Explanation---lucene中对于评分细节描述的类

一般通过IndexSearch.explain(query,docId)----》weight.explain(reader, doc) 方法得到一个文档的评分的具体信息 。

Explanation的信息如下：

4.803122 = (MATCH) fieldWeight(keywords:奶粉 in 457), product of: 
2.0 = tf(termFreq(keywords:奶粉)=4)
4.803122 = idf(docFreq=414, maxDocs=18609)
0.5 = fieldNorm(field=keywords, doc=457)

第一行表示总得分：document（docId=457）的score为4.803122，它由下面的三个值相乘得来
第二行表示项频率：在document（docId=457）keywords这个filed中共出现了4个“奶粉”关键词，2.0是由根号4算出来的。
第三行表示反转文档频率:含有“奶粉”关键词的document共有414个，总的document有18609个，4.803122是由
ln(18609/(414+ 1) )+ 1.0 =ln(18609) -ln(415) +1 = 9.831400 - 6.028278 = 4.803122
第三行表示域的加权长度因子:fieldNorm = fieldboost / sqrt(fieldlength),其中fieldlength为keywords这个field的token(分词)数量  。

对于Weight的一个实现TermWeight中的weight.explain(reader, doc)实现如下 :

public Explanation explain(IndexReader reader, int doc)
      throws IOException {
      ComplexExplanation result = new ComplexExplanation();
      result.setDescription("weight("+getQuery()+" in "+doc+"), product of:");
      Explanation idfExpl = new Explanation(idf, "idf(docFreq=" + reader.docFreq(term) + ")");
      // explain query weight
      Explanation queryExpl = new Explanation();
      queryExpl.setDescription("queryWeight(" + getQuery() + "), product of:");
      Explanation boostExpl = new Explanation(getBoost(), "boost");
      if (getBoost() != 1.0f)
        queryExpl.addDetail(boostExpl);
      queryExpl.addDetail(idfExpl);
      Explanation queryNormExpl = new Explanation(queryNorm,"queryNorm");
      queryExpl.addDetail(queryNormExpl);
      queryExpl.setValue(boostExpl.getValue() *idfExpl.getValue() *queryNormExpl.getValue());
      result.addDetail(queryExpl);
      // 说明Field的权重
      String field = term.field();
      ComplexExplanation fieldExpl = new ComplexExplanation();
      fieldExpl.setDescription("fieldWeight("+term+" in "+doc+"), product of:");
      Explanation tfExpl = scorer(reader).explain(doc);
      fieldExpl.addDetail(tfExpl);
      fieldExpl.addDetail(idfExpl);
      Explanation fieldNormExpl = new Explanation();
      byte[] fieldNorms = reader.norms(field);
      float fieldNorm =
        fieldNorms!=null ? Similarity.decodeNorm(fieldNorms[doc]) : 0.0f;
      fieldNormExpl.setValue(fieldNorm);
      fieldNormExpl.setDescription("fieldNorm(field="+field+", doc="+doc+")");
      fieldExpl.addDetail(fieldNormExpl);
      
      fieldExpl.setMatch(Boolean.valueOf(tfExpl.isMatch()));
      fieldExpl.setValue(tfExpl.getValue() *idfExpl.getValue() *fieldNormExpl.getValue());
      result.addDetail(fieldExpl);
      result.setMatch(fieldExpl.getMatch());
      
      // combine them
      result.setValue(queryExpl.getValue() * fieldExpl.getValue());
      if (queryExpl.getValue() == 1.0f)
        return fieldExpl;
      return result;
    }

参考文章：

http://www.cnblogs.com/lvpei/articles/1732475.html

http://archive.cnblogs.com/a/1906353/

http://lucene.apache.org/java/2_9_0/api/core/org/apache/lucene/search/Weight.html