Stemmer.java

package edu.stanford.nlp.process;


import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Iterator;

import java.util.function.Function;


import edu.stanford.nlp.ling.Word;

/**
 * Stemmer, implementing the Porter Stemming Algorithm
 * <br>
 * The Stemmer class transforms a word into its root form.  The input
 * word can be provided a character at time (by calling add()), or at once
 * by calling one of the various stem(something) methods.
 *
 * @author Sepandar Kamvar (sdkamvar@stanford.edu)
 */

public class Stemmer implements Function<Word,Word> {
  private char[] b;
  private int i,     /* offset into b */
  i_end, /* offset to end of stemmed word */
  j, k;
  private static final int INC = 50;

  /* unit of size whereby b is increased */
  public Stemmer() {
    b = new char[INC];
    i = 0;
    i_end = 0;
  }

  /**
   * Add a character to the word being stemmed.  When you are finished
   * adding characters, you can call stem(void) to stem the word.
   */

  private void add(char ch) {
    if (i == b.length) {
      char[] new_b = new char[i + INC];
      for (int c = 0; c < i; c++) {
        new_b[c] = b[c];
      }
      b = new_b;
    }
    b[i++] = ch;
  }


  /**
   * After a word has been stemmed, it can be retrieved by toString(),
   * or a reference to the internal buffer can be retrieved by getResultBuffer
   * and getResultLength (which is generally more efficient.)
   */
  @Override
  public String toString() {
    return new String(b, 0, i_end);
  }


  /* cons(i) is true <=> b[i] is a consonant. */
  private final boolean cons(int i) {
    switch (b[i]) {
      case 'a':
      case 'e':
      case 'i':
      case 'o':
      case 'u':
        return false;
      case 'y':
        return (i == 0) ? true : !cons(i - 1);
      default:
        return true;
    }
  }

  /* m() measures the number of consonant sequences between 0 and j. 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
        ....
  */

  private final int m() {
    int n = 0;
    int i = 0;
    while (true) {
      if (i > j) {
        return n;
      }
      if (!cons(i)) {
        break;
      }
      i++;
    }
    i++;
    while (true) {
      while (true) {
        if (i > j) {
          return n;
        }
        if (cons(i)) {
          break;
        }
        i++;
      }
      i++;
      n++;
      while (true) {
        if (i > j) {
          return n;
        }
        if (!cons(i)) {
          break;
        }
        i++;
      }
      i++;
    }
  }

  /* vowelinstem() is true <=> 0,...j contains a vowel */

  private final boolean vowelinstem() {
    int i;
    for (i = 0; i <= j; i++) {
      if (!cons(i)) {
        return true;
      }
    }
    return false;
  }

  /* doublec(j) is true <=> j,(j-1) contain a double consonant. */

  private final boolean doublec(int j) {
    if (j < 1) {
      return false;
    }
    if (b[j] != b[j - 1]) {
      return false;
    }
    return cons(j);
  }

  /* cvc(i) is true <=> i-2,i-1,i has the form consonant - vowel - consonant
     and also if the second c is not w,x or y. this is used when trying to
     restore an e at the end of a short word. e.g.

        cav(e), lov(e), hop(e), crim(e), but
        snow, box, tray.

  */

  private final boolean cvc(int i) {
    if (i < 2 || !cons(i) || cons(i - 1) || !cons(i - 2)) {
      return false;
    }
    {
      int ch = b[i];
      if (ch == 'w' || ch == 'x' || ch == 'y') {
        return false;
      }
    }
    return true;
  }

  private final boolean ends(String s) {
    int l = s.length();
    int o = k - l + 1;
    if (o < 0) {
      return false;
    }
    for (int i = 0; i < l; i++) {
      if (b[o + i] != s.charAt(i)) {
        return false;
      }
    }
    j = k - l;
    return true;
  }

  /* setto(s) sets (j+1),...k to the characters in the string s, readjusting
     k. */

  private void setto(String s) {
    int l = s.length();
    int o = j + 1;
    for (int i = 0; i < l; i++) {
      b[o + i] = s.charAt(i);
    }
    k = j + l;
  }

