How to keep memory low when using a hierarchical object structure

Question

I have a simple object thing that is able to have children of the same type.

This object has a toHTML method, which does something like:

$html  = '<div>' . $this->name . '</div>';
$html .= '<ul>';

foreach($this->children as $child)
  $html .= '<li>' . $child->toHTML() . '</li>';

$html .= '</ul>';

return $html;

The problem is that when the object is complex, like lots of children with children with children etc, memory usage skyrockets.

If I simply print_r the multidimensional array that feeds this object I get like 1 MB memory usage, but after I convert the array to my object and do print $root->toHtml() it takes 10 MB !!

How can I fix this?

====================================

Made a simple class that is similar to my real code (but smaller):

class obj{
  protected $name;
  protected $children = array();    
  public function __construct($name){
    $this->name = $name;
  }
  public static function build($name, $array = array()){
    $obj = new self($name);
    if(is_array($array)){
      foreach($array as $k => $v)
        $obj->addChild(self::build($k, $v));
    }  
    return $obj;
  }
  public function addChild(self $child){
    $this->children[] = $child;
  }
  public function toHTML(){
    $html  = '<div>' . $this->name . '</div>';
    $html .= '<ul>';
    foreach($this->children as $child)
      $html .= '<li>' . $child->toHTML() . '</li>';
    $html .= '</ul>';

    return $html;    
  }
}

And tests:

$big = array_fill(0, 500, true);
$big[5] = array_fill(0, 200, $big);

print_r($big);
// memory_get_peak_usage() shows 0.61 MB

$root = obj::build('root', $big);
// memory_get_peak_usage() shows 18.5 MB wtf lol

print $root->toHTML();
// memory_get_peak_usage() shows 24.6 MB

Answer 1

The problem is that you're buffering all the data in memory, which you don't actually need to do, as you're just outputting the data, rather than actually processing it.

Rather than buffering everything in memory, if all you want to do is output it you should just output it to wherever it's going to:

public function toHTMLOutput($outputStream){
    fwrite($outputStream, '<div>' . $this->name . '</div>';
    fwrite($outputStream, '<ul>');

    foreach($this->children as $child){
      fwrite($outputStream, '<li>');
      $child->toHTMLOutput($outputStream);
      fwrite($outputStream, '</li>');}
    } 

    fwrite($outputStream, '</ul>');
}

$stdout = fopen('php://stdout', 'w');
print $root->toHTMLOutput($stdout);

or if you want to save the output to a file

$stdout = fopen('htmloutput.html', 'w');
print $root->toHTMLOutput($stdout);

Obviously I've only implemented it for the toHTML() function but the same principle should be done for the build function, which could lead to you skipping a separate toHTML function at all.