<?php
/**
* SeekQuarry/Yioop --
* Open Source Pure PHP Search Engine, Crawler, and Indexer
*
* Copyright (C) 2009 - 2026 Chris Pollett chris@pollett.org
*
* LICENSE:
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <https://www.gnu.org/licenses/>.
*
* END LICENSE
*
* @author Mangesh Dahale mangeshadahale@gmail.com
* @license https://www.gnu.org/licenses/ GPL3
* @link https://www.seekquarry.com/
* @copyright 2009 - 2026
* @filesource
*/
namespace seekquarry\yioop\library\summarizers;
use seekquarry\yioop\configs as C;
use seekquarry\yioop\library as L;
use seekquarry\yioop\library\CrawlConstants;
use seekquarry\yioop\library\PhraseParser;
use seekquarry\yioop\library\LinearAlgebra;
/**
* Class which may be used by TextProcessors to get a summary for a text
* document that may later be used for indexing. This is done by
* the @see getSummmary method. getSummary does this splitting
* the document into sentences and computing inverse sentence frequency
* (should be ISL, but we call IDF) scores for each term. It then computes
* an average document vector (we call centroid) with components
* (total number of occurrences of term) * (IDF score of term).
* It also generates a word cloud for a document. Notice if we divided
* this by number of documents, we would have components
* average term frequency * IDF. As ranking by either won't affect out
* results, we don't divide. We then compute the cosine similarity of
* each sentence vector with this average and choose the top sentences
* to make our summary. Here a sentence vector has components
* term frequency in sentence * IDF score of term.
*
* @author Mangesh Dahale mangeshadahale@gmail.com
*/
class CentroidSummarizer extends Summarizer
{
/**
* Generates a summary, word cloud, and sentence scoring for a provides
* web page. To do this the page is split into sentences and inverse
* sentence frequency (should be ISL, but we call IDF) scores for each term
* term are computed. Then an average document vector (we call centroid)
* with components
* (total number of occurrences of term) * (IDF score of term)
* is found. We then compute the cosine similarity of
* each sentence vector with this average and choose the top sentences
* to make our summary. Here a sentence vector has components
* term frequency in sentence * IDF score of term.
*
* @param object $dom document object model of page to summarize
* @param string $page complete raw page to generate the summary from.
* @param string $lang language of the page to decide which stop words to
* call proper tokenizer.php of the specified language.
* @return array a triple (string summary, array word cloud, array
* of position => scores for positions within the summary)
*/
public static function getSummary($dom, $page, $lang)
{
list($original_sentences, $sentences) =
self::getPunctuatedUnpunctuatedSentences($dom, $page, $lang);
$terms = self::getTermsFromSentences($sentences, $lang);
$num_sentences = count($original_sentences);
$formatted_doc = self::formatDoc($page);
list($centroid, $idf) = self::computeCentroidIdfFromSentences($terms,
$sentences, $formatted_doc, $lang);
$word_cloud = self::wordCloudFromTermVector($centroid, $terms);
$sorted_sentence_scores = self::scoreSentencesVersusPageTerms(
$sentences, $centroid, $idf, $terms);
list($summary, $summary_scores) = self::getSummaryFromSentenceScores(
$sorted_sentence_scores, $original_sentences, $lang);
return [$summary, $word_cloud, $summary_scores];
}
/**
* Computes a number of occurrences of term * inverse sentence frequency
* vector over all terms in the document as well as inverse sentence
* frequencies for each term in a document.
* @param array $terms distinct terms in a document
* @param array $sentences sentences of a document
* @param string $formatted_doc original document with some punctuation
* removed
* @param string $lang locale tag for document
* @return array [truncated to maximal self::CENTROID_COMPONENTS
* number of occurrences of term * inverse sentence frequency
* vector, array of inverse sentence frequencies for each term
* in document]
*/
public static function computeCentroidIdfFromSentences($terms,
$sentences, $formatted_doc, $lang)
{
$num_sentences = count($sentences);
$num_terms = count($terms);
if ($num_terms == 0) {
return [[], [], 0];
}
/* Initialize Nk [Number of sentences the term occurs] */
$nk = [];
$nk = array_fill(0, $num_terms, 0);
for ($j = 0; $j < $num_terms; $j++) {
for ($i = 0; $i < $num_sentences; $i++) {
if (is_string($terms[$j]) &&
str_contains($sentences[$i], $terms[$j])) {
$nk[$j]++;
}
}
}
/* Calculate IDF (inverse document frequency) score for each term
*/
$idf = [];
for ($k = 0; $k < $num_terms; $k++) {
$idf[$k] = ($nk[$k] == 0) ? 0 : log($num_sentences / $nk[$k]);
}
/* Count TF for finding centroid */
$b = "\b"; //term break character
if (in_array($lang, ["zh-CN", "ja", "ko"])) {
$b = ""; // some asian languages don't use
}
set_error_handler(null);
// Calculate term frequency whole doc (nt) * IDF (sentence) scores
$ntidf = [];
for ($j = 0; $j < $num_terms; $j++) {
$quoted = preg_quote($terms[$j], "/");
$nt = @preg_match_all("/$b(" . $quoted . ")$b/ui", $formatted_doc,
$matches); //$matches included for backwards compatibility
$ntidf[$j] = $nt * $idf[$j];
if (is_nan($ntidf[$j]) || is_infinite($ntidf[$j])) {
$ntidf[$j] = 0;
}
}
restore_error_handler();
/* Calculate centroid */
arsort($ntidf);
/* pick top self::CENTROID_COMPONENTS components of the ntidf vector
as centroid preserving term_index => value association
*/
$centroid = array_slice($ntidf, 0, self::CENTROID_COMPONENTS, true);
return [$centroid, $idf];
}
/**
* Calculates scores for an array of sentences using normalized
* tf-idf score vector of sentence dot centroid vector.
*
* @param array $sentences unpunctated sentences from a source in the
* order they originally appeared in the source
* @param array $centroid an array of term_index => nt *idf scores for that
* term. Here nt number of times term appear in whole document
* idf is inverse document frequency for that term amongst the
* sentences
* @param array $idf array of pairs of form term_index =>
* inverse document frequencies of term amongst sentences
* @param array $terms an array of terms from the sentences that
* term_indexes mentioned above index into
* @return array scores for each sentence
*/
public static function scoreSentencesVersusPageTerms($sentences,
$centroid, $idf, $terms)
{
$centroid_norm = LinearAlgebra::length($centroid);
/* Calculate similarity measure between centroid and each sentence */
$num_terms = count($terms);
$sentence_scores = [];
foreach ($sentences as $sentence) {
$sentence_tfidf_dot_centroid = 0;
$sentence_tfidf_norm_square = 0;
foreach($centroid as $k => $ntidf_k) {
$idf_k = $idf[$k];
//term frequency of term k in current sentence
$tf_k = substr_count($sentence, $terms[$k]);
// TFIDF score of term k in current centence
$tfidf_k = ($tf_k > 0) ?
(1 + log($tf_k)) * $idf_k : 0;
$sentence_tfidf_dot_centroid += ($tfidf_k * $ntidf_k);
$sentence_tfidf_norm_square += ($tfidf_k * $tfidf_k);
}
$normalization = sqrt($sentence_tfidf_norm_square) * $centroid_norm;
$sentence_scores[] = ($normalization == 0) ? 0 :
$sentence_tfidf_dot_centroid / $normalization;
}
return $sentence_scores;
}
}
(c) 2026 Seekquarry - Open Source Search Engine Software