From: Darshan Ishaya <darshan.ishaya@...>
Date: 2006-11-25T01:30:59+09:00
Subject: Re: Problems with String#hash

darshan@shadow ~ $ ri -T Object#hash
------------------------------------------------------------ Object#hash
     obj.hash    => fixnum
------------------------------------------------------------------------
     Generates a Fixnum hash value for this object. This function must
     have the property that a.eql?(b) implies a.hash == b.hash. The
     hash value is used by class Hash. Any hash value that exceeds the
     capacity of a Fixnum will be truncated before being used.
darshan@shadow ~ $ ri -T "String#hash"
------------------------------------------------------------ String#hash
     str.hash   => fixnum
------------------------------------------------------------------------
     Return a hash based on the string's length and content.

I don't see any reason you should assume the hash to be the same on 
different machines, or even across different instances of ruby.  I'd 
only assume the hashes of eql? objects to be equal if they are computed 
within the same run of the script. 

Your nasty problem seems to be that you were making an assumption not 
guaranteed to be true.

What are you actually trying to accomplish?  Maybe we can help you find 
another way.

Darshan

BTW I get the same negative value as you do in your last (4th!) 
example.  Could be a 64-bit thing.

George Moschovitis wrote:
> Dear devs,
>
> I am facing a very NASTY problem with String#hash
>
> Here is the same code that I run on 3 different computers:
>
> root@kosh:~ # irb
> irb(main):001:0> "User".hash
> => 690465448
> irb(main):002:0> VERSION
> => "1.8.2"
>
> C:\Documents and Settings\drak>irb
> irb(main):001:0> "User".hash
> => 690465448
> irb(main):002:0> VERSION
> => "1.8.4"
>
> http://tryruby.hobix.com/
> Interactive ruby ready.
>>> "User".hash
> => 690465448
>>> VERSION
> => "1.8"
>
> root@victory:~# irb
> irb(main):001:0> "User".hash
> => -1457018200
> irb(main):002:0> VERSION
> => "1.8.4"
>
> Please nottice that on the last case I get a different result! Any
> idea why this happens?
>
> thanks in advance for any help!
>
> George.
>
>