On Mon, 29 Nov 2010 01:26:18 +0100 Joerg Sonnenberger <jo...@britannica.bec.de> wrote:
> On Sun, Nov 28, 2010 at 11:37:12PM +0000, Sad Clouds wrote: > > I've had some issues with a hash table that was power of 2 in size, and > > most of the keys were multiples of 4, or 8 integers, for example. > > However making hash table size a prime number and using a good hash > > function, results in very good key distribution. This works especially > > well when hashing simple intergers, e.g. file descriptors, etc. > > If you have issues with a power of 2 hash table size, you do not have a > good hash function in first place. A simple multiplicative hash function > like ((uint64_t)X * random32) >> 32 % size is known to provide good > results for most choices of random32. The suggestion to use a prime > number goes back to using primitive hash functions that don't properly > avoid funnels. With a good hash function, using a non power of 2 size > just tends to be slower. > > Joerg Joerg I was specifically talking about particular sequences of numbers that result in high collision rates, not random numbers. For example, if you take the following: 4, 8, 12, 16, 20 ... Every integer is a multiple of 4. Can you give me a full example of a hash function for power of 2 hash table, that will perform as well as a simple: interger % prime
