// Convenience method for getting the edit distance of two strings
int edit_dist(const char *a, const char *b) {
unsigned int a_len = 0, b_len = 0;
if (a)
a_len = strlen(a);
if (b)
b_len = strlen(b);
return edit_distn(a, a_len, b, b_len);
}
/*
this is the low level string scoring algorithm. It takes two strings
and scores them on a scale of 0-100 where 0 is a terrible match and
100 is a great match. The block_size is used to cope with very small
messages.
*/
static unsigned score_strings(const char *s1, const char *s2, u32 block_size)
{
u32 score;
u32 len1, len2;
int edit_distn(const char *from, int from_len, const char *to, int to_len);
len1 = strlen(s1);
len2 = strlen(s2);
if (len1 > SPAMSUM_LENGTH || len2 > SPAMSUM_LENGTH) {
/* not a real spamsum signature? */
return 0;
}
/* the two strings must have a common substring of length
ROLLING_WINDOW to be candidates */
if (has_common_substring(s1, s2) == 0) {
return 0;
}
/* compute the edit distance between the two strings. The edit distance gives
us a pretty good idea of how closely related the two strings are */
score = edit_distn(s1, len1, s2, len2);
/* scale the edit distance by the lengths of the two
strings. This changes the score to be a measure of the
proportion of the message that has changed rather than an
absolute quantity. It also copes with the variability of
the string lengths. */
score = (score * SPAMSUM_LENGTH) / (len1 + len2);
/* at this stage the score occurs roughly on a 0-64 scale,
* with 0 being a good match and 64 being a complete
* mismatch */
/* rescale to a 0-100 scale (friendlier to humans) */
score = (100 * score) / 64;
/* it is possible to get a score above 100 here, but it is a
really terrible match */
if (score >= 100) return 0;
/* now re-scale on a 0-100 scale with 0 being a poor match and
100 being a excellent match. */
score = 100 - score;
/* when the blocksize is small we don't want to exaggerate the match size */
if (score > block_size/MIN_BLOCKSIZE * MIN(len1, len2)) {
score = block_size/MIN_BLOCKSIZE * MIN(len1, len2);
}
return score;
}
