[fpc-devel] 2 Ideas for generic TDictionary and other hash containers

[Stefan Glienke]

For your second feature request it just needs a method added to THashSet<T> that retrieves the stored item based on the passed key.

> On 27/03/2025 12:28 CET Martin Frb via fpc-devel <fpc-devel at lists.freepascal.org> wrote:
> 
>  
> There are 2 features, I am missing.
> 
> And I can't see a way to add them via subclassing, as the internal 
> FItems array is private, and neither TItem, nor TPair can be gotten by 
> the integer index into that array.
> 
> 1) AddOrGetPointerToEmpty
> This would actually just be to try and see if it gives any benefit in 
> performance.
> 
> There is    TryAdd()
> But, the caller must already have a new value, that it can pass for that 
> key.
> 
> However, if the dictionary already has a value for that key, then the 
> existing value may be enough,
> So the caller ends up
>     NewVal := CreaterNewValSpendingTime;
>    if not Dictionary.TryAdd(TheKey, NewVal) then
>      NewVal.Destroy;
> 
> Alternatively the caller can do
>    If not Dictionary.ContainsKey(TheKey) then ...
> 
> But the latter then means, that "TheKey" need to be hashed twice, if a 
> new value needs to be added (Even though the hash could already be known).
> 
> If there was a function
>    GetItemPointer(AKey: TKey; out APtr: PValue): boolean;
> 
> which returns True, if Akey was found and therefore APtr points to 
> existing Data, while otherwise APtr points to the empty slot, where the 
> data can be copied to.
> 
> 
> 
> 2) Key and Data to be the same.
> 
> Lets say I have lots of data, to which I have pointers (e.g. instances 
> of some object).
> Now when I have such a pointer, I want to know if I had a previous 
> pointer to the same data. (even if the new pointer points to a copy of 
> the data at a new location).
> 
> I could do TDictionary<TMyClass, TMClass>
> I need to have my own IEqualityComparer, so I don't compare the 
> pointers, but the data in the object. (That I can do).
> 
> But now I need to do
>    MyDict.Add(val, val);
> Which means the same value is stored as pointer and data => doubling mem 
> requirements.
> 
> What I would need is
> - set TData to "record end" (zero size)
> - be able to retrieve the existing key, for another key that returns 
> equal to the stored key
> 
> So instead of
>    MyVal := Dict.Items[AKey];
> 
> I need
>    ExsitingKey := Dict.Keys[ANewKey];
> 
> Obviously since the key is the data, my IEqualityComparer can do proper 
> comparison, so even if the hash clashes, the pointed to data can be 
> compared.
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