stemmer.php

<?php

/**
 * Copyright (c) 2005 Richard Heyes (http://www.phpguru.org/)
 *
 * All rights reserved.
 *
 * This script is free software.
 */

/**
 * PHP5 Implementation of the Porter Stemmer algorithm. Certain elements
 * were borrowed from the (broken) implementation by Jon Abernathy.
 *
 * Usage:
 *
 *  $stem = PorterStemmer::Stem($word);
 *
 * How easy is that?
 */

class PorterStemmer
{
	/**
	 * Regex for matching a consonant
	 * @var string
	 */
	private static $regex_consonant = '(?:[bcdfghjklmnpqrstvwxz]|(?<=[aeiou])y|^y)';


	/**
	 * Regex for matching a vowel
	 * @var string
	 */
	private static $regex_vowel = '(?:[aeiou]|(?<![aeiou])y)';


	/**
	 * Stems a word. Simple huh?
	 *
	 * @param  string $word Word to stem
	 * @return string	Stemmed word
	 */
	public static function Stem($word)
	{
		if (strlen($word) <= 2) {
			return $word;
		}

		$word = self::step1ab($word);
		$word = self::step1c($word);
		$word = self::step2($word);
		$word = self::step3($word);
		$word = self::step4($word);
		$word = self::step5($word);

		return $word;
	}


	/**
	 * Step 1
	 */
	private static function step1ab($word)
	{
		// Part a
		if (substr($word, -1) == 's') {

			self::replace($word, 'sses', 'ss')
			OR self::replace($word, 'ies', 'i')
			OR self::replace($word, 'ss', 'ss')
			OR self::replace($word, 's', '');
		}

		// Part b
		if (substr($word, -2, 1) != 'e' OR !self::replace($word, 'eed', 'ee', 0)) { // First rule
			$v = self::$regex_vowel;

			// ing and ed
			if (   preg_match("#$v+#", substr($word, 0, -3)) && self::replace($word, 'ing', '')
				OR preg_match("#$v+#", substr($word, 0, -2)) && self::replace($word, 'ed', '')) { // Note use of && and OR, for precedence reasons

				// If one of above two test successful
				if (	!self::replace($word, 'at', 'ate')
					AND !self::replace($word, 'bl', 'ble')
					AND !self::replace($word, 'iz', 'ize')) {

					// Double consonant ending
					if (	self::doubleConsonant($word)
						AND substr($word, -2) != 'll'
						AND substr($word, -2) != 'ss'
						AND substr($word, -2) != 'zz') {

						$word = substr($word, 0, -1);

					} else if (self::m($word) == 1 AND self::cvc($word)) {
						$word .= 'e';
					}
				}
			}
		}

		return $word;
	}


	/**
	 * Step 1c
	 *
	 * @param string $word Word to stem
	 */
	private static function step1c($word)
	{
		$v = self::$regex_vowel;

		if (substr($word, -1) == 'y' && preg_match("#$v+#", substr($word, 0, -1))) {
			self::replace($word, 'y', 'i');
		}

		return $word;
	}


	/**
	 * Step 2
	 *
	 * @param string $word Word to stem
	 */
	private static function step2($word)
	{
		switch (substr($word, -2, 1)) {
			case 'a':
				self::replace($word, 'ational', 'ate', 0)
				OR self::replace($word, 'tional', 'tion', 0);
				break;

			case 'c':
				self::replace($word, 'enci', 'ence', 0)
				OR self::replace($word, 'anci', 'ance', 0);
				break;

			case 'e':
				self::replace($word, 'izer', 'ize', 0);
				break;

			case 'g':
				self::replace($word, 'logi', 'log', 0);
				break;

			case 'l':
				self::replace($word, 'entli', 'ent', 0)
				OR self::replace($word, 'ousli', 'ous', 0)
				OR self::replace($word, 'alli', 'al', 0)
				OR self::replace($word, 'bli', 'ble', 0)
				OR self::replace($word, 'eli', 'e', 0);
				break;

			case 'o':
				self::replace($word, 'ization', 'ize', 0)
				OR self::replace($word, 'ation', 'ate', 0)
				OR self::replace($word, 'ator', 'ate', 0);
				break;

			case 's':
				self::replace($word, 'iveness', 'ive', 0)
				OR self::replace($word, 'fulness', 'ful', 0)
				OR self::replace($word, 'ousness', 'ous', 0)
				OR self::replace($word, 'alism', 'al', 0);
				break;

			case 't':
				self::replace($word, 'biliti', 'ble', 0)
				OR self::replace($word, 'aliti', 'al', 0)
				OR self::replace($word, 'iviti', 'ive', 0);
				break;
		}

		return $word;
	}


	/**
	 * Step 3
	 *
	 * @param string $word String to stem
	 */
	private static function step3($word)
	{
		switch (substr($word, -2, 1)) {
			case 'a':
				self::replace($word, 'ical', 'ic', 0);
				break;

			case 's':
				self::replace($word, 'ness', '', 0);
				break;

			case 't':
				self::replace($word, 'icate', 'ic', 0)
				OR self::replace($word, 'iciti', 'ic', 0);
				break;

