[Kde-pim] Nepomukfeeder Caching

[Christian Mollekopf]

> 
> * The hashing function simply xor's the conjunction of the hashes of uri
> and 
> value of each resource-property together to calculate the hash of the
> full 
> resource.  I don't know the math behind that but simply copied it from 
> nepomuk-core, input would be appreciated:
> propertycache.cpp:50
> 

The hashing function looks like this:

    uint hash = 0;
    QHashIterator<QUrl, QVariant> it( properties );
    while( it.hasNext() ) {
        it.next();
        hash ^= qHash( it.key() ) & qHash( it.value().toString() );
    }
    return hash;

The (hash ^= newhash) should be ok, but I don't think the conjunction
is, and I think it would make more sense to XOR that as well. That the
hash is initialized with 0 doesn't hurt IMO as there is a 50/50 chance
we for 0 or 1 when using an XOR.

So I'll change this to:

    uint hash = 0;
    QHashIterator<QUrl, QVariant> it( properties );
    while( it.hasNext() ) {
        it.next();
        hash ^= qHash( it.key() ) ^ qHash( it.value().toString() );
    }
    return hash;

Note that XOR is commutative, so the hash does not preserve the
information of order, but that should be fine for our usecase.
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