Significance of the 8 byte prefix of the fnv hash?

[rljacobson]

Hi Cory,

I notice you are feeding in a different 8 byte prefix to the fnv hash function before hashing the data for each atom type. This method for String is typical:

func (str *String) Hash() uint64 {
    h := fnv.New64a()
    h.Write([]byte{102, 206, 57, 172, 207, 100, 198, 133})
    h.Write([]byte(str.Val))
    return h.Sum64()
}

What's the significance of this prefix? Are you just avoiding collisions in case of coincidental equality of data being hashed for different types? Are your choices of prefix chosen randomly?

[corywalker]

Hi Robert,

The prefixes are indeed chosen randomly. I do believe the original intent was, as you suspect, to avoid collisions of data being hashed for different types. For example, if a String happened to have the same text as a Symbol, this allows it to hash to a different value. I don't remember why exactly I chose 8 bytes to do this instead of 1. Eight seems like overkill right now. That's what I remember.

[rljacobson]

Thanks, Cory! I hope you are doing well.