How does similar_text work?

This was actually a very interesting question, thank you for giving me a puzzle that turned out to be very rewarding.

Let me start out by explaining how similar_text actually works.

Similar Text: The Algorithm

It's a recursion based divide and conquer algorithm. It works by first finding the longest common string between the two inputs and breaking the problem into subsets around that string.

The examples you have used in your question, actually all perform only one iteration of the algorithm. The only ones not using one iteration and the ones giving different results are from the php.net comments.

Here is a simple example to understand the main issue behind simple_text and hopefully give some insight into how it works.

Similar Text: The Flaw

eeeefaaaaafddddd
ddddgaaaaagbeeee

Iteration 1:
Max    = 5
String = aaaaa
Left : eeeef and ddddg
Right: fddddd and geeeee

I hope the flaw is already apparent. It will only check directly to the left and to the right of the longest matched string in both input strings. This example

$s1='eeeefaaaaafddddd';
$s2='ddddgaaaaagbeeee';

echo similar_text($s1, $s2).'|'.similar_text($s2, $s1);
// outputs 5|5, this is due to Iteration 2 of the algorithm
// it will fail to find a matching string in both left and right subsets

To be honest, I'm uncertain how this case should be treated. It can be seen that only 2 characters are different in the string. But both eeee and dddd are on opposite ends of the two strings, uncertain what NLP enthusiasts or other literary experts have to say about this specific situation.

Similar Text: Inconsistent results on argument swapping

The different results you were experiencing based on input order was due to the way the alogirthm actually behaves (as mentioned above). I'll give a final explination on what's going on.

echo similar_text('test','wert'); // 1
echo similar_text('wert','test'); // 2

On the first case, there's only one Iteration:

test
wert

Iteration 1:
Max    = 1
String = t
Left :  and wer
Right: est and 

We only have one iteration because empty/null strings return 0 on recursion. So this ends the algorithm and we have our result: 1

On the second case, however, we are faced with multiple Iterations:

wert
test

Iteration 1:
Max    = 1
String = e
Left : w and t
Right: rt and st

We already have a common string of length 1. The algorithm on the left subset will end in 0 matches, but on the right:

rt
st

Iteration 1:
Max    = 1
String = t
Left : r and s
Right:  and 

This will lead to our new and final result: 2

I thank you for this very informative question and the opportunity to dabble in C++ again.

Similar Text: JavaScript Edition

The short answer is: The javascript code is not implementing the correct algorithm

sum += this.similar_text(first.substr(0, pos2), second.substr(0, pos2));

Obviously it should be first.substr(0,pos1)

Note: The JavaScript code has been fixed by eis in a previous commit. Thanks @eis

Demystified!

It would indeed seem the function uses different logic depending of the parameter order. I think there are two things at play.

First, see this example:

echo similar_text('test','wert'); // 1
echo similar_text('wert','test'); // 2

It seems to be that it is testing "how many times any distinct char on param1 is found in param2", and thus result would be different if you swap the params around. It has been reported as a bug, which has been closed as "working as expected".

Now, the above is the same for both PHP and javascript implementations - paremeter order has an impact, so saying that JS code wouldn't do this is wrong. This is argued in the bug entry as intended behaviour.

Second - what doesn't seem correct is the MYSQL/PHP word example. With that, javascript version gives 3 irrelevant of the order of params, whereas PHP gives 2 and 3 (and due to that, percentage is equally different). Now, the phrases "PHP IS GREAT" and "WITH MYSQL" should have 5 characters in common, irrelevant of which way you compare: H, I, S and T, one each, plus one for empty space. In order they have 3 characters, 'H', ' ' and 'S', so if you look at the ordering, correct answer should be 3 both ways. I modified the C code to a runnable version, and added some output, so one can see what is happening there (codepad link):

#include<stdio.h>

/* {{{ php_similar_str
 */
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;
  for (p = (char *) txt1; p < end1; p++) {
    for (q = (char *) txt2; q < end2; q++) {
      for (l = 0; (p + l < end1) && (q + l < end2) && (p[l] == q[l]); l++);
      if (l > *max) {
        *max = l;
        *pos1 = p - txt1;
        *pos2 = q - txt2;
      }
    }
  }
}
/* }}} */


/* {{{ php_similar_char
 */
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) {
      printf("txt here %s,%s\n", txt1, txt2);
      sum += php_similar_char(txt1, pos1,
                  txt2, pos2);
    }
    if ((pos1 + max < len1) && (pos2 + max < len2)) {
      printf("txt here %s,%s\n", txt1+ pos1 + max, txt2+ pos2 + max);
      sum += php_similar_char(txt1 + pos1 + max, len1 - pos1 - max,
                  txt2 + pos2 + max, len2 - pos2 - max);
    }
  }

  return sum;
}
/* }}} */
int main(void)
{
    printf("Found %d similar chars\n",
        php_similar_char("PHP IS GREAT", 12, "WITH MYSQL", 10));
    printf("Found %d similar chars\n",
        php_similar_char("WITH MYSQL", 10,"PHP IS GREAT", 12));
    return 0;
}

the result is output:

txt here PHP IS GREAT,WITH MYSQL
txt here P IS GREAT, MYSQL
txt here IS GREAT,MYSQL
txt here IS GREAT,MYSQL
txt here  GREAT,QL
Found 3 similar chars
txt here WITH MYSQL,PHP IS GREAT
txt here TH MYSQL,S GREAT
Found 2 similar chars

So one can see that on the first comparison, the function found 'H', ' ' and 'S', but not 'T', and got the result of 3. The second comparison found 'I' and 'T' but not 'H', ' ' or 'S', and thus got the result of 2.

The reason for these results can be seen from the output: algorithm takes the first letter in the first string that second string contains, counts that, and throws away the chars before that from the second string. That is why it misses the characters in-between, and that's the thing causing the difference when you change the character order.

What happens there might be intentional or it might not. However, that's not how javascript version works. If you print out the same things in the javascript version, you get this:

txt here: PHP, WIT
txt here: P IS GREAT,  MYSQL
txt here: IS GREAT, MYSQL
txt here: IS, MY
txt here:  GREAT, QL
Found 3 similar chars
txt here: WITH, PHP 
txt here: W, P
txt here: TH MYSQL, S GREAT
Found 3 similar chars

showing that javascript version does it in a different way. What the javascript version does is that it finds 'H', ' ' and 'S' being in the same order in the first comparison, and the same 'H', ' ' and 'S' also on the second one - so in this case the order of params doesn't matter.

As the javascript is meant to duplicate the code of PHP function, it needs to behave identically, so I submitted bug report based on analysis of @Khez and the fix, which has been merged now.

first String = aaaaaaaaaa = 10 letters
second String = aaaaa = 5 letters

first five letters are similar
a+a
a+a
a+a
a+a
a+a
a
a
a
a
a


( <similar_letters> * 200 ) / (<letter_count_first_string> + <letter_count_second_string>)

( 5 * 200 ) / (10 + 5);
= 66.6666666667