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# When sorting a “StringList” is very costly

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The following code looks innocuous but slows down a program significantly:

 ```type TJCHListSortCompare = function(Item1, Item2: Integer): Integer of object; TCheckListBoxWithHints = class(TCheckListBox) private procedure QuickSort(L, R: Integer; SCompare: TJCHListSortCompare); // [...] procedure TCheckListBoxWithHints.QuickSort(L, R: Integer; SCompare: TJCHListSortCompare); var I, J, P: Integer; tmpObj: TObject; tmpStr: string; tmpChecked: Boolean; begin repeat I := L; J := R; P := (L + R) shr 1; repeat while SCompare(I, P) < 0 do Inc(I); while SCompare(J, P) > 0 do Dec(J); if I <= J then begin // exchange I and J tmpStr := Items[I]; tmpObj := Items.Objects[I]; tmpChecked := Self.Checked[I]; Items[I] := Items[J]; Items.Objects[I] := Items.Objects[J]; Self.Checked[I] := Self.Checked[J]; Items[J] := tmpStr; Items.Objects[J] := tmpObj; Self.Checked[J] := tmpChecked; if P = I then P := J else if P = J then P := I; Inc(I); Dec(J); end; until I > J; if L < J then QuickSort(L, J, SCompare); L := I; until I >= R; end;``` Yes it’s Quicksort and it sorts strings in a TCheckListBox’s Items property, swapping not only the strings but also the objects and the Checked values. Now, run this with, lets say 100 entries. That shouldn’t be any problem for Quicksort, should it? But it takes about 2 seconds on my computer which is muuuuuuch longer than I expected. The same code running on a simple TStringList takes less than 1/10 of a second. Why is that?   read on in the blog post

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If the TStrings.Objects isn't used for anything then, instead of separating the strings and checked states into two list and then sort both simultaneously, you could just stuff the checked state into TStrings.Objects and then use the standard TStrings.Sort.

Or you could sort a TList<integer> of indices, using TList<>.Sort and then reassemble the sorted string list based on the result.

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I'd suspect that you call onclick every time you set checked.

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Posted (edited)

Yes @Anders Melander, you are right in all points. Unfortunately none of the options you mention was possible in this case (Delphi 6 -> no Generics, Objects already used).

I could also have used TStringList.Objects to store the old index and use Sort, just as you describe for TList<integer>, but that wouldn't have helped much in this particular case. The point of this post was that everything else is better than directly using the Items property and that it is not obvious why.

A virtual CheckListBox would have been even better, but that code is broken since like forever.

Edited by dummzeuch

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Posted (edited)
8 minutes ago, Attila Kovacs said:

I'd suspect that you call onclick every time you set checked.

Hm, good point, I'll have to check that too.

Edit: No, doesn't happen.

Edited by dummzeuch

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Posted (edited)

Oh I see, this component is just a wrapper for the windows component, so you are causing a message storm. There is not even a beginupdate/endupdate.

Edited by Attila Kovacs

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Paying the price for using a GUI control to hold your data, rather than to view your data.

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27 minutes ago, David Heffernan said:

Paying the price for using a GUI control to hold your data, rather than to view your data.

As I said: A virtual ChecklistBox would have been ideal, but unfortunately that mode has been broken since forever. So it's either live with that, or use a different control (ListView?). I'm still considering the latter, but I'm not sure it's worth the effort since it's fast enough now and newer IDEs already come with a similar functionality.

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7 minutes ago, dummzeuch said:

A virtual ChecklistBox

I doubt that will be better, on contrary i think it will be slower, because almost all operations with ListBox are messages, so making it virtual will add additional message to retrieve the data.

It will be faster many folds if you read all the data then cleared the listbox then sorted after that added the data back to the list box.

Switch to ListView or VirtualTreeView, or don't read the data from the list box, keep a copy in the memory to work with, then clear and add after sorting your copy.

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A ListView would also require lots of messages, but from experience i know that it can easily handle thousands of lines and still be reasonable fast (we're talking about less than 200 here). A VirtualTreeview would be an option, but meaning to add yet another component which i am rather reluctant to do in this case. Another option would be a (virtual) StringGrid, I actually started down that road but decided to try and improve that sorting first. Now I've got a solution that's fast enough and works, so I'll probably leave it as is.

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You don't need a new control. You just need not to use it to store your data. Store your data in a non visual control.

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9 hours ago, David Heffernan said:

You don't need a new control. You just need not to use it to store your data. ﻿Store your data in a non visual control.

Dear David, I think you are right for the sorting issue.

Since I like to use the "Tag" or "Object" property for having an simple integer ID to each entry, beside the display function,
I'm curious how you would handle this.

Do you also use an external storage, if you just need the "Tag" for identification of the entry, but not the sorting ?

When would you use a "Tag" locally, and when would you use it externally ?

I think if the "Tag" only would exists in the ListView then it should be better hold externally, to have the data separated,
but most of the time it comes already from external sources, which were not so convenient to access or need more processing.

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Quick sort is known to suffer performance-wise on already sorted lists, but with only 100 to 200 elements, that should not be the problem.

Other than that, keeping your working data outside UI controls is sound advice.

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34 minutes ago, Lars Fosdal said:

Quick sort is known to suffer performance-wise on already sorted lists, but with only 100 to 200 elements, that should not be the problem.

Other than that, keeping your working data outside UI controls is sound advice.

These days I use Timsort for this reason.

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22 minutes ago, David Heffernan said:

These days I use Timsort for this reason.

I still use Shell's Sort - but with a bit of extra work to keep the original ordering intact for identical elements.

Is there a good implementation of Timsort for Delphi to be found?

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Posted (edited)
3 minutes ago, Lars Fosdal said:

Is there a good implementation of Timsort for Delphi to be found﻿﻿?

We've talked about this before I think. Stefan and I developed one for spring4d. You'll find it in the develop branch.

As I'm sure you know, Timsort is stable.

Edited by David Heffernan
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Ah, yeah... It is a bit embarrassing, but I still have not even looked at spring4d.  I really should.

I did follow it on GitHub, but never got around to actually delving into it.

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4 hours ago, Lars Fosdal said:

Is there a good implementation of Timsort for Delphi to be found?

Don't know whether it's any good, but here is one for free pascal:

Unit lgMiscUtils

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When we all will use quantum computers - and when Windows 12 will run on quantum computers - then this kind of speed problems will be a thing of the past. 🔮

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9 minutes ago, PeterPanettone said:

When we all will use quantum computers - and when Windows 12 will run on quantum computers - then this kind of speed problems will be a thing of the past. 🔮

Nicklaus Wirth disagrees: https://en.wikipedia.org/wiki/Wirth's_law

Bonus quote:

What Intel giveth, Microsoft taketh away: https://en.wikipedia.org/wiki/Andy_and_Bill's_law

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Posted (edited)
19 hours ago, dummzeuch said:

Don't know whether it's any good, but here is one for free pascal:

Unit lgMiscUtils

Impressive:

