Question

I have had numerous cases where I need access to a decent hashing algorithm in C#, from overriding GetHashCode to performing quick comparisons/lookups against data.

I have found the FNV hash to be a really easy/good/quick hash algorithm. However, I have never seen a good example of a C# implementation.

The core of the FNV-1a hash algorithm is as follows:

 hash = OFFSET_BASIS
 foreach (object value in object) 
 {
     hash = hash ^ value.GetHashCode()
     hash = hash * FNV_PRIME
 }

So, when I override GetHashCode for a class I end up doing something like:

public static class FNVConstants
{
    public static readonly int OffsetBasis = unchecked((int)2166136261);
    public static readonly int Prime = 16777619;
}

public override int GetHashCode()
{
    int hash = Constants.FNVConstants.OffsetBasis;
    hash = (hash ^ EntityId.GetHashCode()) * Constants.FNVConstants.Prime;
    hash = (hash ^ FromDate.GetHashCode()) * Constants.FNVConstants.Prime;
    hash = (hash ^ ToDate.GetHashCode()) * Constants.FNVConstants.Prime;
    return hash;
}

What do people think of this?

Answer 1

You could add this to your FNVConstants class

public static int CreateHash(params object[] objs)
{
    return objs.Aggregate(OffsetBasis, (r, o) => (r ^ o.GetHashCode()) * Prime);
}

Then call it like

public override int GetHashCode()
{
    return FNVConstants.CreateHash(EntityId, FromDate, ToDate);
}