + mu;

         return Math.log(numerator / denominator);

     }

 }