MergeSort.cs

        private SpriteBatchItem[] _leftArray;
        private SpriteBatchItem[] _rightArray;

        private void MergeSort(SpriteBatchItem[] input, int left, int right)
        {
            if (left >= right)
                return;

            var middle = (left + right) / 2;
            MergeSort(input, left, middle);
            MergeSort(input, middle + 1, right);

            var leftCount = middle - left + 1;
            if (_leftArray == null || _leftArray.Length < leftCount)
                _leftArray = new SpriteBatchItem[leftCount];

            var rightCount = right - middle;
            if (_rightArray == null || _rightArray.Length < rightCount)
                _rightArray = new SpriteBatchItem[rightCount];

            Array.Copy(input, left, _leftArray, 0, leftCount);
            Array.Copy(input, middle + 1, _rightArray, 0, rightCount);

            var i = 0;
            var j = 0;
            for (var k = left; k < right + 1; k++)
            {
                if (i == leftCount)
                {
                    input[k] = _rightArray[j];
                    j++;
                }
                else if (j == rightCount)
                {
                    input[k] = _leftArray[i];
                    i++;
                }
                else if (_leftArray[i].SortKey > _rightArray[j].SortKey)
                {
                    input[k] = _rightArray[j];
                    j++;
                }
                else
                {
                    input[k] = _leftArray[i];
                    i++;
                }
            }
        }