<?php

function GetStem($word) {return GermanStemmer::stem($word);}

mb_internal_encoding("utf-8");

/**

 * Copyright (c) 2013 Aris Buzachis (buzachis.aris@gmail.com)
 *
 * All rights reserved.
 *
 * This script is free software.
 * 
 * DISCLAIMER:
 *
 * IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
 * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
 * ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

 /**
 * Takes a word and reduces it to its German stem using the Porter stemmer algorithm.
 * 
 * References:
 *  - http://snowball.tartarus.org/algorithms/porter/stemmer.html
 *  - http://snowball.tartarus.org/algorithms/german/stemmer.html
 *
 * Usage:
 *  $stem = GermanStemmer::stem($word);
 *
 * @author Aris Buzachis <buzachis.aris@gmail.com>
 * @author Pascal Landau <kontakt@myseosolution.de>
 */

class GermanStemmer
{
    /**
    *  R1 and R2 regions (see the Porter algorithm)
    */
    private static $R1;
    private static $R2;

    private static $cache = array();

    private static $vowels = array('a','e','i','o','u','y','ä','ö','ü');
    private static $s_ending = array('b','d','f','g','h','k','l','m','n','r','t');
    private static $st_ending = array('b','d','f','g','h','k','l','m','n','t');


    /**
     * Gets the stem of $word.
     * @param string $word
     * @return string
     */
    public static function stem($word) {
        $word = mb_strtolower($word);
        //check for invalid characters
        preg_match("#.#u",$word);
        if(preg_last_error() !== 0){
            throw new \InvalidArgumentException("Word '$word' seems to be errornous. Error code from preg_last_error(): ".preg_last_error());
        }
        if (!isset(self::$cache[$word])) {
            $result = self::getStem($word);
            self::$cache[$word] = $result;
        }

        return self::$cache[$word];
    }

    /**
     * @param $word
     * @return string
     */
    private static function getStem($word) {
        $word = self::step0a($word);
        $word = self::step1($word);
        $word = self::step2($word);
        $word = self::step3($word);
        $word = self::step0b($word);

        return $word;
    }

    /**
     * Replaces to protect some characters
     * @param string $word
     * @return string mixed
     */
    private static function step0a($word) {
        $vstr = implode('',self::$vowels);
        $word = preg_replace('#(['.$vstr.'])u(['.$vstr.'])#u', '$1U$2',$word);
        $word = preg_replace('#(['.$vstr.'])y(['.$vstr.'])#u', '$1Y$2',$word);

        return $word;
    }

    /**
     * Undo the initial replaces
     * @param string $word
     * @return string
     */
    private static function step0b($word) {
        $word = str_replace(array('ä','ö','ü', 'U','Y'),array('a','o','u','u','y'),$word);

        return $word;
    }

    private static function step1($word) {
        $word = str_replace('ß','ss',$word);

        self::getR($word);

        $replaceCount = 0;

        $arr = array('em','ern','er');
        foreach ($arr as $s) {
            self::$R1 = preg_replace('#'.$s.'$#u','',self::$R1,-1,$replaceCount);
            if ($replaceCount > 0) {
                $word = preg_replace('#'.$s.'$#u','',$word);
            }
        }

        $arr = array('en','es','e');
        foreach ($arr as $s) {
            self::$R1 = preg_replace('#'.$s.'$#u','',self::$R1,-1,$replaceCount);
            if ($replaceCount > 0) {
                $word = preg_replace('#'.$s.'$#u','',$word);
                $word = preg_replace('#niss$#u', 'nis', $word);
            }
        }

        $word = preg_replace('/(['.implode('',self::$s_ending).'])s$/u','$1',$word);

        return $word;
    }

    private static function step2($word) {
        self::getR($word);

        $replaceCount = 0;

        $arr = array('est','er','en');
        foreach ($arr as $s) {
            self::$R1 = preg_replace('#'.$s.'$#u','',self::$R1,-1,$replaceCount);
            if ($replaceCount > 0) {
                $word = preg_replace('#'.$s.'$#u','',$word);
            }
        }

        if (strpos(self::$R1,'st') !== false) {
            self::$R1 = preg_replace('#st$#u','',self::$R1);
            $word = preg_replace('#(...['.implode('',self::$st_ending).'])st$#u','$1',$word);
        }

        return $word;
    }

    private static function step3($word) {
        self::getR($word);

        $replaceCount = 0;

        $arr = array('end', 'ung');
        foreach ($arr as $s) {
            if (preg_match('#'.$s.'$#u',self::$R2)) {
                $word = preg_replace('#([^e])'.$s.'$#u','$1',$word, -1, $replaceCount);
                if ($replaceCount > 0) {
                    self::$R2 = preg_replace('#'.$s.'$#u','',self::$R2,-1,$replaceCount);
                }
            }
        }

        $arr = array('isch', 'ik', 'ig');
        foreach ($arr as $s) {
            if (preg_match('#'.$s.'$#u',self::$R2)) {
                $word = preg_replace('#([^e])'.$s.'$#u','$1',$word, -1, $replaceCount);
                if ($replaceCount > 0) {
                    self::$R2 = preg_replace('#'.$s.'$#u','',self::$R2);
                }
            }
        }

        $arr = array('lich', 'heit');
        foreach ($arr as $s) {
            self::$R2 = preg_replace('#'.$s.'$#u','',self::$R2,-1,$replaceCount);
            if ($replaceCount > 0) {
                $word = preg_replace('#'.$s.'$#u','',$word);
            } else {
                if (preg_match('#'.$s.'$#u',self::$R1)) {
                    $word = preg_replace('#(er|en)'.$s.'$#u','$1',$word, -1, $replaceCount);
                    if ($replaceCount > 0) {
                        self::$R1 = preg_replace('#'.$s.'$#u','',self::$R1);
                    }
                }
            }
        }

        $arr = array('keit');
        foreach ($arr as $s) {
            self::$R2 = preg_replace('#'.$s.'$#u','',self::$R2,-1,$replaceCount);
            if ($replaceCount > 0) {
                $word = preg_replace('#'.$s.'$#u','',$word);
            }
        }

        return $word;
    }

    /**
     * Find R1 and R2
     * @param string $word
     */
    private static function getR($word) {
        self::$R1 = "";
        self::$R2 = "";

        $vowels = implode("", self::$vowels);
        $vowelGroup = "[{$vowels}]";
        $nonVowelGroup = "[^{$vowels}]";
        // R1 is the region after the first non-vowel following a vowel, or is the null region at the end of the word if there is no such non-vowel.
        $pattern = "#(?P<rest>.*?{$vowelGroup}{$nonVowelGroup})(?P<r>.*)#u";
        if(preg_match($pattern, $word, $match)){
            $rest = $match["rest"];
            $r1 = $match["r"];
            // [...], but then R1 is adjusted so that the region before it contains at least 3 letters.
            $cutOff = 3 - mb_strlen($rest);
            if($cutOff > 0){
                $r1 = mb_substr($r1, $cutOff);
            }
            self::$R1 = $r1;
        }

        //R2 is the region after the first non-vowel following a vowel in R1, or is the null region at the end of the word if there is no such non-vowel.
        if(preg_match($pattern, self::$R1, $match)){
            self::$R2 = $match["r"];
        }
    }
}