			case 'u':
				self::replace($word, 'ful', '', 0);
				break;

			case 'v':
				self::replace($word, 'ative', '', 0);
				break;

			case 'z':
				self::replace($word, 'alize', 'al', 0);
				break;
		}

		return $word;
	}


	/**
	 * Step 4
	 *
	 * @param string $word Word to stem
	 */
	private static function step4($word)
	{
		switch (substr($word, -2, 1)) {
			case 'a':
				self::replace($word, 'al', '', 1);
				break;

			case 'c':
				self::replace($word, 'ance', '', 1)
				OR self::replace($word, 'ence', '', 1);
				break;

			case 'e':
				self::replace($word, 'er', '', 1);
				break;

			case 'i':
				self::replace($word, 'ic', '', 1);
				break;

			case 'l':
				self::replace($word, 'able', '', 1)
				OR self::replace($word, 'ible', '', 1);
				break;

			case 'n':
				self::replace($word, 'ant', '', 1)
				OR self::replace($word, 'ement', '', 1)
				OR self::replace($word, 'ment', '', 1)
				OR self::replace($word, 'ent', '', 1);
				break;

			case 'o':
				if (substr($word, -4) == 'tion' OR substr($word, -4) == 'sion') {
				self::replace($word, 'ion', '', 1);
				} else {
					self::replace($word, 'ou', '', 1);
				}
				break;

			case 's':
				self::replace($word, 'ism', '', 1);
				break;

			case 't':
				self::replace($word, 'ate', '', 1)
				OR self::replace($word, 'iti', '', 1);
				break;

			case 'u':
				self::replace($word, 'ous', '', 1);
				break;

			case 'v':
				self::replace($word, 'ive', '', 1);
				break;

			case 'z':
				self::replace($word, 'ize', '', 1);
				break;
		}

		return $word;
	}


	/**
	 * Step 5
	 *
	 * @param string $word Word to stem
	 */
	private static function step5($word)
	{
		// Part a
		if (substr($word, -1) == 'e') {
			if (self::m(substr($word, 0, -1)) > 1) {
				self::replace($word, 'e', '');

			} else if (self::m(substr($word, 0, -1)) == 1) {

				if (!self::cvc(substr($word, 0, -1))) {
					self::replace($word, 'e', '');
				}
			}
		}

		// Part b
		if (self::m($word) > 1 AND self::doubleConsonant($word) AND substr($word, -1) == 'l') {
			$word = substr($word, 0, -1);
		}

		return $word;
	}

	/**
	 * Replaces the first string with the second, at the end of the string. If third
	 * arg is given, then the preceding string must match that m count at least.
	 *
	 * @param  string $str   String to check
	 * @param  string $check Ending to check for
	 * @param  string $repl  Replacement string
	 * @param  int	$m	 Optional minimum number of m() to meet
	 * @return bool		  Whether the $check string was at the end
	 *					of the $str string. True does not necessarily mean
	 *					that it was replaced.
	 */
	private static function replace(&$str, $check, $repl, $m = null)
	{
		$len = 0 - strlen($check);

		if (substr($str, $len) == $check) {
			$substr = substr($str, 0, $len);
			if (is_null($m) OR self::m($substr) > $m) {
				$str = $substr . $repl;
			}

			return true;
		}

		return false;
	}

	/**
	 * What, you mean it's not obvious from the name?
	 *
	 * m() measures the number of consonant sequences in $str. if c is
	 * a consonant sequence and v a vowel sequence, and <..> indicates arbitrary
	 * presence,
	 *
	 * <c><v>	gives 0
	 * <c>vc<v>	 gives 1
	 * <c>vcvc<v>   gives 2
	 * <c>vcvcvc<v> gives 3
	 *
	 * @param  string $str The string to return the m count for
	 * @return int		 The m count
	 */
	private static function m($str)
	{
		$c = self::$regex_consonant;
		$v = self::$regex_vowel;

		$str = preg_replace("#^$c+#", '', $str);
		$str = preg_replace("#$v+$#", '', $str);

		preg_match_all("#($v+$c+)#", $str, $matches);

		return count($matches[1]);
	}

	/**
	 * Returns true/false as to whether the given string contains two
	 * of the same consonant next to each other at the end of the string.
	 *
	 * @param  string $str String to check
	 * @return bool		Result
	 */
	private static function doubleConsonant($str)
	{
		$c = self::$regex_consonant;

		return preg_match("#$c{2}$#", $str, $matches) AND $matches[0]{0} == $matches[0]{1};
	}

	/**
	 * Checks for ending CVC sequence where second C is not W, X or Y
	 *
	 * @param  string $str String to check
	 * @return bool		Result
	 */
	private static function cvc($str)
	{
		$c = self::$regex_consonant;
		$v = self::$regex_vowel;

		return	 preg_match("#($c$v$c)$#", $str, $matches)
			AND strlen($matches[1]) == 3
			AND $matches[1]{2} != 'w'
			AND $matches[1]{2} != 'x'
			AND $matches[1]{2} != 'y';
	}
}