问题
Is it possible to somehow "hash" a given String with length n to a hash value of an arbitrary length m? I want to achieve something like follows:
let s1 = "<UNIQUE_USER_IDENTIFIER_1>"
let s2 = "<UNIQUE_USER_IDENTIFIER_2>"
let x1 = s1.hashValue(length: 4)
let x2 = s2.hashValue(length: 4)
I want to assign each given user a (e.g. four-digit) number, that is based on its unique UID. Is that possible?
回答1:
First, I want to be clear that you mean "hash" and don't mean "(lossless) compress." You should expect some collisions where x1 and x2 are the same value for different s1 and s2. If you really mean a mapping so that there are no collisions, then we have to know a lot more about the problem. It is impossible to achieve that in the general case (see the Pigeonhole principle). But it can be achieved in some special cases where there is sufficient redundancy in the input. Or it can be done by maintaining a table (i.e. a database or the like). The rest of this answer is about hashing.
If your UID is a UUID created on iOS (or any v4 UUID), then its bits are already quite high quality, and the last four digits should be fine without doing any hashing at all. There are a couple of bytes in the middle that you should avoid, but the whole end section is random and so an ideal hash.
If your UUID is not random, you can try using the default hashes and pulling the required number of bits out of them, but non-cryptographic hashes don't always have good independence between their bits, so this may collide more than you like.
In that case use a cryptographic hash larger than the size you need and truncate it (or take the least-significant bits; either set are fine). This is commonly done in cryptography. For example SHA-512/256 is a commonly used hash that computes a 512-bit hash and extracts 256 bits from it. Cryptographic hashes require high independence of all their bits, so any subset of bits will also be collision resistant.
BTW, if you mean "4 decimal digits," then you should expect a collision about 1 time out 100. If you mean 16 bits (4 hex digits), you should expect a collision about one time in 300. These are your best-case scenarios and mean your hash is working well. See Birthday Attack for a table of expectations and some helpful approximations.
来源：https://stackoverflow.com/questions/50350425/generate-custom-length-hash-values-of-a-string-in-swift