Sorting TCollection (via it's private TList)

We have a large collection stored in a TCollection descendant for which I am implementing a sort.

A solution is to :

- copy the TCollection to a TObjectList
- call the TObjectList's sort (the algorithm is a really quick Quicksort)
- cycle through the TObjectList and reset the TCollectionItem.Index property for each item

This works but it is still a little sluggish for large collections, the big overhead being setting the index which does a Move.

However TCollection actually uses a TList itself but _unfortunately it's private_. We tried copying the VCL code for TCollection, opened access to the List and called the sort directly and it works lightening fast!

We don't know a way to have a TCollection descendant access the private TList without creating a new class in the VCL source unit. Does a hack exist?

Has anyone come up with a solution for sorting a TCollection that does not involve adding to the VCL source or setting the Items Index?

I would appreciate any suggestions,

Thanks, Tom

Delphi 6 UP2
boardtcAsked:
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ITugayConnect With a Mentor Commented:
Hi tomcorcoran,

it is possible, but it can be differ for different Delphi versions.

e.g D5 TCollection declaration:

  TCollection = class(TPersistent)
  private
    FItemClass: TCollectionItemClass;
    FItems: TList;
....

you can see that FItems located bellow FItemClass. So, hack code to access FItems:

function GetCollectionList(C: TCollection): TList;
var
  M: TMethod;
  L: TList absolute M;
  P: Pointer;
begin
  // offset of FItemClass
  P := @C.ItemClass;
  // add size of FItemClass
  inc(Integer(P), SizeOf(TCollectionItemClass));
  // now P point to FItems
  M := TMethod(P^);
  Result := L;
end;


now an example:

  L := TCollection.Create(TCollectionItem);
  L.Add;
  L.Add;
  L.Add;
  GetCollectionList(L).Clear;
  Caption := IntToStr(L.Count);

------
Igor.
   
PS: Can be a problem regarding TCollectionItem.Index. It stored inside an item and will not be changed after sorting.
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swift99Commented:
To me it looks like you will end up doing exactly what you did - creating your clone of the Borland code base and  either exposing the FList or creating a sort method that in turn invokes FList.Sort.

Hacks are always a dangerous thing - I'm in the process of attempting to upgrade about 250,000 lines of code from a hacked D3 VCL base to a standard D6 code base.

We have another 500,000 or so lines that two other gentlemen are responsible for upgrading.  It's a real pain - you don't want to go there.
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aikimarkCommented:
idea 1:
1. Pull the list into an array of type TObject
2. do a Quicksort on the array (your code)
3. rebuild your list

==============================================
on what criteria (object property) are basing your sort comparisons?

==============================================
idea 2:
1. build an array of the criteria and (double-)linked pointer data.
2. sort the array
3. "move" the the list items directly to their correct (sorted) location in the list, if they aren't already in the correct location in the list.  You only need to update the pointers.
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boardtcAuthor Commented:
Thanks guys. Got a possibility today by adding a TObjectList member to the collection and adding (integer) mappers for both the collection and the list to the collectionitem object. Ugly but a possibility.

Igor, that is exactly what I was hoping for :-) _But_ your PS of the index not being set is worrying. I will look at it tomorrow.

The sorting of the collection is done via a ListView where they can click on a number of columns for ascending / descending order. How can you rebuild a TCollection with the same TCollectionItems? There would be too much design overhead in idea 2 if I understand it correctly we would prefer not to change from using a TCollection. The tough part every time is the TCollectonItem.Index.

As the access to the collection is through the ListView the fact the index from that won't be talking with the correct index in the Collection means trouble :-) I will investigate tomorrow.

Thanks for the mails. Tom.

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aikimarkCommented:
Tom,

Does this other solution create an "index" to the list?
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boardtcAuthor Commented:
Can I give an A++ :-)

You beauty, what a sick hack!!!!!!

On a test list of 1 million items it sorted in 15 seconds! It our real world situtation it would not be this fast as we use RTTI to get the column clicked on...

Anyway, we were concerned about 16,000 items which with the old optimised insertion sort was taking up to a minute, now it's a second odd.

I can't thank you enough!

Experts-exchamge rules :-)

Cheers, Tom.
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