Tuning bm25, adding sort order

Modify suggestion mode flow

src/search.cpp

@@ -354,6 +354,14 @@ Search::iterator Search::begin() const {
     Xapian::Query query;
     try {
       query = queryParser->parse_query(this->query, flags, prefix);
+
+      // In suggestion mode, parsing the query with OP_PHRASE provides
+      // better results.Combined with the subquery above provides single
+      // term and phrase compatibility.
+      if(suggestion_mode){
+        queryParser->set_default_op(Xapian::Query::op::OP_PHRASE);
+        query = Xapian::Query(Xapian::Query::OP_OR, queryParser->parse_query(this->query), query);
+      }
     } catch (Xapian::QueryParserError& e) {
       estimated_matches_number = 0;
       return nullptr;
@@ -379,6 +387,16 @@ Search::iterator Search::begin() const {
         }
     }
 
+    // In suggestion mode, we are searching over a separate title index.
+    // Default BM25 is not adapted for this case, tuning down wdf factor
+    // (k1) and increasing length normalization factor(b) is necessary.
+    // The document set is first sorted by their relevance score then by 
+    // value so that suggestion results are closer to search string.
+    if (suggestion_mode) {
+      enquire.set_weighting_scheme(Xapian::BM25Weight(0.001,0,1,1,0.5));
+      enquire.set_sort_by_relevance_then_value(valuesmap["title"], true);
+    }
+
     enquire.set_query(query);
 
 #if WITH_LEV