Porter Stemmer

porter_stemmer.php

<?php
/**
* porter_stemmer.php
*
* In linguistic morphology and information retrieval, stemming is the process
* for reducing inflected (or sometimes derived) words to their stem, base or
* root form—generally a written word form.
*
* Porter Stemmer Steps:
* - Step 1: Gets rid of plurals and -ed or -ing suffixes
* - Step 2: Turns terminal y to i when there is another vowel in the stem
* - Step 3: Maps double suffixes to single ones: -ization, -ational, etc.
* - Step 4: Deals with suffixes, -full, -ness etc.
* - Step 5: Takes off -ant, -ence, etc.
* - Step 6: Removes a final -e
*
* Porter Mishaps & Shortcomings:
* - Severing vs. several => sever
* - University vs. universe => univers
* - Iron vs. ironic => iron
* - Animal vs. animated
* - Stemmers are rudimentary
* - No word sense disambiguation (“bats” vs “batting”)
* - No POS disambiguation (“Batting” could be noun or verb, but “hitting” could only be verb)
* - Cannot handle irregular conjungation/inflection (“to be”, etc.)
*
* @example
* $word = "stemming";
* $ps = new PorterStemmer($word);
* $stem = $ps->get_stem();
* echo $stem;
*/
class PorterStemmer
{
private $regex_consonant = '(?:[bcdfghjklmnpqrstvwxz]|(?<=[aeiou])y|^y)';
private $regex_vowel = '(?:[aeiou]|(?<![aeiou])y)';

private $stems = array();
private $words = array();

private $input;
private $stem;
private $word;

public function __construct($input)
{
$this->words = preg_split("/\s/", $input);
$this->stems();
}

public function writeln($str)
{
if (is_array($str))
{
echo "<pre>";
print_r($str);
echo "</pre>";
}
else
{
echo $str;
}
echo "<br>";
}

public function stems ()
{
foreach ($this->words as $this->stem)
{
$this->step1();
$this->step2();
$this->step3();
$this->step4();
$this->step5();
$this->step6();
$this->stems[] = $this->stem;
}
}

/**
* Step 1: Gets rid of plurals and -ed or -ing suffixes
*/
public function step1()
{
if (substr($this->stem, -1) == 's')
{
$this->replace($this->stem, 'sses', 'ss')
OR $this->replace($this->stem, 'ies', 'i')
OR $this->replace($this->stem, 'ss', 'ss')
OR $this->replace($this->stem, 's', '');
}

if (substr($this->stem, -2, 1) != 'e' OR !$this->replace($this->stem, 'eed', 'ee', 0))
{
$v = $this->regex_vowel;

if (preg_match("#$v+#", substr($this->stem, 0, -3)) && $this->replace($this->stem, 'ing', '')
OR preg_match("#$v+#", substr($this->stem, 0, -2)) && $this->replace($this->stem, 'ed', ''))
{
if (!$this->replace($this->stem, 'at', 'ate')
AND !$this->replace($this->stem, 'bl', 'ble')
AND !$this->replace($this->stem, 'iz', 'ize'))
{
// Double consonant ending
if ($this->doubleConsonant($this->stem)
AND substr($this->stem, -2) != 'll'
AND substr($this->stem, -2) != 'ss'
AND substr($this->stem, -2) != 'zz')
{
$this->stem = substr($this->stem, 0, -1);

}
elseif ($this->m($this->stem) == 1 AND $this->cvc($this->stem))
{
$this->stem .= 'e';
}
}
}
}
}

/**
* Step 1c: Turns terminal y to i when there is another vowel in the stem
*/
public function step2()
{
$v = $this->regex_vowel;

if (substr($this->stem, -1) == 'y' && preg_match("#$v+#", substr($this->stem, 0, -1)))
{
$this->replace($this->stem, 'y', 'i');
}
}

