Andrew's git - gitweb.git/blob

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