<?php
/*
***************************************************************************
*   Copyright (C) 2008 by Felipe Ribeiro                                  *
*   felipernb@gmail.com                                                   *
*   http://www.feliperibeiro.com                                          *
*                                                                         *
*   Permission is hereby granted, free of charge, to any person obtaining *
*   a copy of this software and associated documentation files (the       *
*   "Software"), to deal in the Software without restriction, including   *
*   without limitation the rights to use, copy, modify, merge, publish,   *
*   distribute, sublicense, and/or sell copies of the Software, and to    *
*   permit persons to whom the Software is furnished to do so, subject to *
*   the following conditions:                                             *
*                                                                         *
*   The above copyright notice and this permission notice shall be        *
*   included in all copies or substantial portions of the Software.       *
*                                                                         *
*   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,       *
*   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF    *
*   MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.*
*   IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR     *
*   OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, *
*   ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR *
*   OTHER DEALINGS IN THE SOFTWARE.                                       *
***************************************************************************
*/


/**
 * This class implements the Spell correcting feature, useful for the
 * "Did you mean" functionality on the search engine. Using a dicionary of words
 * extracted from the product catalog.
 *
 * Based on the concepts of Peter Norvig: http://norvig.com/spell-correct.html
 *
 * @author Felipe Ribeiro <felipernb@gmail.com>
 * @date September 18th, 2008
 * @package catalog
 *
 */
class SpellCorrector {
	private static $NWORDS;

	/**
	 * Reads a text and extracts the list of words
	 *
	 * @param string $text
	 * @return array The list of words
	 */
	private static function  words($text) {
		$matches = array();
		preg_match_all("/[a-z]+/",strtolower($text),$matches);
		return $matches[0];
	}

	/**
	 * Creates a table (dictionary) where the word is the key and the value is it's relevance
	 * in the text (the number of times it appear)
	 *
	 * @param array $features
	 * @return array
	 */
	private static function train(array $features) {
		$model = array();
		$count = count($features);
		for($i = 0; $i<$count; $i++) {
			$f = $features[$i];
			$parts = explode("=",$f);
			$model[$parts[0]] = intval($parts[1]);
		}
		return $model;
	}

	/**
	 * Generates a list of possible "disturbances" on the passed string
	 *
	 * @param string $word
	 * @return array
	 */
	private static function edits1($word) {
		$alphabet = 'abcdefghijklmnopqrstuvwxyz';
		$alphabet = str_split($alphabet);
		$n = strlen($word);
		$edits = array();
		for($i = 0 ; $i<$n;$i++) {
			$edits[] = substr($word,0,$i).substr($word,$i+1); 		//deleting one char
			foreach($alphabet as $c) {
				$edits[] = substr($word,0,$i) . $c . substr($word,$i+1); //substituting one char
			}
		}
		for($i = 0; $i < $n-1; $i++) {
			$edits[] = substr($word,0,$i).$word[$i+1].$word[$i].substr($word,$i+2); //swapping chars order
		}
		for($i=0; $i < $n+1; $i++) {
			foreach($alphabet as $c) {
				$edits[] = substr($word,0,$i).$c.substr($word,$i); //inserting one char
			}
		}

		return $edits;
	}

	/**
	 * Generate possible "disturbances" in a second level that exist on the dictionary
	 *
	 * @param string $word
	 * @return array
	 */
	private static function known_edits2($word) {
		$known = array();
		foreach(self::edits1($word) as $e1) {
			foreach(self::edits1($e1) as $e2) {
				if(array_key_exists($e2,self::$NWORDS)) $known[] = $e2;
			}
		}
		return $known;
	}

	/**
	 * Given a list of words, returns the subset that is present on the dictionary
	 *
	 * @param array $words
	 * @return array
	 */
	private static function known(array $words) {
		$known = array();
		foreach($words as $w) {
			if(array_key_exists($w,self::$NWORDS)) {
				$known[] = $w;
			}
		}
		return $known;
	}


	/**
	 * Returns the word that is present on the dictionary that is the most similar (and the most relevant) to the
	 * word passed as parameter,
	 *
	 * @param string $word
	 * @return string
	 */
	public static function correct($word) {
		$word = trim($word);
		if(empty($word)) return;

		$word = strtolower($word);
		/* To optimize performance, the serialized dictionary can be saved on a file
		instead of parsing every single execution */

		if(empty(self::$NWORDS)) {

			if(!file_exists('serialized_dictionary.txt')) {
				echo "ERROR!! Please keep serialized_dictionary.txt in the same directory!!";
			} else {
				$contents = array();
				$contents = file("serialized_dictionary.txt");
				self::$NWORDS = self::train($contents);
			}
		}

		$candidates = array();
		if(self::known(array($word))) {
			return $word;
		} elseif(($tmp_candidates = self::known(self::edits1($word)))) {
			foreach($tmp_candidates as $candidate) {
				$candidates[] = $candidate;
			}
		} elseif(($tmp_candidates = self::known_edits2($word))) {
			foreach($tmp_candidates as $candidate) {
				$candidates[] = $candidate;
			}
		} else {
			return $word;
		}

		$max = 0;
		foreach($candidates as $c) {
			$value = self::$NWORDS[$c];
			if( $value > $max) {
				$max = $value;
				$word = $c;
			}
		}
		return $word;
	}
}

?>