On 15 Feb 2013, at 01:25, Ken Thomases <k...@codeweavers.com> wrote:
>
>
> On Feb 14, 2013, at 8:30 AM, Uli Kusterer wrote:
>
>> On Feb 14, 2013, at 3:57 AM, Ken Thomases <k...@codeweavers.com> wrote:
>>> Your question prompted me to try to design an analog of NSKeyedArchiver,
>>> NSCode, and NSCoding that would generate the new object graph on the fly as
>>> it went instead of producing a data object. The idea is that the copier
>>> (the analog of the archiver/coder) would know which objects had already
>>> been copied and so would avoid over-duplicating them in the new graph.
>>> However, that ends up being hard because each object has to copy its
>>> related object before it's complete enough to be registered with the
>>> copier. So, it isn't successful in avoiding the potential for infinite
>>> recursion.
>>
>> What NSKeyedArchiver probably does is have a dictionary that maps the
>> original object pointer values to the copied objects. So instead of just
>> straight-out copying an object, it does:
>>
>> NSString* theKey = [NSString stringWithFormat: @"%p", theOriginal];
>> id theCopy = [objectCopies objectForKey: theKey];
>> if( !theCopy )
>> {
>> theCopy = [theOriginal copy];
>> [objectCopies setObject: theCopy forKey: theKey];
>> }
>>
>> That way, every object only gets copied once, and the copy re-used in other
>> spots. That may be part of why it is slower. (NB - you could probably use an
>> NSValue +valueWithUnretainedPointer: or whatever as the key, I just quickly
>> typed this untested code into the e-mail)
>
> That's what I considered but it doesn't work. You can't add the copy into
> the archiver's database of already-copied objects until it's complete. But,
> for the case where the graph has cycles, making the complete copy will try to
> make copies of all its related objects first. When the cycle loops back to
> the same original, that original still does not have a copy in the database,
> so it tries to make another copy. Infinite recursion.
Why not keep a mutable dictionary M and on encountering an object (in the
scanning phase) do:
if object not in mutable dictionary M then
add to M: key = address of object, value = NSNull
continue walking the object tree
else
do nothing
endif
And in the second (copy) phase do:
if value of object = NSNull then copy it and set the value in M to the copied
object
else just store a link to the already copied object.
Should work with cycles. Might not be faster than NSArchive.
Kind regards,
Gerriet.
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