+++ /dev/null
-package org.apache.lucene.search.spell;
-
-/**
-* Licensed to the Apache Software Foundation (ASF) under one or more
-* contributor license agreements. See the NOTICE file distributed with
-* this work for additional information regarding copyright ownership.
-* The ASF licenses this file to You under the Apache License, Version 2.0
-* (the "License"); you may not use this file except in compliance with
-* the License. You may obtain a copy of the License at
-*
-* http://www.apache.org/licenses/LICENSE-2.0
-*
-* Unless required by applicable law or agreed to in writing, software
-* distributed under the License is distributed on an "AS IS" BASIS,
-* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-* See the License for the specific language governing permissions and
-* limitations under the License.
-*/
-
-/**
- * N-Gram version of edit distance based on paper by Grzegorz Kondrak,
- * "N-gram similarity and distance". Proceedings of the Twelfth International
- * Conference on String Processing and Information Retrieval (SPIRE 2005), pp. 115-126,
- * Buenos Aires, Argentina, November 2005.
- * http://www.cs.ualberta.ca/~kondrak/papers/spire05.pdf
- *
- * This implementation uses the position-based optimization to compute partial
- * matches of n-gram sub-strings and adds a null-character prefix of size n-1
- * so that the first character is contained in the same number of n-grams as
- * a middle character. Null-character prefix matches are discounted so that
- * strings with no matching characters will return a distance of 0.
- *
- */
-public class NGramDistance implements StringDistance {
-
- private int n;
-
- /**
- * Creates an N-Gram distance measure using n-grams of the specified size.
- * @param size The size of the n-gram to be used to compute the string distance.
- */
- public NGramDistance(int size) {
- this.n = size;
- }
-
- /**
- * Creates an N-Gram distance measure using n-grams of size 2.
- */
- public NGramDistance() {
- this(2);
- }
-
- public float getDistance(String source, String target) {
- final int sl = source.length();
- final int tl = target.length();
-
- if (sl == 0 || tl == 0) {
- if (sl == tl) {
- return 1;
- }
- else {
- return 0;
- }
- }
-
- int cost = 0;
- if (sl < n || tl < n) {
- for (int i=0,ni=Math.min(sl,tl);i<ni;i++) {
- if (source.charAt(i) == target.charAt(i)) {
- cost++;
- }
- }
- return (float) cost/Math.max(sl, tl);
- }
-
- char[] sa = new char[sl+n-1];
- float p[]; //'previous' cost array, horizontally
- float d[]; // cost array, horizontally
- float _d[]; //placeholder to assist in swapping p and d
-
- //construct sa with prefix
- for (int i=0;i<sa.length;i++) {
- if (i < n-1) {
- sa[i]=0; //add prefix
- }
- else {
- sa[i] = source.charAt(i-n+1);
- }
- }
- p = new float[sl+1];
- d = new float[sl+1];
-
- // indexes into strings s and t
- int i; // iterates through source
- int j; // iterates through target
-
- char[] t_j = new char[n]; // jth n-gram of t
-
- for (i = 0; i<=sl; i++) {
- p[i] = i;
- }
-
- for (j = 1; j<=tl; j++) {
- //construct t_j n-gram
- if (j < n) {
- for (int ti=0;ti<n-j;ti++) {
- t_j[ti]=0; //add prefix
- }
- for (int ti=n-j;ti<n;ti++) {
- t_j[ti]=target.charAt(ti-(n-j));
- }
- }
- else {
- t_j = target.substring(j-n, j).toCharArray();
- }
- d[0] = j;
- for (i=1; i<=sl; i++) {
- cost = 0;
- int tn=n;
- //compare sa to t_j
- for (int ni=0;ni<n;ni++) {
- if (sa[i-1+ni] != t_j[ni]) {
- cost++;
- }
- else if (sa[i-1+ni] == 0) { //discount matches on prefix
- tn--;
- }
- }
- float ec = (float) cost/tn;
- // minimum of cell to the left+1, to the top+1, diagonally left and up +cost
- d[i] = Math.min(Math.min(d[i-1]+1, p[i]+1), p[i-1]+ec);
- }
- // copy current distance counts to 'previous row' distance counts
- _d = p;
- p = d;
- d = _d;
- }
-
- // our last action in the above loop was to switch d and p, so p now
- // actually has the most recent cost counts
- return 1.0f - (p[sl] / Math.max(tl, sl));
- }
-
-}
fnp / pylucene.git / blobdiff