  /* r(s) is used further down. */

  private final void r(String s) {
    if (m() > 0) {
      setto(s);
    }
  }

  /* step1() gets rid of plurals and -ed or -ing. e.g.

         caresses  ->  caress
         ponies    ->  poni
         ties      ->  ti
         caress    ->  caress
         cats      ->  cat

         feed      ->  feed
         agreed    ->  agree
         disabled  ->  disable

         matting   ->  mat
         mating    ->  mate
         meeting   ->  meet
         milling   ->  mill
         messing   ->  mess

         meetings  ->  meet

  */

  private void step1() {
    if (b[k] == 's') {
      if (ends("sses")) {
        k -= 2;
      } else if (ends("ies")) {
        setto("i");
      } else if (b[k - 1] != 's') {
        k--;
      }
    }
    if (ends("eed")) {
      if (m() > 0) {
        k--;
      }
    } else if ((ends("ed") || ends("ing")) && vowelinstem()) {
      k = j;
      if (ends("at")) {
        setto("ate");
      } else if (ends("bl")) {
        setto("ble");
      } else if (ends("iz")) {
        setto("ize");
      } else if (doublec(k)) {
        k--;
        {
          int ch = b[k];
          if (ch == 'l' || ch == 's' || ch == 'z') {
            k++;
          }
        }
      } else if (m() == 1 && cvc(k)) {
        setto("e");
      }
    }
  }

  /* step2() turns terminal y to i when there is another vowel in the stem. */

  private final void step2() {
    if (ends("y") && vowelinstem()) {
      b[k] = 'i';
    }
  }

  /* step3() maps double suffices to single ones. so -ization ( = -ize plus
     -ation) maps to -ize etc. note that the string before the suffix must give
     m() > 0. */

  private final void step3() {
    if (k == 0) {
      return; /* For Bug 1 */
    }
    switch (b[k - 1]) {
      case 'a':
        if (ends("ational")) {
          r("ate");
          break;
        }
        if (ends("tional")) {
          r("tion");
          break;
        }
        break;
      case 'c':
        if (ends("enci")) {
          r("ence");
          break;
        }
        if (ends("anci")) {
          r("ance");
          break;
        }
        break;
      case 'e':
        if (ends("izer")) {
          r("ize");
          break;
        }
        break;
      case 'l':
        if (ends("bli")) {
          r("ble");
          break;
        }
        if (ends("alli")) {
          r("al");
          break;
        }
        if (ends("entli")) {
          r("ent");
          break;
        }
        if (ends("eli")) {
          r("e");
          break;
        }
        if (ends("ousli")) {
          r("ous");
          break;
        }
        break;
      case 'o':
        if (ends("ization")) {
          r("ize");
          break;
        }
        if (ends("ation")) {
          r("ate");
          break;
        }
        if (ends("ator")) {
          r("ate");
          break;
        }
        break;
      case 's':
        if (ends("alism")) {
          r("al");
          break;
        }
        if (ends("iveness")) {
          r("ive");
          break;
        }
        if (ends("fulness")) {
          r("ful");
          break;
        }
        if (ends("ousness")) {
          r("ous");
          break;
        }
        break;
      case 't':
        if (ends("aliti")) {
          r("al");
          break;
        }
        if (ends("iviti")) {
          r("ive");
          break;
        }
        if (ends("biliti")) {
          r("ble");
          break;
        }
        break;
      case 'g':
        if (ends("logi")) {
          r("log");
          break;
        }
    }
  }

  /* step4() deals with -ic-, -full, -ness etc. similar strategy to step3. */

  private final void step4() {
    switch (b[k]) {
      case 'e':
        if (ends("icate")) {
          r("ic");
          break;
        }
        if (ends("ative")) {
          r("");
          break;
        }
        if (ends("alize")) {
          r("al");
          break;
        }
        break;
      case 'i':
        if (ends("iciti")) {
          r("ic");
          break;
        }
        break;
      case 'l':
        if (ends("ical")) {
          r("ic");
          break;
        }
        if (ends("ful")) {
          r("");
          break;
        }
        break;
      case 's':
        if (ends("ness")) {
          r("");
          break;
        }
        break;
    }
  }

