similar_text算相似性时归一化时的疑问

$str1	= "esca";

$str2	= "bca";

echo	levenshtein($str1,$str2), "\n";

similar_text($str1,$str2,$percent);

echo	$percent;

/*

2

57.142857142857

*/

$str1	= "esca";

$str2	= "sbca";

echo	levenshtein($str1,$str2), "\n";

similar_text($str1,$str2,$percent);

echo	$percent;

/*

2

75

*/

#define LEVENSHTEIN_MAX_LENGTH 255



/* {{{ reference_levdist

 * reference implementation, only optimized for memory usage, not speed */

static int reference_levdist(const char *s1, int l1, const char *s2, int l2, int cost_ins, int cost_rep, int cost_del )

{

	int *p1, *p2, *tmp;

	int i1, i2, c0, c1, c2;



	if (l1 == 0) {

		return l2 * cost_ins;

	}

	if (l2 == 0) {

		return l1 * cost_del;

	}



	if ((l1 > LEVENSHTEIN_MAX_LENGTH) || (l2 > LEVENSHTEIN_MAX_LENGTH)) {

		return -1;

	}

	p1 = safe_emalloc((l2 + 1), sizeof(int), 0);

	p2 = safe_emalloc((l2 + 1), sizeof(int), 0);



	for (i2 = 0; i2 <= l2; i2++) {

		p1[i2] = i2 * cost_ins;

	}

	for (i1 = 0; i1 < l1 ; i1++) {

		p2[0] = p1[0] + cost_del;



		for (i2 = 0; i2 < l2; i2++) {

			c0 = p1[i2] + ((s1[i1] == s2[i2]) ? 0 : cost_rep);

			c1 = p1[i2 + 1] + cost_del;

			if (c1 < c0) {

				c0 = c1;

			}

			c2 = p2[i2] + cost_ins;

			if (c2 < c0) {

				c0 = c2;

			}

			p2[i2 + 1] = c0;

		}

		tmp = p1;

		p1 = p2;

		p2 = tmp;

	}

	c0 = p1[l2];



	efree(p1);

	efree(p2);



	return c0;

}

/* }}} */



/* {{{ custom_levdist

 */

static int custom_levdist(char *str1, char *str2, char *callback_name TSRMLS_DC)

{

	php_error_docref(NULL TSRMLS_CC, E_WARNING, "The general Levenshtein support is not there yet");

	/* not there yet */



	return -1;

}

/* }}} */



/* {{{ proto int levenshtein(string str1, string str2[, int cost_ins, int cost_rep, int cost_del])

   Calculate Levenshtein distance between two strings */

PHP_FUNCTION(levenshtein)

{

	int argc = ZEND_NUM_ARGS();

	char *str1, *str2;

	char *callback_name;

	int str1_len, str2_len, callback_len;

	long cost_ins, cost_rep, cost_del;

	int distance = -1;



	switch (argc) {

		case 2: /* just two strings: use maximum performance version */

			if (zend_parse_parameters(2 TSRMLS_CC, "ss", &str1, &str1_len, &str2, &str2_len) == FAILURE) {

				return;

			}

			distance = reference_levdist(str1, str1_len, str2, str2_len, 1, 1, 1);

			break;



		case 5: /* more general version: calc cost by ins/rep/del weights */

			if (zend_parse_parameters(5 TSRMLS_CC, "sslll", &str1, &str1_len, &str2, &str2_len, &cost_ins, &cost_rep, &cost_del) == FAILURE) {

				return;

			}

			distance = reference_levdist(str1, str1_len, str2, str2_len, cost_ins, cost_rep, cost_del);

			break;



		case 3: /* most general version: calc cost by user-supplied function */

			if (zend_parse_parameters(3 TSRMLS_CC, "sss", &str1, &str1_len, &str2, &str2_len, &callback_name, &callback_len) == FAILURE) {

				return;

			}

			distance = custom_levdist(str1, str2, callback_name TSRMLS_CC);

			break;



		default:

			WRONG_PARAM_COUNT;

	}



	if (distance < 0 && /* TODO */ ZEND_NUM_ARGS() != 3) {

		php_error_docref(NULL TSRMLS_CC, E_WARNING, "Argument string(s) too long");

	}



	RETURN_LONG(distance);

}

/* }}} */

/* {{{ proto int similar_text(string str1, string str2 [, float percent])

   Calculates the similarity between two strings */

PHP_FUNCTION(similar_text)

{

	char *t1, *t2;

	zval **percent = NULL;

	int ac = ZEND_NUM_ARGS();

	int sim;

	int t1_len, t2_len;



	if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "ss|Z", &t1, &t1_len, &t2, &t2_len, &percent) == FAILURE) {

		return;

	}



	if (ac > 2) {

		convert_to_double_ex(percent);

	}



	if (t1_len + t2_len == 0) {

		if (ac > 2) {

			Z_DVAL_PP(percent) = 0;

		}



		RETURN_LONG(0);

	}



	sim = php_similar_char(t1, t1_len, t2, t2_len);



	if (ac > 2) {

		Z_DVAL_PP(percent) = sim * 200.0 / (t1_len + t2_len);

	}



	RETURN_LONG(sim);

}









static int php_similar_char(const char *txt1, int len1, const char *txt2, int len2)

{

	int sum;

	int pos1, pos2, max;



	php_similar_str(txt1, len1, txt2, len2, &pos1, &pos2, &max);

	if ((sum = max)) {

		if (pos1 && pos2) {

			sum += php_similar_char(txt1, pos1,

									txt2, pos2);

		}

		if ((pos1 + max < len1) && (pos2 + max < len2)) {

			sum += php_similar_char(txt1 + pos1 + max, len1 - pos1 - max,

									txt2 + pos2 + max, len2 - pos2 - max);

		}

	}



	return sum;

}