Шанс выпадения

$arr1 = array(1,2);// при выборе рандомом у единицы шанс выпадения 50%
$arr2 = array(1,1,2);//  а теперь 67%
$arr3 = array(1,1,1,2);//  а теперь 75%

class RandomBox {
    public function __construct($things) {
        foreach ($things as $thing) {
            if ($thing['amount'] < 1) {
                throw new Exception();
            }
            $this->things[] = array(
                'thing' => $thing['thing'],
                'amount' => $thing['amount'],
                'chance' => 0.0
            );
        }
        $this->calculateBounds();
    }
    public function get() {
        $random = mt_rand() / mt_getrandmax();
        $size = count($this->things);
        $n = 0;
        for ($i=1; $i < $size; ++$i) {
            $cur = $this->things[$i];
            if ($cur['chance'] < $random) {
                $n = $i;
                continue;
            }
            break;
        }
        $res = $this->things[$n]['thing'];
        if (!--$this->things[$n]['amount']) {
            unset($this->things[$n]);
            $this->things = array_values($this->things);
        }
        $this->calculateBounds();
        return $res;
    }
    private function calculateBounds() {
        $total = 0.0;
        foreach ($this->things as $thing) {
            $total += $thing['amount'] * $thing['thing']->priority;
        }
        $chance = 0.0;
        foreach ($this->things as &$thing) {
            $thing['chance'] = $chance;
            $chance += $thing['thing']->priority * $thing['amount'] / $total;
        }
    }
    private $things;
}
 
class Something {
    public function __construct($name, $priority = 1) {
        if ($priority < 0) {
            throw new Exception();
        }
        $this->name     = $name;
        $this->priority = $priority;
    }
    public function __get($property) {
        if (property_exists($this, $property)) {
            return $this->$property;
        }
    }
    private $name;
    private $priority;
}
 
$box = new RandomBox(array(
    array(
        'thing' => new Something('qwerty', 10),
        'amount' => 1
    ),
    array(
        'thing' => new Something('test', 1),
        'amount' => 10
    ),
    array(
        'thing' => new Something('asdf', 2.5),
        'amount' => 5
    )
));
 
for ($i = 0; $i < 16; ++$i) {
    $thing = $box->get();
    echo $thing->name . '<br>';
}