Hash function on list independant of order of item

Question

I want to have a dictionary that assigns a value to a set of integers.

For example key is [1 2 3] and value will have certain value.

The thing is that [3 2 1] needs to be treated the same in my case so hash needs to be equal, if I go with hash approach.

The set will have 2 to 10 items.

Sum of items is usually fixed so we cannot make hashcode according to sum, which is a first natural idea here.

Not a homework task, actually facing this problem in my code.

This set is basically IEnumerable<int> in C# so any data structure is fine to store them.

Any help appreciated. Performance is pretty important here too.

An immediate thought: we could sum up items^2 and already get some kind of better hash, but still I would like to hear some thoughts.

EDIT: hmm really sorry guys, everyone suggests ordering, didn't come to my mind that I needed to say that actually ordering and hashing is the current solution I use and I am considering faster alternatives.

Answer 1

You could sort the numbers and select a sample from predetermined indices and leave rest as zero if current value has less numbers. Or you could xor them, or whatever.

Answer 2

One possibility: sort the items in the list, then hash that.

Answer 3

I think your squaring idea is going in the right direction, but a poor choice of function. I'd try something more like the PRNG functions or just multiplication by a large prime, followed by XOR of all the resulting values.