More Like This: ensures selection of best terms is indeed O(n)

Previously the size of the priority queue was wrongly set to the total number
of terms. Instead, it should be set to 'maxQueryTerms'. This makes the
selection of best terms O(n), instead of O(n*log(n)).

Jira patch: https://issues.apache.org/jira/browse/LUCENE-5795

Closes #6657

src/main/java/org/elasticsearch/common/lucene/search/MoreLikeThisQuery.java

@@ -21,7 +21,6 @@
 
 import org.apache.lucene.analysis.Analyzer;
 import org.apache.lucene.index.IndexReader;
-import org.apache.lucene.queries.mlt.MoreLikeThis;
 import org.apache.lucene.search.BooleanClause;
 import org.apache.lucene.search.BooleanQuery;
 import org.apache.lucene.search.Query;
@@ -50,14 +49,14 @@ public class MoreLikeThisQuery extends Query {
     private String[] moreLikeFields;
     private Analyzer analyzer;
     private float percentTermsToMatch = DEFAULT_PERCENT_TERMS_TO_MATCH;
-    private int minTermFrequency = MoreLikeThis.DEFAULT_MIN_TERM_FREQ;
-    private int maxQueryTerms = MoreLikeThis.DEFAULT_MAX_QUERY_TERMS;
-    private Set<?> stopWords = MoreLikeThis.DEFAULT_STOP_WORDS;
-    private int minDocFreq = MoreLikeThis.DEFAULT_MIN_DOC_FREQ;
-    private int maxDocFreq = MoreLikeThis.DEFAULT_MAX_DOC_FREQ;
-    private int minWordLen = MoreLikeThis.DEFAULT_MIN_WORD_LENGTH;
-    private int maxWordLen = MoreLikeThis.DEFAULT_MAX_WORD_LENGTH;
-    private boolean boostTerms = MoreLikeThis.DEFAULT_BOOST;
+    private int minTermFrequency = XMoreLikeThis.DEFAULT_MIN_TERM_FREQ;
+    private int maxQueryTerms = XMoreLikeThis.DEFAULT_MAX_QUERY_TERMS;
+    private Set<?> stopWords = XMoreLikeThis.DEFAULT_STOP_WORDS;
+    private int minDocFreq = XMoreLikeThis.DEFAULT_MIN_DOC_FREQ;
+    private int maxDocFreq = XMoreLikeThis.DEFAULT_MAX_DOC_FREQ;
+    private int minWordLen = XMoreLikeThis.DEFAULT_MIN_WORD_LENGTH;
+    private int maxWordLen = XMoreLikeThis.DEFAULT_MAX_WORD_LENGTH;
+    private boolean boostTerms = XMoreLikeThis.DEFAULT_BOOST;
     private float boostTermsFactor = 1;
 
 
@@ -135,7 +134,7 @@ public boolean equals(Object obj) {
 
     @Override
     public Query rewrite(IndexReader reader) throws IOException {
-        MoreLikeThis mlt = new MoreLikeThis(reader, similarity == null ? new DefaultSimilarity() : similarity);
+        XMoreLikeThis mlt = new XMoreLikeThis(reader, similarity == null ? new DefaultSimilarity() : similarity);
 
         mlt.setFieldNames(moreLikeFields);
         mlt.setAnalyzer(analyzer);