cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
git clone https://git.sinitax.com/sinitax/cachepc-linux
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levenshtein.c (2670B)

      1// SPDX-License-Identifier: GPL-2.0
      2#include "levenshtein.h"
      3#include <errno.h>
      4#include <stdlib.h>
      5#include <string.h>
      6
      7/*
      8 * This function implements the Damerau-Levenshtein algorithm to
      9 * calculate a distance between strings.
     10 *
     11 * Basically, it says how many letters need to be swapped, substituted,
     12 * deleted from, or added to string1, at least, to get string2.
     13 *
     14 * The idea is to build a distance matrix for the substrings of both
     15 * strings.  To avoid a large space complexity, only the last three rows
     16 * are kept in memory (if swaps had the same or higher cost as one deletion
     17 * plus one insertion, only two rows would be needed).
     18 *
     19 * At any stage, "i + 1" denotes the length of the current substring of
     20 * string1 that the distance is calculated for.
     21 *
     22 * row2 holds the current row, row1 the previous row (i.e. for the substring
     23 * of string1 of length "i"), and row0 the row before that.
     24 *
     25 * In other words, at the start of the big loop, row2[j + 1] contains the
     26 * Damerau-Levenshtein distance between the substring of string1 of length
     27 * "i" and the substring of string2 of length "j + 1".
     28 *
     29 * All the big loop does is determine the partial minimum-cost paths.
     30 *
     31 * It does so by calculating the costs of the path ending in characters
     32 * i (in string1) and j (in string2), respectively, given that the last
     33 * operation is a substitution, a swap, a deletion, or an insertion.
     34 *
     35 * This implementation allows the costs to be weighted:
     36 *
     37 * - w (as in "sWap")
     38 * - s (as in "Substitution")
     39 * - a (for insertion, AKA "Add")
     40 * - d (as in "Deletion")
     41 *
     42 * Note that this algorithm calculates a distance _iff_ d == a.
     43 */
     44int levenshtein(const char *string1, const char *string2,
     45		int w, int s, int a, int d)
     46{
     47	int len1 = strlen(string1), len2 = strlen(string2);
     48	int *row0 = malloc(sizeof(int) * (len2 + 1));
     49	int *row1 = malloc(sizeof(int) * (len2 + 1));
     50	int *row2 = malloc(sizeof(int) * (len2 + 1));
     51	int i, j;
     52
     53	for (j = 0; j <= len2; j++)
     54		row1[j] = j * a;
     55	for (i = 0; i < len1; i++) {
     56		int *dummy;
     57
     58		row2[0] = (i + 1) * d;
     59		for (j = 0; j < len2; j++) {
     60			/* substitution */
     61			row2[j + 1] = row1[j] + s * (string1[i] != string2[j]);
     62			/* swap */
     63			if (i > 0 && j > 0 && string1[i - 1] == string2[j] &&
     64					string1[i] == string2[j - 1] &&
     65					row2[j + 1] > row0[j - 1] + w)
     66				row2[j + 1] = row0[j - 1] + w;
     67			/* deletion */
     68			if (row2[j + 1] > row1[j + 1] + d)
     69				row2[j + 1] = row1[j + 1] + d;
     70			/* insertion */
     71			if (row2[j + 1] > row2[j] + a)
     72				row2[j + 1] = row2[j] + a;
     73		}
     74
     75		dummy = row0;
     76		row0 = row1;
     77		row1 = row2;
     78		row2 = dummy;
     79	}
     80
     81	i = row1[len2];
     82	free(row0);
     83	free(row1);
     84	free(row2);
     85
     86	return i;
     87}