Principle of PHP function similar_text()_PHP Tutorial

PHP has a function similar_text() that calculates the similarity of two strings. It can get a percentage to express the similarity of the two strings. The effect is as follows:

similar_text('aaaa', 'aaaa', $percent);

var_dump($percent);

//float(100)

similar_text('aaaa', 'aaaabbbb', $percent);

var_dump($percent);

//float(66.666666666667)

similar_text('abcdef', 'aabcdefg', $percent);

var_dump($percent);

//float(85.714285714286)

Using this function, you can use it to perform fuzzy search functions, or other functions that require fuzzy matching. Recently, I have involved this function in the feature matching step in the research on verification code recognition.

But what kind of algorithm does this function use? I studied its underlying implementation and summarized it in three steps:

(1) Find the longest segment with the same part in the two strings;

(2) Use the same method to find the longest segment with the same parts in the remaining two paragraphs, and so on until there are no more identical parts;

　(3) Similarity = the sum of the lengths of all the same parts * 2 / the sum of the lengths of the two strings;

The source code version I studied is PHP 5.4.6, and the relevant code is located in lines 2951~3031 of the file php-5.4.6/ext/standard/string.c. The following is the source code after I added comments.

//Find the longest segment with the same part in the two strings

static void php_similar_str(const char *txt1, int len1, const char *txt2, int len2, int *pos1, int *pos2, int *max)

　{

Char *p, *q;

Char *end1 = (char *) txt1 + len1;

Char *end2 = (char *) txt2 + len2;

int l;

*max = 0;

//Start traversing based on the first string

　for (p = (char *) txt1; p < end1; p++) {

//Traverse the second string

　for (q = (char *) txt2; q < end2; q++) {

// Found that there are the same characters, continue to loop through the search, l is the length of the same part

　for (l = 0; (p + l < end1) && (q + l < end2) && (p[l] == q[l]); l++);

//Bubble method to find the longest l and remember the starting position of the same part

　if (l > *max) {

*max = l;

　*pos1 = p - txt1;

　*pos2 = q - txt2;

　}

　}

　}

　}

// Calculate the total length of the same parts of the two strings

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

　{

int sum;

　int pos1, pos2, max;

//Find the longest segment of the same part between two strings

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

//Here is the initial assignment to sum and the judgment of the max value

//If max is zero, it means that the two strings do not have any same characters, and if

will jump out

　if ((sum = max)) {

//Recurse for the first half of the segment, and the length of the same segment is accumulated

　if (pos1 && pos2) {

sum += php_similar_char(txt1, pos1,

　txt2, pos2);

　}

//Recurse for the second half of the segment, and the length of the same segment is accumulated

　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;

　}

　//PHP function definition

PHP_FUNCTION(similar_text)

　{

Char *t1, *t2;

zval **percent = NULL;

int ac = ZEND_NUM_ARGS();

int sim;

　int t1_len, t2_len;

// Check parameter validity

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

return;

　}

//If there is a third parameter

　if (ac > 2) {

convert_to_double_ex(percent);

　}

//If the length of both strings is 0, return 0

　if (t1_len + t2_len == 0) {

　if (ac > 2) {

　Z_DVAL_PP(percent) = 0;

　}

RETURN_LONG(0);

　}

//Call the above function to calculate the similarity library of two strings

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

//You can see the calculation formula of the third parameter percent

　if (ac > 2) {

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

　}

RETURN_LONG(sim);

　}

In addition, PHP also provides another function levenshtein() for calculating string similarity. It expresses string similarity by calculating the edit distance of two strings. This is also a very common algorithm. The performance of levenshtein() is better than similar_text(), because from the previous code analysis, we can see that the complexity of similar_text() is O(n^3), n represents the length of the longest string, and levenshtein() The complexity is O(m*n), m and n are the lengths of the two strings respectively.

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