porterstemmer.java

一个自然语言处理的Java开源工具包。LingPipe目前已有很丰富的功能

package com.aliasi.tokenizer;

/**
 * The <code>PorterStemmer</code> class is Martin Porter's Java
 * implementation of his English stemmer.  See the class {@link
 * PorterStemmerFilterTokenizer} for a wrapper that stems the
 * tokens in a tokenizer.
 *
 * <p>Porter's stemmer computes an approximation of converting words
 * to their morphological base form.  This class provides a single
 * top-level static method, {@link #stem(String)}, which returns a
 * stemmed form of an input string.
 *
 * <p>
 * The original paper describing Porter's stemmer is:
 *
 * <blockquote>
 *   Porter, Martin. 1980. An algorithm for suffix stripping. <i>Program</i>.
 *    <b>14</b>:3. 130--137.
 * </blockquote>
 * </p>
 *
 * <p>
 * <blockquote>
 *   <a href="http://www.tartarus.org/~martin/PorterStemmer">Porter
 *     Stemmer Home Page</a>
 * </blockquote>
 * </p>
 *
 * @author  Martin Porter
 * @version 4 (Porter's Version Numbering)
 * @since   LingPipe1.0
 */
public class PorterStemmer {

    /*
     * Release 1. Bug 1 (reported by Gonzalo Parra 16/10/99) fixed as
     * marked below.  The words 'aed', 'eed', 'oed' leave k at 'a' for
     * step 3, and b[k-1] is then out outside the bounds of b.
     *
     * Release 2. Bug 2 (reported by Steve Dyrdahl 22/2/00) fixed as
     *  marked below.  'ion' by itself leaves j = -1 in the test for
     * 'ion' in step 5, and b[j] is then outside the bounds of b.
     *
     * Release 3.  Considerably revised 4/9/00 in the light of many
     * helpful suggestions from Brian Goetz of Quiotix Corporation
     * ([e-mail]).
     *
     * Release 4.  No comments from authors.
     *
     * Release 5.  (Local to LingPipe) Made all methods, variables and
     * constructors private.  Added top level static stem(String)
     * method.
     */

    private char[] b;
    private int i;
    private int i_end;
    private int j;
    private int k;

    /**
     * unit of size whereby b is increased
     */
    private static final int INC = 50;

    /** Construct a stemmer.
     */
    private PorterStemmer() {
        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.)
     */
    public String toString() { return new String(b,0,i_end); }

    // EDITED OUT BECAUSE NOT USED
    // /** Adds wLen characters to the word being stemmed contained in 
    // * a portion
    // * of a char[] array. This is like repeated calls of add(char ch), but
    // * faster.
    // */
    // private void add(char[] w, int wLen)
    // {  if (i+wLen >= b.length)
    // {  char[] new_b = new char[i+wLen+INC];
    // for (int c = 0; c < i; c++) new_b[c] = b[c];
    // b = new_b;
    // }
    // for (int c = 0; c < wLen; c++) b[i++] = w[c];
    // }

    // /**
    // * Returns the length of the word resulting from the stemming process.
    // */
    // private int getResultLength() { return i_end; }

    // /**
    // * Returns a reference to a character buffer containing the results of
    // * the stemming process.  You also need to consult getResultLength()
    // * to determine the length of the result.
    // */
    // private char[] getResultBuffer() { return b; }

    /* 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 final 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 final 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; }
        default: return;
        } 
    }

    /** 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;
            default: return;
            } 
    }

    /** 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;
    }

    public static String stem(String in) {
        PorterStemmer stemmer = new PorterStemmer();
        if (in == null)
            throw new IllegalArgumentException("Input must not be null: " + in);
        char[] cs = in.toCharArray();
        if (cs.length < 1 || !Character.isLetter(cs[0])) return in;
        for (int i = 0; i < cs.length; ++i)
            stemmer.add(cs[i]);
        stemmer.stem();
        return stemmer.toString();
    }

}