[fpc-devel] Want to remove AVL_Tree from DOM

[Michael Van Canneyt]

On Sat, 24 Oct 2009, Sergei Gorelkin wrote:

> Hello,
>
> I want to remove the avl_tree-related stuff from DOM unit. The reasons are:
>
> 1) It is targeted to optimize one particular type of documents, "configs",
>   that require all node names within a single parent to be unique (strictly
>   speaking, this isn't xml, it is more like ini-files or Windows registry).
>
>   For all other xml documents (which have repeating names) it simply doesn't 
> work.
>
> 2) It is a severe performance and memory bottleneck. This maybe wasn't too 
> noticeable
>   in the past, but things had improved over time. Here are some numbers:

Since avl_tree was built to improve performance, this alone is reason enough
to replace it with something more performant.

> Given the above, I see no sense in further keeping this feature. However, the 
> configs likely
> need some sort of optimization anyway. I see the following options:
>
> 1) Since all names in DOM document are now stored in a hashtable, it is easy 
> to modify
>   FindNode() so it compares pointers rather than strings.
>
> 2) The new xmlconf unit has the OpenKey/CloseKey methods which improve 
> performance by limiting
>   the search range to a specific element. Utilizing this feature, however, 
> need modifications to
>   the user code so it actually calls OpenKey/CloseKey (note that is works 
> perfectly without the
>   modifications, it's only the matter of performance).
>
> 3) It is possible to create even faster xmlconfig class, consisting of 
> TXMLDocument + THashTable,
>   offering O(1) access to any element. The problem is that it is somewhat 
> mutually exclusive
>   to the previous solution (OpenKey/CloseKey).
>
> 4) Finally, the xpath, being sufficiently mature, can supersede it all and 
> provide
>   a much more powerful solution.

In the long run, I think XPath is the way to go, it's a standard, after all.

Michael.