波特词干算法
/* Porter stemmer in Java. The original paper is in Porter, 1980, An algorithm for suffix stripping, Program, Vol. 14, no. 3, pp 130-137, See also http://www.tartarus.org/~martin/PorterStemmer History: 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 Similarly, 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 (brian@quiotix.com). Release 4 */ import java.io.*; /** * Stemmer, implementing the Porter Stemming Algorithm * * 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. */ class Stemmer { 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. */ public 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; } /** 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. */ public 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]; } /** * 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); } /** * Returns the length of the word resulting from the stemming process. */ public 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. */ public 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; } } } /* 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(). */ public 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) { char[] w = new char[501]; Stemmer s = new Stemmer(); for (int i = 0; i < args.length; i++) try { FileInputStream in = new FileInputStream(args[i]); try { while(true) { int ch = in.read(); if (Character.isLetter((char) ch)) { int j = 0; while(true) { ch = Character.toLowerCase((char) ch); w[j] = (char) ch; if (j < 500) j++; ch = in.read(); if (!Character.isLetter((char) ch)) { /* to test add(char ch) */ for (int c = 0; c < j; c++) s.add(w[c]); /* or, to test add(char[] w, int j) */ /* s.add(w, j); */ s.stem(); { String u; /* and now, to test toString() : */ u = s.toString(); /* to test getResultBuffer(), getResultLength() : */ /* u = new String(s.getResultBuffer(), 0, s.getResultLength()); */ System.out.print(u); } break; } } } if (ch < 0) break; System.out.print((char)ch); } } catch (IOException e) { System.out.println("error reading " + args[i]); break; } } catch (FileNotFoundException e) { System.out.println("file " + args[i] + " not found"); break; } } }
算法源代码主页：http://tartarus.org/~martin/PorterStemmer/
