detecting infinite array recursion in PHP?

Question

i've just reworked my recursion detection algorithm in my pet project dump_r()

https://github.com/leeoniya/dump_r.php

detecting object recursion is not too difficult - you use spl_object_hash() to get the unique internal id of the object instance, store it in a dict and compare against it while dumping other nodes.

for array recursion detection, i'm a bit puzzled, i have not found anything helpful. php itself is able to identify recursion, though it seems to do it one cycle too late. EDIT: nvm, it occurs where it needs to :)

$arr = array();
$arr[] = array(&$arr);
print_r($arr);

does it have to resort to keeping track of everything in the recursion stack and do shallow comparisons against every other array element?

any help would be appreciated,
thanks!

Answer 1

Because of PHP's call-by-value mechanism, the only solution I see here is to iterate the array by reference, and set an arbitrary value in it, which you later check if it exists to find out if you were there before:

function iterate_array(&$arr){

  if(!is_array($arr)){
    print $arr;
    return;
  }

  // if this key is present, it means you already walked this array
  if(isset($arr['__been_here'])){
    print 'RECURSION';
    return;
  }

  $arr['__been_here'] = true;

  foreach($arr as $key => &$value){

    // print your values here, or do your stuff
    if($key !== '__been_here'){
      if(is_array($value)){
        iterate_array($value);
      }

      print $value;
    }
  }

  // you need to unset it when done because you're working with a reference...
  unset($arr['__been_here']);

}

You could wrap this function into another function that accepts values instead of references, but then you would get the RECURSION notice from the 2nd level on. I think print_r does the same too.

Answer 2

Someone will correct me if I am wrong, but PHP is actually detecting recursion at the right moment. Your assignation simply creates the additional cycle. The example should be:

$arr    = array();
$arr    = array(&$arr);

Which will result in

array(1) { [0]=> &array(1) { [0]=> *RECURSION* } } 

As expected.

---

Well, I got a bit curious myself how to detect recursion and I started to Google. I found this article http://noteslog.com/post/detecting-recursive-dependencies-in-php-composite-values/ and this solution:

function hasRecursiveDependency($value)
{
    //if PHP detects recursion in a $value, then a printed $value 
    //will contain at least one match for the pattern /\*RECURSION\*/
    $printed = print_r($value, true);
    $recursionMetaUser = preg_match_all('@\*RECURSION\*@', $printed, $matches);
    if ($recursionMetaUser == 0)
    {
        return false;
    }
    //if PHP detects recursion in a $value, then a serialized $value 
    //will contain matches for the pattern /\*RECURSION\*/ never because
    //of metadata of the serialized $value, but only because of user data
    $serialized = serialize($value);
    $recursionUser = preg_match_all('@\*RECURSION\*@', $serialized, $matches);
    //all the matches that are user data instead of metadata of the 
    //printed $value must be ignored
    $result = $recursionMetaUser > $recursionUser;
    return $result;
}