Application: gvSIG desktop

/* gvSIG. Geographic Information System of the Valencian Government

 *

 * Copyright (C) 2007-2008 Infrastructures and Transports Department

 * of the Valencian Government (CIT)

 * 

 * This program is free software; you can redistribute it and/or

 * modify it under the terms of the GNU General Public License

 * as published by the Free Software Foundation; either version 2

 * of the License, or (at your option) any later version.

 * 

 * This program is distributed in the hope that it will be useful,

 * but WITHOUT ANY WARRANTY; without even the implied warranty of

 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

 * GNU General Public License for more details.

 * 

 * You should have received a copy of the GNU General Public License

 * along with this program; if not, write to the Free Software

 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, 

 * MA  02110-1301, USA.

 * 

 */

/*

 * AUTHORS (In addition to CIT):

 * 2008 Prodevelop S.L  vsanjaime   programador

 */

package org.gvsig.geocoding.distance.impl;

import org.gvsig.geocoding.distance.StringsDistance;

/**

 * This class calculate the levenshtein distance between two strings

 * 

 * @author <a href="mailto:jsanz@prodevelop.es"> Jorge Gaspar Sanz Salinas</a>

 * @author <a href="mailto:vsanjaime@prodevelop.es"> Vicent Sanjaime Calvet</a>

 */

public class LevenshteinDistance implements StringsDistance{

        /**

         * calculate levenshtein distance between two strings

         * 

         * @param str1

         * @param str2

         * @return

         */

        public static int computeLevenshteinDistance(String str1, String str2) {

                return computeLevenshteinDistance(str1.toCharArray(), str2

                                .toCharArray());

        }

        /**

         * get levenshtein distance between two strings

         * 

         * @param s

         * @param t

         * @return

         */

        public static int getLevenshteinDistance(String s, String t) {

                if (s == null || t == null) {

                        throw new IllegalArgumentException("Strings must not be null");

                }

                /*

                 * 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.

                 */

                int n = s.length(); // length of s

                int m = t.length(); // length of t

                if (n == 0) {

                        return m;

                } else if (m == 0) {

                        return n;

                }

                int p[] = new int[n + 1]; // 'previous' cost array, horizontally

                int d[] = new int[n + 1]; // cost array, horizontally

                int _d[]; // placeholder to assist in swapping p and d

                // 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 = t.charAt(j - 1);

                        d[0] = j;

                        for (i = 1; i <= n; i++) {

                                cost = s.charAt(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 p[n];

        }

        /**

         * 

         * @param a

         * @param b

         * @param c

         * @return

         */

        private static int minimum(int a, int b, int c) {

                if (a <= b && a <= c)

                        return a;

                if (b <= a && b <= c)

                        return b;

                return c;

        }

        /**

         * calculate levenshtein distance between two chars

         * 

         * @param str1

         * @param str2

         * @return

         */

        private static int computeLevenshteinDistance(char[] str1, char[] str2) {

                int[][] distance = new int[str1.length + 1][];

                for (int i = 0; i <= str1.length; i++) {

                        distance[i] = new int[str2.length + 1];

                        distance[i][0] = i;

                }

                for (int j = 0; j < str2.length + 1; j++)

                        distance[0][j] = j;

                for (int i = 1; i <= str1.length; i++)

                        for (int j = 1; j <= str2.length; j++)

                                distance[i][j] = minimum(distance[i - 1][j] + 1,

                                                distance[i][j - 1] + 1, distance[i - 1][j - 1]

                                                                + ((str1[i - 1] == str2[j - 1]) ? 0 : 1));

                return distance[str1.length][str2.length];

        }

        /**

         * get levenshtein distance between two strings

         * @param s first string

         * @param t second string

         */

        public int getStringsDistance(String s, String t) {

                return computeLevenshteinDistance(s, t);

        }

}