> > At a previous job, we tested a 32-bit hash function by running it > > against hundreds of thousands of unique URL's stored in our > > database. We found one collision. A 64-bit hash is billions of > > times better (4 billion, to be exact). > > Good to know. I wonder how many collisions I'd find if I ran it over > every URL listed in the directory www.yahoo.com. > > Which 64 bit hash function did you use? Invent your own, or something > "off the shelf"? >
We found a public domain one on the net see http://www.burtleburtle.net/bob/hash/evahash.html for some sample code. It's only a 32-bit hash though. However, that same page appears to have instructions for a 64-bit hash function as well, but I haven't tried it at all. I'd be curious to know how many collisions you find hashing all the URL's in yahoo's database :) I don't know how long that would take, but if you do it I'd like to hear the results. Since the hash function takes a key and an initial value, you could try running it with two different initial values and/or keys. This would give you effectively a 128-bit hash, which you could store across two fields in MySQL. I'm guessing that the 64-bit hash will probably be good enough though. Steve Meyers --------------------------------------------------------------------- Before posting, please check: http://www.mysql.com/manual.php (the manual) http://lists.mysql.com/ (the list archive) To request this thread, e-mail <[EMAIL PROTECTED]> To unsubscribe, e-mail <[EMAIL PROTECTED]> Trouble unsubscribing? Try: http://lists.mysql.com/php/unsubscribe.php