  /* step5() takes off -ant, -ence etc., in context <c>vcvc<v>. */

  private final void step5() {
    if (k == 0) {
      return; /* for Bug 1 */
    }
    switch (b[k - 1]) {
      case 'a':
        if (ends("al")) {
          break;
        }
        return;
      case 'c':
        if (ends("ance")) {
          break;
        }
        if (ends("ence")) {
          break;
        }
        return;
      case 'e':
        if (ends("er")) {
          break;
        }
        return;
      case 'i':
        if (ends("ic")) {
          break;
        }
        return;
      case 'l':
        if (ends("able")) {
          break;
        }
        if (ends("ible")) {
          break;
        }
        return;
      case 'n':
        if (ends("ant")) {
          break;
        }
        if (ends("ement")) {
          break;
        }
        if (ends("ment")) {
          break;
        }
        /* element etc. not stripped before the m */
        if (ends("ent")) {
          break;
        }
        return;
      case 'o':
        if (ends("ion") && j >= 0 && (b[j] == 's' || b[j] == 't')) {
          break;
        }
        /* j >= 0 fixes Bug 2 */
        if (ends("ou")) {
          break;
        }
        return;
        /* takes care of -ous */
      case 's':
        if (ends("ism")) {
          break;
        }
        return;
      case 't':
        if (ends("ate")) {
          break;
        }
        if (ends("iti")) {
          break;
        }
        return;
      case 'u':
        if (ends("ous")) {
          break;
        }
        return;
      case 'v':
        if (ends("ive")) {
          break;
        }
        return;
      case 'z':
        if (ends("ize")) {
          break;
        }
        return;
      default:
        return;
    }
    if (m() > 1) {
      k = j;
    }
  }

  /* step6() removes a final -e if m() > 1. */

  private final void step6() {
    j = k;
    if (b[k] == 'e') {
      int a = m();
      if (a > 1 || a == 1 && !cvc(k - 1)) {
        k--;
      }
    }
    if (b[k] == 'l' && doublec(k) && m() > 1) {
      k--;
    }
  }

  /**
   * Stem the word placed into the Stemmer buffer through calls to add().
   * Returns true if the stemming process resulted in a word different
   * from the input.  You can retrieve the result with
   * getResultLength()/getResultBuffer() or toString().
   */
  private void stem() {
    k = i - 1;
    if (k > 1) {
      step1();
      step2();
      step3();
      step4();
      step5();
      step6();
    }
    i_end = k + 1;
    i = 0;
  }

  /**
   * Test program for demonstrating the Stemmer.  It reads text from a
   * a list of files, stems each word, and writes the result to standard
   * output. Note that the word stemmed is expected to be in lower case:
   * forcing lower case must be done outside the Stemmer class.
   * Usage: Stemmer file-name file-name ...
   */
  public static void main(String[] args) throws IOException {
    Stemmer s = new Stemmer();
    if (args[0].equals("-file")) {
      Iterator<Word> it = PTBTokenizer.newPTBTokenizer(new InputStreamReader(new FileInputStream(args[1]), "utf-8"));
      while (it.hasNext()) {
        Word token = it.next();
        System.out.print(s.stem(token.word()));
        System.out.print(' ');
      }
    } else {
      for (String arg : args) {
        System.out.print(s.stem(arg));
        System.out.print(' ');
      }
    }
    System.out.println();
  }

  /**
   * Stems <code>s</code> and returns stemmed <code>String</code>.
   */

  public String stem(String s) {
    char[] characters = s.toCharArray();
    for (char character : characters) {
      add(character);
    }
    stem();
    return toString();
  }

  /**
   * Stems <code>w</code> and returns stemmed <code>Word</code>.
   */

  public Word stem(Word w) {
    return (new Word(stem(w.word())));
  }

  /**
   * Stems <code>word</code> (which must be a <code>Word</code>,
   * or else
   * a ClassCastException will be thrown, and returns stemmed
   * <code>Word</code>.
   */
  public Word apply(Word word) {
    return stem(word);
  }

}