[eru]

Sorry, what?

That might we one very particular way to write a hash function, but it's far from the only one.

Believe it or not, for some purposes taking the first few bytes of a string or even just the length of the string are good hash functions.

[andai]

Well, that's technically also a deterministic random number generator! (I want to say it's not a great one, but... that's apparently context-dependent!)

What are those purposes?

[eru]

If your input is i.i.d. random, then truncating works great. Eg if your keys are UUIDs then truncating can work well.

Another use:

Suppose you write a tool like rmlint that is looking for duplicate files. Generally, you compute some hash for each file, see if you got any duplicates, and then compare the relevant files directly.

A traditional hash like crc or sha256 takes O(n) to compute. But for files you can start with some cheaper hashes, like file length. After all, files of different length can't have the same content. Taking the first few bytes of your file is another cheap 'hash' you can compute.

Only when these cheap 'hashes' show that you have a potential duplicate, do you go and pay for a more expensive hash.