From: Rick DeNatale <rick.denatale@...>
Date: 2010-12-04T02:28:56+09:00
Subject: Re: hash method how-to?

On Thu, Dec 2, 2010 at 5:35 PM, Robert Klemme
<shortcutter@googlemail.com> wrote:
> 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

Note that Set, like hash depends on that implication relationship
between #hash and #eql?

class X
 attr_reader :name
 def initialize(name)
   @name = name
 end

 def hash
   @name.hash
 end

 def inspect
   "X:#{@name.inspect}"
 end
end

require 'set'

[X.new("foo"), X.new("foo")].to_set # => #<Set: {X:"foo", X:"foo"}>

class X
 def eql?(other)
   other.class == X  &&
   name.eql?(other.name )
 end
end

[X.new("foo"), X.new("foo")].to_set # => #<Set: {X:"foo"}>



-- 
Rick DeNatale

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