perf/util/levenshtein.c

0001 // SPDX-License-Identifier: GPL-2.0
0002 #include "levenshtein.h"
0003 #include <errno.h>
0004 #include <stdlib.h>
0005 #include <string.h>
0006
0007 /*
0008  * This function implements the Damerau-Levenshtein algorithm to
0009  * calculate a distance between strings.
0010  *
0011  * Basically, it says how many letters need to be swapped, substituted,
0012  * deleted from, or added to string1, at least, to get string2.
0013  *
0014  * The idea is to build a distance matrix for the substrings of both
0015  * strings.  To avoid a large space complexity, only the last three rows
0016  * are kept in memory (if swaps had the same or higher cost as one deletion
0017  * plus one insertion, only two rows would be needed).
0018  *
0019  * At any stage, "i + 1" denotes the length of the current substring of
0020  * string1 that the distance is calculated for.
0021  *
0022  * row2 holds the current row, row1 the previous row (i.e. for the substring
0023  * of string1 of length "i"), and row0 the row before that.
0024  *
0025  * In other words, at the start of the big loop, row2[j + 1] contains the
0026  * Damerau-Levenshtein distance between the substring of string1 of length
0027  * "i" and the substring of string2 of length "j + 1".
0028  *
0029  * All the big loop does is determine the partial minimum-cost paths.
0030  *
0031  * It does so by calculating the costs of the path ending in characters
0032  * i (in string1) and j (in string2), respectively, given that the last
0033  * operation is a substitution, a swap, a deletion, or an insertion.
0034  *
0035  * This implementation allows the costs to be weighted:
0036  *
0037  * - w (as in "sWap")
0038  * - s (as in "Substitution")
0039  * - a (for insertion, AKA "Add")
0040  * - d (as in "Deletion")
0041  *
0042  * Note that this algorithm calculates a distance _iff_ d == a.
0043  */
0044 int levenshtein(const char *string1, const char *string2,
0045         int w, int s, int a, int d)
0046 {
0047     int len1 = strlen(string1), len2 = strlen(string2);
0048     int *row0 = malloc(sizeof(int) * (len2 + 1));
0049     int *row1 = malloc(sizeof(int) * (len2 + 1));
0050     int *row2 = malloc(sizeof(int) * (len2 + 1));
0051     int i, j;
0052
0053     for (j = 0; j <= len2; j++)
0054         row1[j] = j * a;
0055     for (i = 0; i < len1; i++) {
0056         int *dummy;
0057
0058         row2[0] = (i + 1) * d;
0059         for (j = 0; j < len2; j++) {
0060             /* substitution */
0061             row2[j + 1] = row1[j] + s * (string1[i] != string2[j]);
0062             /* swap */
0063             if (i > 0 && j > 0 && string1[i - 1] == string2[j] &&
0064                     string1[i] == string2[j - 1] &&
0065                     row2[j + 1] > row0[j - 1] + w)
0066                 row2[j + 1] = row0[j - 1] + w;
0067             /* deletion */
0068             if (row2[j + 1] > row1[j + 1] + d)
0069                 row2[j + 1] = row1[j + 1] + d;
0070             /* insertion */
0071             if (row2[j + 1] > row2[j] + a)
0072                 row2[j + 1] = row2[j] + a;
0073         }
0074
0075         dummy = row0;
0076         row0 = row1;
0077         row1 = row2;
0078         row2 = dummy;
0079     }
0080
0081     i = row1[len2];
0082     free(row0);
0083     free(row1);
0084     free(row2);
0085
0086     return i;
0087 }