Is the hash of a GUID unique?

Question

I create a GUID (as a string) and get the hash of it. Can I consider this hash to be unique?

Solution 1

Not as reliably unique as the GUID itself, no.

Just to expand, you are reducing your uniqueness by a factor of 4, going from 16 bytes to 4 bytes of possible combinations.

As pointed out in the comments the hash size will make a difference. The 4 byte thing was an assumption, horrible at best I know, that it may be used in .NET, where the default hash size is 4 bytes (int). So you can replace what I said above with whatever byte size your hash may be.

Solution 2

Nope.

See here, if you want a mini GUID: https://devblogs.microsoft.com/oldnewthing/20080627-00/?p=21823

Solution 3

In a word, no.

Let's assume that your hash has fewer bits than the GUID, by the pigeon hole principle, there must exist more than one mapping of some GUID -> hash simply because there are fewer hashes than GUIDS.

If we assume that the hash has a larger number of bits than the GUID, there is a very small--but finite--chance of a collision, assuming you're using a good hash function.

Solution 4

No hash function that reduces an arbitrary sized data block to a fixed size number of bits will produce a 1-to-1 mapping between the two. There will always exist a chance of having two different data blocks be reduced to the same sequence of bits in the hash.

Good hash algorithms minimizes the likelihood of this happening, and generally, the more bits in the hash, the less chance of a collision.

Solution 5

No, and I wouldn't assume uniqueness of any hash value. That shouldn't matter because hash values don't need to unique, they just need to evenly distributed across their range. The more even the distribution, the fewer collisions occur (in the hashtable). Fewer collisions mean better hashtable performance.

fyi For a good description of how hash tables work, read the accepted answer to What are hashtables and hashmaps and their typical use cases?

View more solutions