Złożoność obliczeniowa algorytmu

$dane = Array
(
    [0] => 130
    [1] => 4
    [2] => 12
    [3] => 32
    [4] => 40
    [5] => 3
    [6] => 14
    [7] => 19
    [8] => 27
    [9] => 25
    [10] => 31
    [11] => 49
    [12] => 51
    [13] => 55
    [14] => 50
    [15] => 100
)

function merge_sort($arr){
  if(count($arr) == 1 ) return $arr;
  $mid = count($arr) / 2;
  $left = array_slice($arr, 0, $mid);
  $right = array_slice($arr, $mid);
  $left = merge_sort($left);
  $right = merge_sort($right);
  return merge($left, $right);
}
function merge($left, $right){
  $res = array(); // wynik
  while (count($left) > 0 && count($right) > 0){
    if($left[0] > $right[0]){
      $res[] = $right[0];
      $right = array_slice($right , 1);
    } else {
      $res[] = $left[0];
      $left = array_slice($left, 1);
    }
  }
  while (count($left) > 0){
    $res[] = $left[0];
    $left = array_slice($left, 1);
  }
  while (count($right) > 0){
    $res[] = $right[0];
    $right = array_slice($right, 1);
  }
  return $res;
}
$dane = merge_sort($dane);
echo "Posortowana tablica:";
echo "<pre>\$dane = ";
print_r($dane);
echo "</pre><hr>";
Posortowana tablica:
$dane = Array
(
    [0] => 3
    [1] => 4
    [2] => 12
    [3] => 14
    [4] => 19
    [5] => 25
    [6] => 27
    [7] => 31
    [8] => 32
    [9] => 40
    [10] => 49
    [11] => 50
    [12] => 51
    [13] => 55
    [14] => 100
    [15] => 130
)
Złożoność obliczeniowa algorytmu O(n⋅log n).