Is it possible to combine hash codes for private members to generate a new hash code?

Question

I have an object for which I want to generate a unique hash (override GetHashCode()) but I want to avoid overflows or something unpredictable.

The code should be the result of combining the hash codes of a small collection of strings.

The hash codes will be part of generating a cache key, so ideally they should be unique however the number of possible values that are being hashed is small so I THINK probability is in my favour here.

Would something like this be sufficient AND is there a better way of doing this?

int hash = 0;
foreach(string item in collection){
    hash += (item.GetHashCode() / collection.Count)
}
return hash;

EDIT: Thanks for answers so far. @Jon Skeet: No, order is not important

I guess this is almost a another question but since I am using the result to generate a cache key (string) would it make sense to use a crytographic hash function like MD5 or just use the string representation of this int?

Answer 1

The fundamentals pointed out by Marc and Jon are not bad but they are far from optimal in terms of their evenness of distribution of the results. Sadly the 'multiply by primes' approach copied by so many people from Knuth is not the best choice in many cases better distribution can be achieved by cheaper to calculate functions (though this is very slight on modern hardware). In fact throwing primes into many aspects of hashing is no panacea.

If this data is used for significantly sized hash tables I recommend reading of Bret Mulvey's excellent study and explanation of various modern (and not so modern) hashing techniques handily done with c#.

Note that the behaviour with strings of various hash functions is heavily biased towards wehther the strings are short (roughly speaking how many characters are hashed before the bits begin to over flow) or long.

One of the simplest and easiest to implement is also one of the best, the Jenkins One at a time hash.

private static unsafe void Hash(byte* d, int len, ref uint h)
{
    for (int i = 0; i < len; i++)
    {
        h += d[i];
        h += (h << 10);
        h ^= (h >> 6);
    }
}

public unsafe static void Hash(ref uint h, string s)
{
    fixed (char* c = s)            
    {
        byte* b = (byte*)(void*)c;
        Hash(b, s.Length * 2, ref h);
    }
}

public unsafe static int Avalanche(uint h)
{
    h += (h<< 3);   
    h ^= (h>> 11);  
    h += (h<< 15);  
    return *((int*)(void*)&h);
}

you can then use this like so:

uint h = 0;
foreach(string item in collection) 
{
    Hash(ref h, item);
}
return Avalanche(h);

you can merge multiple different types like so:

public unsafe static void Hash(ref uint h, int data)
{ 
    byte* d = (byte*)(void*)&data;
    AddToHash(d, sizeof(int), ref h);
}

public unsafe static void Hash(ref uint h, long data)
{ 
    byte* d= (byte*)(void*)&data;
    Hash(d, sizeof(long), ref h);
}

If you only have access to the field as an object with no knowledge of the internals you can simply call GetHashCode() on each one and combine that value like so:

uint h = 0;
foreach(var item in collection) 
{
    Hash(ref h, item.GetHashCode());
}
return Avalanche(h);

Sadly you can't do sizeof(T) so you must do each struct individually.

If you wish to use reflection you can construct on a per type basis a function which does structural identity and hashing on all fields.

If you wish to avoid unsafe code then you can use bit masking techniques to pull out individual bits from ints (and chars if dealing with strings) with not too much extra hassle.

Answer 2

Hashes aren't meant to be unique - they're just meant to be well distributed in most situations. They're just meant to be consistent. Note that overflows shouldn't be a problem.

Just adding isn't generally a good idea, and dividing certainly isn't. Here's the approach I usually use:

int result = 17;
foreach (string item in collection)
{
    result = result * 31 + item.GetHashCode();
}
return result;

If you're otherwise in a checked context, you might want to deliberately make it unchecked.

Note that this assumes that order is important, i.e. that { "a", "b" } should be different from { "b", "a" }. Please let us know if that's not the case.

Answer 3

There is nothing wrong with this approach as long as the members whose hashcodes you are combining follow the rules of hash codes. In short ...

1. The hash code of the private members should not change for the lifetime of the object
2. The container must not change the object the private members point to lest it in turn change the hash code of the container

Answer 4

If the order of the items is not important (i.e. {"a","b"} is the same as {"b","a"}) then you can use exclusive or to combine the hash codes:

hash ^= item.GetHashCode();

[Edit: As Mark pointed out in a comment to a different answer, this has the drawback of also give collections like {"a"} and {"a","b","b"} the same hash code.]

If the order is important, you can instead multiply by a prime number and add:

hash *= 11;
hash += item.GetHashCode();

(When you multiply you will sometimes get an overflow that is ignored, but by multiplying with a prime number you lose a minimum of information. If you instead multiplied with a number like 16, you would lose four bits of information each time, so after eight items the hash code from the first item would be completely gone.)