Random Iterator - Picking n Iterations random out of x Iterations (Reservoir sampling)

RandomIterator.php

<?php
/**
 * Random Iterator - Picking n Iterations random out of x Iterations (Reservoir sampling)
 *
 * TODO Preserve Iteration Keys
 *
 * @author hakre
 */
class RandomIterator implements IteratorAggregate
{
    private $innerIterator;
    private $randomIterations;
    private $count;

    public function __construct(Traversable $iterator, $randomIterations = 1) {

        $this->setRandomIterations($randomIterations);
        $this->innerIterator = $iterator;
    }

    private function setRandomIterations($randomIterations) {

        $number = (int)$randomIterations;

        if ($number < 1) {
            throw new InvalidArgumentException(
                sprintf('Number of iterations must be larger than 0, %d (%s) given.', $number, $randomIterations)
            );
        }

        $this->randomIterations = $number;
    }

    public function getIterator() {

        return $this->randomIterations === 1
            ? $this->getRandomSingle()
            : $this->getRandomMultiple();
    }

    public function getCount() {

        return $this->count;
    }

    /**
     * SOLID This belongs into it's own type
     *
     * @return ArrayIterator
     */
    private function getRandomSingle() {

        $result = null;
        $count  = 0;
        foreach ($this->innerIterator as $current) {
            mt_rand(0, $count++) || $result = $current;
        }

        $this->count = $count;

        return new ArrayIterator(array($result));
    }

    /**
     * SOLID This belongs into it's own type
     *
     * @return ArrayIterator
     */
    private function getRandomMultiple() {

        $iterator = new IteratorIterator($this->innerIterator);
        $iterator->rewind();
        $it = new NoRewindIterator($iterator);

        $result = array();

        $pickCount = $this->randomIterations;
        $count     = $pickCount;

        while ($count-- && $it->valid()) {
            $result[$count] = $it->current();
            $it->next();
        }

        shuffle($result);

        $count = $pickCount;

        foreach ($it as $current) {
            $random = mt_rand(0, $count++);
            ($random < $pickCount) && $result[$random] = $current;
        }

        $this->count = $count;

        return new ArrayIterator($result);
    }

}

Instead of getCount why not implement countable ??

@olekukonko I also thought about it, but didn't do it because the count is only available after aggregation of the iterator, not before. So it would not always be available, count can not return NULL officially, so would require error / exception handling and I just needed it for some functional tests I did run (e.g. if the number of iteration counts matches).

<3