From: Robert Klemme <shortcutter@...>
Date: 2010-12-03T07:35:18+09:00
Subject: Re: hash method how-to?

On 12/02/2010 10:46 PM, Yossef Mendelssohn wrote:

> Although I still think what I was trying to point out is something to
> consider, that simply delegating #hash to an internal element isn't
> all that good because on some level it makes them the same.

Ultimately all hash values are derived from member hash values.  The key 
point - and I believe this is what you want to stress - is _how_ values 
are derived.  The methodology applied should strive to yield different 
hash values for things that are considered non equivalent.  For example, 
hash value of an Array or list implementation should somehow consider 
position of elements in order to not return the same hash for two 
collections which only differ in ordering.  We can see that Array#hash 
obeys this:

irb(main):001:0> [1,2].hash
=> 11
irb(main):002:0> [2,1].hash
=> 1

On the other hand, since Set conceptually is agnostic of position 
different ordering of elements should not create different hash codes:

irb(main):001:0> require 'set'
=> true
irb(main):002:0> s1 = [1,2].to_set
=> #<Set: {1, 2}>
irb(main):003:0> s2 = [2,1].to_set
=> #<Set: {2, 1}>
irb(main):004:0> s1.hash
=> 14
irb(main):005:0> s2.hash
=> 14
irb(main):006:0> s1 == s2
=> true

Kind regards

	robert