--- /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.
+ */
+
+/**
+ * Levenstein edit distance class.
+ */
+public final class LevensteinDistance implements StringDistance {
+
+ /**
+ * Optimized to run a bit faster than the static getDistance().
+ * In one benchmark times were 5.3sec using ctr vs 8.5sec w/ static method, thus 37% faster.
+ */
+ public LevensteinDistance () {
+ }
+
+
+ //*****************************
+ // Compute Levenshtein distance: see org.apache.commons.lang.StringUtils#getLevenshteinDistance(String, String)
+ //*****************************
+ public float getDistance (String target, String other) {
+ char[] sa;
+ int n;
+ int p[]; //'previous' cost array, horizontally
+ int d[]; // cost array, horizontally
+ int _d[]; //placeholder to assist in swapping p and d
+
+ /*
+ The difference between this impl. and the previous is that, rather
+ than creating and retaining a matrix of size s.length()+1 by t.length()+1,
+ we maintain two single-dimensional arrays of length s.length()+1. The first, d,
+ is the 'current working' distance array that maintains the newest distance cost
+ counts as we iterate through the characters of String s. Each time we increment
+ the index of String t we are comparing, d is copied to p, the second int[]. Doing so
+ allows us to retain the previous cost counts as required by the algorithm (taking
+ the minimum of the cost count to the left, up one, and diagonally up and to the left
+ of the current cost count being calculated). (Note that the arrays aren't really
+ copied anymore, just switched...this is clearly much better than cloning an array
+ or doing a System.arraycopy() each time through the outer loop.)
+
+ Effectively, the difference between the two implementations is this one does not
+ cause an out of memory condition when calculating the LD over two very large strings.
+ */
+
+ sa = target.toCharArray();
+ n = sa.length;
+ p = new int[n+1];
+ d = new int[n+1];
+
+ final int m = other.length();
+ if (n == 0 || m == 0) {
+ if (n == m) {
+ return 1;
+ }
+ else {
+ return 0;
+ }
+ }
+
+
+ // indexes into strings s and t
+ int i; // iterates through s
+ int j; // iterates through t
+
+ char t_j; // jth character of t
+
+ int cost; // cost
+
+ for (i = 0; i<=n; i++) {
+ p[i] = i;
+ }
+
+ for (j = 1; j<=m; j++) {
+ t_j = other.charAt(j-1);
+ d[0] = j;
+
+ for (i=1; i<=n; i++) {
+ cost = sa[i-1]==t_j ? 0 : 1;
+ // 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]+cost);
+ }
+
+ // 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 - ((float) p[n] / Math.max(other.length(), sa.length));
+ }
+
+}
fnp / pylucene.git / blobdiff