Hi, It’s actually an NSInteger so it’s 64 bits isn’t it? But I take your point, the hash isn’t going to change.
Any one have any other techniques for telling if a CFTypeRef has changed? All the Best Dave > On 11 Jan 2016, at 18:08, Jens Alfke <j...@mooseyard.com> wrote: > > >> On Jan 11, 2016, at 9:57 AM, Jens Alfke <j...@mooseyard.com >> <mailto:j...@mooseyard.com>> wrote: >> >> No. If the object has changed, its hash isn’t guaranteed to change; there >> exist hash collisions where two different values have the same hash. (This >> is inevitable since there are only 2^32 hash values, but a nearly infinite >> number of different strings, arrays, etc.) > > FYI, since hash functions can be tricky, a useful technique I use to reason > about them is to imagine a deliberately stupid hash function. For instance, > imagine that the hash function for strings simply returns the first 4 bytes > of the string. (Yes, this meets the criteria for a correct hash function, > it’s just going to be inefficient in use.) If you used the ‘compare the > old/new hashes’ trick, you would only detect changes to the string that > altered the first four characters. That’s obviously not going to work. > > —Jens _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
