1b2441318SGreg Kroah-Hartman // SPDX-License-Identifier: GPL-2.0
286470930SIngo Molnar #include "levenshtein.h"
3175729fcSArnaldo Carvalho de Melo #include <errno.h>
4175729fcSArnaldo Carvalho de Melo #include <stdlib.h>
5175729fcSArnaldo Carvalho de Melo #include <string.h>
686470930SIngo Molnar
786470930SIngo Molnar /*
886470930SIngo Molnar * This function implements the Damerau-Levenshtein algorithm to
986470930SIngo Molnar * calculate a distance between strings.
1086470930SIngo Molnar *
1186470930SIngo Molnar * Basically, it says how many letters need to be swapped, substituted,
1286470930SIngo Molnar * deleted from, or added to string1, at least, to get string2.
1386470930SIngo Molnar *
1486470930SIngo Molnar * The idea is to build a distance matrix for the substrings of both
1586470930SIngo Molnar * strings. To avoid a large space complexity, only the last three rows
1686470930SIngo Molnar * are kept in memory (if swaps had the same or higher cost as one deletion
1786470930SIngo Molnar * plus one insertion, only two rows would be needed).
1886470930SIngo Molnar *
1986470930SIngo Molnar * At any stage, "i + 1" denotes the length of the current substring of
2086470930SIngo Molnar * string1 that the distance is calculated for.
2186470930SIngo Molnar *
2286470930SIngo Molnar * row2 holds the current row, row1 the previous row (i.e. for the substring
2386470930SIngo Molnar * of string1 of length "i"), and row0 the row before that.
2486470930SIngo Molnar *
2586470930SIngo Molnar * In other words, at the start of the big loop, row2[j + 1] contains the
2686470930SIngo Molnar * Damerau-Levenshtein distance between the substring of string1 of length
2786470930SIngo Molnar * "i" and the substring of string2 of length "j + 1".
2886470930SIngo Molnar *
2986470930SIngo Molnar * All the big loop does is determine the partial minimum-cost paths.
3086470930SIngo Molnar *
3186470930SIngo Molnar * It does so by calculating the costs of the path ending in characters
3286470930SIngo Molnar * i (in string1) and j (in string2), respectively, given that the last
33*4d39c89fSIngo Molnar * operation is a substitution, a swap, a deletion, or an insertion.
3486470930SIngo Molnar *
3586470930SIngo Molnar * This implementation allows the costs to be weighted:
3686470930SIngo Molnar *
3786470930SIngo Molnar * - w (as in "sWap")
3886470930SIngo Molnar * - s (as in "Substitution")
3986470930SIngo Molnar * - a (for insertion, AKA "Add")
4086470930SIngo Molnar * - d (as in "Deletion")
4186470930SIngo Molnar *
4286470930SIngo Molnar * Note that this algorithm calculates a distance _iff_ d == a.
4386470930SIngo Molnar */
levenshtein(const char * string1,const char * string2,int w,int s,int a,int d)4486470930SIngo Molnar int levenshtein(const char *string1, const char *string2,
4586470930SIngo Molnar int w, int s, int a, int d)
4686470930SIngo Molnar {
4786470930SIngo Molnar int len1 = strlen(string1), len2 = strlen(string2);
4886470930SIngo Molnar int *row0 = malloc(sizeof(int) * (len2 + 1));
4986470930SIngo Molnar int *row1 = malloc(sizeof(int) * (len2 + 1));
5086470930SIngo Molnar int *row2 = malloc(sizeof(int) * (len2 + 1));
5186470930SIngo Molnar int i, j;
5286470930SIngo Molnar
5386470930SIngo Molnar for (j = 0; j <= len2; j++)
5486470930SIngo Molnar row1[j] = j * a;
5586470930SIngo Molnar for (i = 0; i < len1; i++) {
5686470930SIngo Molnar int *dummy;
5786470930SIngo Molnar
5886470930SIngo Molnar row2[0] = (i + 1) * d;
5986470930SIngo Molnar for (j = 0; j < len2; j++) {
6086470930SIngo Molnar /* substitution */
6186470930SIngo Molnar row2[j + 1] = row1[j] + s * (string1[i] != string2[j]);
6286470930SIngo Molnar /* swap */
6386470930SIngo Molnar if (i > 0 && j > 0 && string1[i - 1] == string2[j] &&
6486470930SIngo Molnar string1[i] == string2[j - 1] &&
6586470930SIngo Molnar row2[j + 1] > row0[j - 1] + w)
6686470930SIngo Molnar row2[j + 1] = row0[j - 1] + w;
6786470930SIngo Molnar /* deletion */
6886470930SIngo Molnar if (row2[j + 1] > row1[j + 1] + d)
6986470930SIngo Molnar row2[j + 1] = row1[j + 1] + d;
7086470930SIngo Molnar /* insertion */
7186470930SIngo Molnar if (row2[j + 1] > row2[j] + a)
7286470930SIngo Molnar row2[j + 1] = row2[j] + a;
7386470930SIngo Molnar }
7486470930SIngo Molnar
7586470930SIngo Molnar dummy = row0;
7686470930SIngo Molnar row0 = row1;
7786470930SIngo Molnar row1 = row2;
7886470930SIngo Molnar row2 = dummy;
7986470930SIngo Molnar }
8086470930SIngo Molnar
8186470930SIngo Molnar i = row1[len2];
8286470930SIngo Molnar free(row0);
8386470930SIngo Molnar free(row1);
8486470930SIngo Molnar free(row2);
8586470930SIngo Molnar
8686470930SIngo Molnar return i;
8786470930SIngo Molnar }
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