class SearchIndex

class SearchIndex implements SearchIndexInterface {
    
    /**
     * The config factory.
     *
     * @var \Drupal\Core\Config\ConfigFactoryInterface
     */
    protected $configFactory;
    
    /**
     * The database connection.
     *
     * @var \Drupal\Core\Database\Connection
     */
    protected $connection;
    
    /**
     * The database replica connection.
     *
     * @var \Drupal\Core\Database\Connection
     */
    protected $replica;
    
    /**
     * The cache tags invalidator.
     *
     * @var \Drupal\Core\Cache\CacheTagsInvalidatorInterface
     */
    protected $cacheTagsInvalidator;
    
    /**
     * SearchIndex constructor.
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The config factory.
     * @param \Drupal\Core\Database\Connection $connection
     *   The database connection.
     * @param \Drupal\Core\Database\Connection $replica
     *   The database replica connection.
     * @param \Drupal\Core\Cache\CacheTagsInvalidatorInterface $cache_tags_invalidator
     *   The cache tags invalidator.
     */
    public function __construct(ConfigFactoryInterface $config_factory, Connection $connection, Connection $replica, CacheTagsInvalidatorInterface $cache_tags_invalidator) {
        $this->configFactory = $config_factory;
        $this->connection = $connection;
        $this->replica = $replica;
        $this->cacheTagsInvalidator = $cache_tags_invalidator;
    }
    
    /**
     * {@inheritdoc}
     */
    public function index($type, $sid, $langcode, $text, $update_weights = TRUE) {
        $settings = $this->configFactory
            ->get('search.settings');
        $minimum_word_size = $settings->get('index.minimum_word_size');
        // Keep track of the words that need to have their weights updated.
        $current_words = [];
        // Multipliers for scores of words inside certain HTML tags. The weights are
        // stored in config so that modules can overwrite the default weights.
        // Note: 'a' must be included for link ranking to work.
        $tags = $settings->get('index.tag_weights');
        // Strip off all ignored tags to speed up processing, but insert space
        // before and after them to keep word boundaries.
        $text = str_replace([
            '<',
            '>',
        ], [
            ' <',
            '> ',
        ], $text);
        $text = strip_tags($text, '<' . implode('><', array_keys($tags)) . '>');
        // Split HTML tags from plain text.
        $split = preg_split('/\\s*<([^>]+?)>\\s*/', $text, -1, PREG_SPLIT_DELIM_CAPTURE);
        // Note: PHP ensures the array consists of alternating delimiters and
        // literals and begins and ends with a literal (inserting $null as
        // required).
        // Odd/even counter. Tag or no tag.
        $tag = FALSE;
        // Starting score per word.
        $score = 1;
        // Accumulator for cleaned up data.
        $accum = ' ';
        // Stack with open tags.
        $tagstack = [];
        // Counter for consecutive words.
        $tagwords = 0;
        // Focus state.
        $focus = 1;
        // Accumulator for words for index.
        $scored_words = [];
        foreach ($split as $value) {
            if ($tag) {
                // Increase or decrease score per word based on tag.
                list($tagname) = explode(' ', $value, 2);
                $tagname = mb_strtolower($tagname);
                // Closing or opening tag?
                if ($tagname[0] == '/') {
                    $tagname = substr($tagname, 1);
                    // If we encounter unexpected tags, reset score to avoid incorrect
                    // boosting.
                    if (!count($tagstack) || $tagstack[0] != $tagname) {
                        $tagstack = [];
                        $score = 1;
                    }
                    else {
                        // Remove from tag stack and decrement score.
                        $score = max(1, $score - $tags[array_shift($tagstack)]);
                    }
                }
                else {
                    if (isset($tagstack[0]) && $tagstack[0] == $tagname) {
                        // None of the tags we look for make sense when nested identically.
                        // If they are, it's probably broken HTML.
                        $tagstack = [];
                        $score = 1;
                    }
                    else {
                        // Add to open tag stack and increment score.
                        array_unshift($tagstack, $tagname);
                        $score += $tags[$tagname];
                    }
                }
                // A tag change occurred, reset counter.
                $tagwords = 0;
            }
            else {
                // Note: use of PREG_SPLIT_DELIM_CAPTURE above will introduce empty
                // values.
                if ($value != '') {
                    $words = search_index_split($value, $langcode);
                    foreach ($words as $word) {
                        // Add word to accumulator.
                        $accum .= $word . ' ';
                        // Check word length.
                        if (is_numeric($word) || mb_strlen($word) >= $minimum_word_size) {
                            if (!isset($scored_words[$word])) {
                                $scored_words[$word] = 0;
                            }
                            $scored_words[$word] += $score * $focus;
                            // Focus is a decaying value in terms of the amount of unique
                            // words up to this point. From 100 words and more, it decays, to
                            // e.g. 0.5 at 500 words and 0.3 at 1000 words.
                            $focus = min(1, 0.01 + 3.5 / (2 + count($scored_words) * 0.015));
                        }
                        $tagwords++;
                        // Too many words inside a single tag probably mean a tag was
                        // accidentally left open.
                        if (count($tagstack) && $tagwords >= 15) {
                            $tagstack = [];
                            $score = 1;
                        }
                    }
                }
            }
            $tag = !$tag;
        }
        // Remove the item $sid from the search index, and invalidate the relevant
        // cache tags.
        $this->clear($type, $sid, $langcode);
        try {
            // Insert cleaned up data into dataset.
            $this->connection
                ->insert('search_dataset')
                ->fields([
                'sid' => $sid,
                'langcode' => $langcode,
                'type' => $type,
                'data' => $accum,
                'reindex' => 0,
            ])
                ->execute();
            // Insert results into search index.
            foreach ($scored_words as $word => $score) {
                // If a word already exists in the database, its score gets increased
                // appropriately. If not, we create a new record with the appropriate
                // starting score.
                $this->connection
                    ->merge('search_index')
                    ->keys([
                    'word' => $word,
                    'sid' => $sid,
                    'langcode' => $langcode,
                    'type' => $type,
                ])
                    ->fields([
                    'score' => $score,
                ])
                    ->expression('score', 'score + :score', [
                    ':score' => $score,
                ])
                    ->execute();
                $current_words[$word] = TRUE;
            }
        } catch (\Exception $e) {
            throw new SearchIndexException("Failed to insert dataset in index for type '{$type}', sid '{$sid}' and langcode '{$langcode}'", 0, $e);
        } finally {
            if ($update_weights) {
                $this->updateWordWeights($current_words);
            }
        }
        return $current_words;
    }
    
