> On Jan 11, 2016, at 9:57 AM, Jens Alfke <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
