On 2007-04-11, "Leon Mergen" <leon@xxxxxxxxxxx> wrote: > Now, my question is: how big is the chance that a collision happens > between hashes ? I noticed that the function only returns a 32 bit > number, so I figure it must be at least once in the 4 billion values. Assuming it's a uniform random hash, 32 bits long, then if you have 65536 values, you have a ~40% chance of at least one collision. Any defects in the hash function only increase that probability. This is a result of what's known as the "birthday paradox" (so-called because in a group of 23 people, there is a better than even chance that two of them share a birthday). The number of rows needed to have an approximately even chance of at least one collision grows as the _square root_ of the number of hash buckets; or to put it another way, you always need _more than twice as many bits_ in your hash value than you think you do. (e.g. using md5(), which is a 128-bit hash) -- Andrew, Supernews http://www.supernews.com - individual and corporate NNTP services