/**
* Step 2: Maps double suffixes to single ones
*/
public function step3()
{
switch (substr($this->stem, -2, 1))
{
case 'a':
$this->replace($this->stem, 'ational', 'ate', 0)
OR $this->replace($this->stem, 'tional', 'tion', 0);
break;

case 'c':
$this->replace($this->stem, 'enci', 'ence', 0)
OR $this->replace($this->stem, 'anci', 'ance', 0);
break;

case 'e':
$this->replace($this->stem, 'izer', 'ize', 0);
break;

case 'g':
$this->replace($this->stem, 'logi', 'log', 0);
break;

case 'l':
$this->replace($this->stem, 'entli', 'ent', 0)
OR $this->replace($this->stem, 'ousli', 'ous', 0)
OR $this->replace($this->stem, 'alli', 'al', 0)
OR $this->replace($this->stem, 'bli', 'ble', 0)
OR $this->replace($this->stem, 'eli', 'e', 0);
break;

case 'o':
$this->replace($this->stem, 'ization', 'ize', 0)
OR $this->replace($this->stem, 'ation', 'ate', 0)
OR $this->replace($this->stem, 'ator', 'ate', 0);
break;

case 's':
$this->replace($this->stem, 'iveness', 'ive', 0)
OR $this->replace($this->stem, 'fulness', 'ful', 0)
OR $this->replace($this->stem, 'ousness', 'ous', 0)
OR $this->replace($this->stem, 'alism', 'al', 0);
break;

case 't':
$this->replace($this->stem, 'biliti', 'ble', 0)
OR $this->replace($this->stem, 'aliti', 'al', 0)
OR $this->replace($this->stem, 'iviti', 'ive', 0);
break;
}
}

/**
* Step 3: Deals with suffixes, -full, -ness etc.
*/
public function step4()
{
switch (substr($this->stem, -2, 1))
{
case 'a':
$this->replace($this->stem, 'ical', 'ic', 0);
break;

case 's':
$this->replace($this->stem, 'ness', '', 0);
break;

case 't':
$this->replace($this->stem, 'icate', 'ic', 0)
OR $this->replace($this->stem, 'iciti', 'ic', 0);
break;

case 'u':
$this->replace($this->stem, 'ful', '', 0);
break;

case 'v':
$this->replace($this->stem, 'ative', '', 0);
break;

case 'z':
$this->replace($this->stem, 'alize', 'al', 0);
break;
}
}

/**
* Step 4: Takes off -ant, -ence, etc.
*/
public function step5()
{
switch (substr($this->stem, -2, 1))
{
case 'a':
$this->replace($this->stem, 'al', '', 1);
break;

case 'c':
$this->replace($this->stem, 'ance', '', 1)
OR $this->replace($this->stem, 'ence', '', 1);
break;

case 'e':
$this->replace($this->stem, 'er', '', 1);
break;

case 'i':
$this->replace($this->stem, 'ic', '', 1);
break;

case 'l':
$this->replace($this->stem, 'able', '', 1)
OR $this->replace($this->stem, 'ible', '', 1);
break;

case 'n':
$this->replace($this->stem, 'ant', '', 1)
OR $this->replace($this->stem, 'ement', '', 1)
OR $this->replace($this->stem, 'ment', '', 1)
OR $this->replace($this->stem, 'ent', '', 1);
break;

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

case 's':
$this->replace($this->stem, 'ism', '', 1);
break;

case 't':
$this->replace($this->stem, 'ate', '', 1)
OR $this->replace($this->stem, 'iti', '', 1);
break;

case 'u':
$this->replace($this->stem, 'ous', '', 1);
break;

case 'v':
$this->replace($this->stem, 'ive', '', 1);
break;

case 'z':
$this->replace($this->stem, 'ize', '', 1);
break;
}
}

/**
* Step 5: Removes a final -e
*/
public function step6()
{
// Part a
if (substr($this->stem, -1) == 'e')
{
if ($this->m(substr($this->stem, 0, -1)) > 1)
{
$this->replace($this->stem, 'e', '');

}
elseif ($this->m(substr($this->stem, 0, -1)) == 1)
{
if (!$this->cvc(substr($this->stem, 0, -1)))
{
$this->replace($this->stem, 'e', '');
}
}
}

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

/**
* 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 $str string. True does not necessarily mean
* that it was replaced.
*/
public function replace(&$str, $check, $repl, $m = null)
{
$len = 0 - strlen($check);

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

return true;
}

return false;
}

/**
* m() measures the number of consonant sequences in $str. if c is
* a consonant sequence and v a vowel sequence, and <..> indicates arbitrary
* presence,
*
* <c><v> gives 0
* <c>vc<v> gives 1
* <c>vcvc<v> gives 2
* <c>vcvcvc<v> gives 3
*/
public function m($str)
{
$c = $this->regex_consonant;
$v = $this->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.
*/
public function doubleConsonant($str)
{
$c = $this->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
*/
public function cvc($str)
{
$c = $this->regex_consonant;
$v = $this->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';
}

public function get_stems()
{
return $this->stems;
}
} // Class
?>