VBAでクイックソート(改良版)

Function quick_sort(arrs)
    Dim ub As Long, ub_front As Long, ub_rear As Long
    Dim pivot As Double
    Dim i As Long, x As Long, y As Long, z As Long
    '要素が1個の場合
    If UBound(arrs) < 1 Then
        quick_sort = arrs
        Exit Function
    End If
    'ピボットとの大小で別の配列に
    ub = UBound(arrs)
    pivot = arrs(0)
    ReDim front(ub)
    ReDim rear(ub)
    x = 0
    y = 0
    For i = 1 To ub
        If arrs(i) < pivot Then
            front(x) = arrs(i)
            x = x + 1
        Else
            rear(y) = arrs(i)
            y = y + 1
        End If
    Next i
    '処理後の配列を生成
    ReDim new_arrs(ub)
    z = 0
    '前方の配列(ピボットより小)
    If x > 0 Then
        ReDim Preserve front(x - 1)
        front = quick_sort(front) '再帰的にソート
        ub_front = UBound(front)
        For i = 0 To ub_front
            new_arrs(z) = front(i)
            z = z + 1
        Next i
    End If
    'ピボット
    new_arrs(z) = pivot
    z = z + 1
    '後方の配列(ピボットより大)
    If y > 0 Then
        ReDim Preserve rear(y - 1)
        rear = quick_sort(rear) '再帰的にソート
        ub_rear = UBound(rear)
        For i = 0 To ub_rear
            new_arrs(z) = rear(i)
            z = z + 1
        Next i
    End If
    quick_sort = new_arrs
End Function

Sub test()
    Dim arrs(5000)
    Dim new_arrs
    Dim i As Long
    For i = 0 To 5000
        arrs(i) = Rnd()
    Next i
    new_arrs = quick_sort(arrs)
    For i = 0 To UBound(new_arrs)
        Cells(i + 1, 1) = new_arrs(i)
    Next i
End Sub