Question

What algorithm yields the best distribution when it comes to mapping hash key --> bucket instance ?

In other words, let’s say I have an hashing function (probably SHA-1) and I have n buckets; what algorithm do I use to map a key to a bucket? E.g. lower bits, upper bits, something else?

Answer 1

Usually, you just mod your hash value with the number of buckets. In the unlikely event that the number of buckets is a power of two, you can use bitwise-and instead.

An excerpt from Wikipedia on hash function:

A common solution is to compute a fixed hash function with a very
large range (say, 0 to 2³² − 1), divide the result by n, and use the
division’s remainder. If n is itself a power of 2, this can be done by
bit masking and bit shifting. When this approach is used, the hash
function must be chosen so that the result has fairly uniform
distribution between 0 and n−1, for any n that may occur in the
application. Depending on the function, the remainder may be uniform
only for certain n, e.g. odd or prime numbers.