Median selection

   protected function selectPivot(array $dataset)
   {
      $x      = $dataset[0];
      $y      = $dataset[(int)(count($dataset)/2)];
      $z      = $dataset[count($dataset)-1];
      if($x>=$y)
      {
         if($y>=$z)
            return [(int)(count($dataset)/2) => $y];
         return $x>=$z
            ?[count($dataset)-1 => $z]
            :[0 => $x];
      }
      if($x>=$z)
         return [0 => $x];
      return $y>=$z
         ?[count($dataset)-1 => $z]
         :[(int)(count($dataset)/2) => $y];
   }