Lucene 评分机制

1. tf(t in d) correlates to the term's frequency, defined as the number of times term t appears in the currently scored document d. Documents that have more occurrences of a given term receive a higher score.

2. idf(t) stands for Inverse Document Frequency. This value correlates to the inverse of docFreq (the number of documents in which the term t appears). This means rarer terms give higher contribution to the total score

3. coord(q,d) is a score factor based on how many of the query terms are found in the specified document

4. norm(t,d) encapsulates a few (indexing time) boost and length factors:

• Document boost - set by calling doc.setBoost() before adding the document to the index.
• Field boost - set by calling field.setBoost() before adding the field to a document.
• lengthNorm(field) - computed when the document is added to the index in accordance with the number of tokens of this field in the document, so that shorter fields contribute more to the score.  When a document is added to the index, all the above factors are multiplied. If the document has multiple fields with the same name, all their boosts are multiplied together

对于wildcard search， Lucene默认会屏蔽掉norm部分的分数。 如果想让它参与的话，可以重新设置它的rewrite method。

for (BooleanClause clause : ((BooleanQuery) query).getClauses()) {
        if (clause.getQuery() instanceof MultiTermQuery) {
        	((MultiTermQuery) clause.getQuery()).setRewriteMethod(MultiTermQuery.SCORING_BOOLEAN_QUERY_REWRITE);
        } else {        		
        	enableCalScore(clause.getQuery());
	}
}