From: David Palm <dvdplm@...>
Date: 2009-05-05T05:57:18+09:00
Subject: Re: String#hash changed in Ruby 1.9?

On Tue, 5 May 2009 02:15:09 +0900, Robert Klemme wrote:
> On 04.05.2009 16:48, David Palm wrote:
>>>> Any tips on how to replace it?
>>> What does it hurt if the hash value of a string does not remain 
>>> constant between runs of the application?
>> 
>> In my case it's pretty bad. I use it in a command line utility to 
>> cache rake tasks. I create one cachefile for each directory, naming 
>> them using the String#hash of the full path (Dir.pwd.hash). If the 
>> hash is different the next time the program runs the cache lookup 
>> fails (and I get a new cache file instead of the old one).
> 
> Hm...  But you do admit that this is a bit abusive, do you?   
> Especially since there are no guarantees that you won't have any 
> collisions with a hash value like the one returned by #hash.

Oh yes, it's just a quick and convenient way of doing it. Dunno if I'd call it "abusive", but it's sure not military grade programming...

> How about storing your cache files with a fixed name in the original 
> directory?  Or have a file with metadata (mapping from path to cache 
> file name)?

Fixed name won't work; most directories are scm tracked so it'd be a mess to keep the cache files out of the way. One big(ish) cache file might work. Maybe even a sqlite db. Have to run some benchmark on that.

>> that stay consistent. Do I need to MD5 it? Feels like overkill. Why 
>> was this changed in the first place?
> 
> That's an interesting question.  I'm curious as well.  Maybe the 
> changes are just a side effect of a new - supposedly better - hashing 
> algorithm.

The link sebastian provided (http://murmurhash.googlepages.com/) was interesting but not exhaustive and I still don't know when/how/why the behaviour was changed. Perhaps the ml archives will tell?