    /**
     * {@inheritdoc}
     */
    public function clear($type = NULL, $sid = NULL, $langcode = NULL) {
        try {
            $query_index = $this->connection
                ->delete('search_index');
            $query_dataset = $this->connection
                ->delete('search_dataset');
            if ($type) {
                $query_index->condition('type', $type);
                $query_dataset->condition('type', $type);
                if ($sid) {
                    $query_index->condition('sid', $sid);
                    $query_dataset->condition('sid', $sid);
                    if ($langcode) {
                        $query_index->condition('langcode', $langcode);
                        $query_dataset->condition('langcode', $langcode);
                    }
                }
            }
            $query_index->execute();
            $query_dataset->execute();
        } catch (\Exception $e) {
            throw new SearchIndexException("Failed to clear index for type '{$type}', sid '{$sid}' and langcode '{$langcode}'", 0, $e);
        }
        if ($type) {
            // Invalidate all render cache items that contain data from this index.
            $this->cacheTagsInvalidator
                ->invalidateTags([
                'search_index:' . $type,
            ]);
        }
        else {
            // Invalidate all render cache items that contain data from any index.
            $this->cacheTagsInvalidator
                ->invalidateTags([
                'search_index',
            ]);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function markForReindex($type = NULL, $sid = NULL, $langcode = NULL) {
        try {
            $query = $this->connection
                ->update('search_dataset')
                ->fields([
                'reindex' => REQUEST_TIME,
            ])
                ->condition('reindex', 0);
            if ($type) {
                $query->condition('type', $type);
                if ($sid) {
                    $query->condition('sid', $sid);
                    if ($langcode) {
                        $query->condition('langcode', $langcode);
                    }
                }
            }
            $query->execute();
        } catch (\Exception $e) {
            throw new SearchIndexException("Failed to mark index for re-indexing for type '{$type}', sid '{$sid}' and langcode '{$langcode}'", 0, $e);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function updateWordWeights(array $words) {
        try {
            // Update word IDF (Inverse Document Frequency) counts for new/changed
            // words.
            $words = array_keys($words);
            foreach ($words as $word) {
                // Get total count.
                $total = $this->replica
                    ->query("SELECT SUM(score) FROM {search_index} WHERE word = :word", [
                    ':word' => $word,
                ])
                    ->fetchField();
                // Apply Zipf's law to equalize the probability distribution.
                $total = log10(1 + 1 / max(1, $total));
                $this->connection
                    ->merge('search_total')
                    ->key('word', $word)
                    ->fields([
                    'count' => $total,
                ])
                    ->execute();
            }
            // Find words that were deleted from search_index, but are still in
            // search_total. We use a LEFT JOIN between the two tables and keep only
            // the rows which fail to join.
            $result = $this->replica
                ->query("SELECT t.word AS realword, i.word FROM {search_total} t LEFT JOIN {search_index} i ON t.word = i.word WHERE i.word IS NULL");
            $or = new Condition('OR');
            foreach ($result as $word) {
                $or->condition('word', $word->realword);
            }
            if (count($or) > 0) {
                $this->connection
                    ->delete('search_total')
                    ->condition($or)
                    ->execute();
            }
        } catch (\Exception $e) {
            throw new SearchIndexException("Failed to update totals for index words.", 0, $e);
        }
    }

}