UDSEntry compression ideas

[David Faure]

On Friday 20 September 2013 00:32:08 Frank Reininghaus wrote:
> Now the obvious question is: "How on earth can this be more efficient
> that the current solution? There is still a QHash in each
> UDSEntryPrivate after all, and the code gets more complicated!"
> 
> The trick is that the mapping from the "uint uds" to the index of the
> corresponding "Field" is usually exactly the same for every UDSEntry
> if we read many of them in a row, and we can make all UDSEntryPrivates
> share this mapping (QHash is implicitly shared). This means that every
> UDSEntryPrivate only has a private QVector<Field>, which only needs a
> rather small overhead compared to the memory that the Fields
> themselves need.
> 
> The patch definitely needs further testing and polishing. I probably
> won't get much work done until next week though.

Nice tricks, very clever.

The patch looks good to me.

Just some minor issues with the patch:
*  contains + value means double lookup, use iterators instead
 (like the code was doing before - with a typedef for the QHash, for 
readability)
* the initialisation of cachedStrings can be simplified with QVector::resize 
which does call the default constructor for each item.


Mark: you're right that in general when using QHash one has to remember the 
time of the hash calculation -- like my first QHash<KUrl, Node*> back when 
KUrl's hash value calculation needed to concatenate the parts of the url first 
(ouch!!!). However in the case of an int as key, the calculation is trivial  
(just a modulo).

-- 
David Faure, faure at kde.org, http://www.davidfaure.fr
Working on KDE, in particular KDE Frameworks 5