```                               sorted     reverse        half      random
QuickSort(10000000):    0.69550     0.72872    not done     1.89622
QuickSort+(12)(10000000):    0.63999     0.65609     3.61170     1.70513
QuickSort+(15)(10000000):    0.63134     0.66484     3.65941     1.72658
QuickSort+(17)(10000000):    0.63200     0.65742     3.58655     1.73312
QuickSort+(20)(10000000):    0.59678     0.62114     3.60640     1.75100
TimSort(10000000):    0.02052     0.03282     0.11939     1.88033

QuickSort(100000000):    8.13450     8.50642    not done    21.52633
QuickSort+(12)(100000000):    7.16910     7.47369    45.95549    19.66967
QuickSort+(15)(100000000):    7.25634     7.49350    45.59182    19.57720
QuickSort+(17)(100000000):    7.15293     7.43410    46.06242    19.69688
QuickSort+(20)(100000000):    7.26109     7.46288    46.09772    20.61769
TimSort(100000000):    0.20487     0.33015     1.12082    23.05309
```

This is the result of a test sorting an array of 10 Million and 100 milliion integers on my computer. The values are the time in seconds.

QuickSort is my regular quicksort implementation

QuickSort+ is my optimized quicksort which:

• choses a pivot by selecting the median between elements Left, Right and (Left + Right) / 2
• uses InsertionSort for small sets, if Right - Left < Cutoff (The value in parenthesis is the cutoff.)

TimSort is the code from the link above, adjusted so it compiles with Delphi 2007 (no generics).

Edit: Here is the source code. Note that It currently can only sort an array of integer.

Edited by dummzeuch
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1 hour ago, dummzeuch said:

I'm not sure whether the license allows me to share that implementation

It does. You just need to keep the attribution, copyright and license notice in the source.

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Posted (edited)
14 hours ago, Anders Melander said:

It does. You just need to keep the attribution, copyright and license notice in the source.

I just read the Apache License (never did before) and have now added a link to the source code above.

Note that the current code can only sort an array of integer and is pretty rough. I'll clean it up a bit today and add more tests.

I also started a new thread on this: TimSort for Delphi without Generics.

Edited by dummzeuch
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I appreciate your interest.

But it still seems to me that your QuicksortPlus is not in a hurry.
Anyway, my own benchmark looks like this:

```                        Sorted   	Reverse  	OrganPipe	Random
QuickSort         	0,0073   	0,0135   	0,2448   	0,7248
IntroSort         	0,0064   	0,0134   	0,2122   	0,7448
DualPivotQuickSort	0,0064   	0,0134   	0,27     	0,7356
PDQSort           	0,0064   	0,0136   	0,549    	0,388
MergeSort         	0,0066   	0,0125   	0,0362   	0,9725
RadixSort         	0,006    	0,013    	0,2692   	0,2554
TimSort           	0,0064   	0,0126   	0,044    	1,255
QuicksortPlus     	0,4123   	0,4246   	2,1564   	1,5676```

for an array of 10 M integers

and

```                        Sorted   	Reverse  	OrganPipe	Random
QuickSort         	0,0643   	0,1363   	2,6703   	8,4403
IntroSort         	0,0793   	0,1557   	2,3547   	8,529
DualPivotQuickSort	0,0643   	0,1363   	2,9583   	8,4367
PDQSort           	0,0643   	0,1363   	6,2103   	4,2567
MergeSort         	0,0647   	0,1263   	0,3347   	11,114
RadixSort         	0,0597   	0,1313   	3,268    	2,4947
TimSort           	0,065    	0,126    	0,4107   	15,0483
QuicksortPlus     	5,1873   	5,3153   	30,6993  	18,7067```

for an array of 100 M integers

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@avk I am sceptical of your numbers. How can the other algorithms match TimSort for sorted and reverse data?

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