Boost results with exact order and LCS by default

Our current way of ranking results is to sort them by total number
of words in the sentence. A problem with this approach is that the
order of the words is ignored. The top result of searching for "you
go there" is "There you go!" because it’s a shorter sentence than
"You may go there."

Ignoring word order is especially catastrophic on languages without
word boundaries, like Chinese, because the searched characters are
randomly reordered into something totally unrelated. For example, the
results for "可不可" in Chinese are cluttered by irrelevant
"不可something". Same for kana words in Japanese.

This commit address this problem by changing the default ranking
algorithm into something that prioritize, in the following order:

1. sentences that contains an exact match (like if searching for
   ="you go there")
2. sentences having the "longest common subsequence" (LCS, [1])
3. sentences having the least number of words

[1] https://docs.manticoresearch.com/latest/html/searching/search_results_ranking.html?highlight=lcs#field-level-ranking-factors

Note that there is still room for improvement, because sorting by
number of words is not ideal, as very short sentences are usually
not the most relevant.

src/Controller/SentencesController.php

@@ -100,7 +100,7 @@ class SentencesController extends AppController
         'trans_unapproved' => '',
         'trans_has_audio' => '',
         'trans_filter' => 'limit',
-        'sort' => 'words',
+        'sort' => 'relevance',
         'sort_reverse' => '',
     );
 

src/Model/Table/SentencesTable.php

@@ -1339,6 +1339,8 @@ public function sphinxOptions($query, $from, $sort, $sort_reverse)
             $sortOrder = $this->orderby('@rank', $sort_reverse);
             if ($sort == 'words') {
                 $rankingExpr = '-text_len';
+            } elseif ($sort == 'relevance') {
+                $rankingExpr = '-text_len+top(lcs+exact_order*100)*100';
             } elseif ($sort == 'created' || $sort == 'modified') {
                 $rankingExpr = $sort;
             }

src/Template/Element/advanced_search_form.ctp

@@ -209,6 +209,7 @@ echo $this->Form->create(
       echo $this->Form->input('sort', array(
           'label' => __('Order:'),
           'options' => array(
+              'relevance' => __('Relevance'),
               'words' => __('Fewest words first'),
               'created' => __('Last created first'),
               'modified' => __('Last modified first'),