Re: Using CFHash to tell if a CFTypeRef has changed?
Re: Using CFHash to tell if a CFTypeRef has changed?
- Subject: Re: Using CFHash to tell if a CFTypeRef has changed?
- From: Jens Alfke <email@hidden>
- Date: Mon, 11 Jan 2016 10:08:47 -0800
> On Jan 11, 2016, at 9:57 AM, Jens Alfke <email@hidden> 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
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