Iterators

Recently I came across a situation where I need to iterate over multiple result sets. Since SPL has a decent collection of iterators I began my search there. So I came across the MultipleIterator thinking it would solve all my problems. It didn't.
The problem with the MultipleIterator is that it doesn't provide a continuous stream of elements of all the attached iterators, instead it 'groups' them. Per iteration you get an array of all n-th elements for all attached iterators.
Given the following piece of code:

<?php
$iter = new MultipleIterator();
$iter->attachIterator(new ArrayIterator(range(0, 5)));
$iter->attachIterator(new ArrayIterator(range(10, 15)));

foreach ($iter as $row) {
    var_dump($row);
}

The output is:

array(2) {
  [0]=>
  int(0)
  [1]=>
  int(10)
}
array(2) {
  [0]=>
  int(1)
  [1]=>
  int(11)
}
array(2) {
  [0]=>
  int(2)
  [1]=>
  int(12)
}
array(2) {
  [0]=>
  int(3)
  [1]=>
  int(13)
}
array(2) {
  [0]=>
  int(4)
  [1]=>
  int(14)
}
array(2) {
  [0]=>
  int(5)
  [1]=>
  int(15)
}

Not exactly what i want. And even worse, all iterators need to be of the same length. It just doesn't cover my use case where I simply want to group a bunch of iterators and interface with them as if it were a single one.

It seems the SPL doesn't provide a solution to my use case, so I wrote my own. This is the implementation I ended up with:

<?php 
class MultiIterator 
implements Iterator 
{
    /**
     * Holds the stack of iterators
     * @var array
     */
    protected $iterators = array();
    /**
     * Holds the index to the current interator
     * @var integer
     */
    protected $current_iterator_idx;
    /**
     * Add a new iterator to the stack
     * @param Iterator $iterator
     */
    public function appendIterator(Iterator $iterator)
    {
        $this->iterators[] = $iterator;
    }
    /**
     * Returns the currently used iterator
     * @return Iterator
     */
    public function iterator()
    {
        return (isset($this->iterators[$this->current_iterator_idx])) ? $this->iterators[$this->current_iterator_idx] : false;
    }
    /**
     * (non-PHPdoc)
     * @see Iterator::current()
     */
    public function current()
    {
        return $this->iterator()->current();
    }
    /**
     * (non-PHPdoc)
     * @see Iterator::next()
     */
    public function next()
    {
        //does nothing
        return $this->iterator()->next();
    }
    /**
     * (non-PHPdoc)
     * @see Iterator::key()
     */
    public function key()
    {
        return $this->iterator()->key();
    }
    /**
     * (non-PHPdoc)
     * @see Iterator::valid()
     */
    public function valid()
    {
        if (!count($this->iterators)) {
            return false;
        }
        //check if current iterator is valid
        $valid = $this->iterator()->valid();
        if (false === $valid) { //this iterator has dried up, try and find a new one
            //increment the index to find a new iterator
            $this->current_iterator_idx++;

            //now check if we have a new iterator
            if (false !== $this->iterator()) {
                //we have a new iterator, valid or not
                return $this->iterator()->valid();
            } else {
                //no more iterators
                return false;
            }
        }
     
        //apparantly, the current iterator is still valid
        return $valid;
    }
    /**
     * (non-PHPdoc)
     * @see Iterator::rewind()
     */
    public function rewind()
    {
        //reset state
        $this->current_iterator_idx = 0;
        array_walk($this->iterators, function($iter) {
            $iter->rewind();
        });
    }
}

With the expected usage and output:

<?php
$iter = new MultiIterator();
$iter->appendIterator(new ArrayIterator(range(0, 6)));
$iter->appendIterator(new ArrayIterator(range(10, 15)));

foreach ($iter as $row) {
    var_dump($row);
}


int(0)
int(1)
int(2)
int(3)
int(4)
int(5)
int(6)
int(10)
int(11)
int(12)
int(13)
int(14)
int(15)

Problem solved.