From: Rick DeNatale <rick.denatale@...>
Date: 2010-12-02T09:11:03+09:00
Subject: Re: hash method how-to?

On Wed, Dec 1, 2010 at 2:27 PM, Yossef Mendelssohn <ymendel@pobox.com> wrote:
> On Dec 1, 1:10 pm, niklas | brueckenschlaeger
> <nik...@brueckenschlaeger.de> wrote:
>> http://en.wikipedia.org/wiki/Hash_function
>>
>> in most cases it's the easiest to delegate the hash method to something
>> else. Example:
>>
>> class X
>>   def initialize(name)
>>     @name = name
>>   end
>>
>>   def hash
>>     @name.hash
>>   end
>> end
>>
>> h = { X.new("foo") => "bar" }
>> h[X.new("foo")] = "baz" # will overwrite the above
>
> And so will `h['foo'] = 'baz'`.

No neither will:

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

  def hash
    @name.hash
  end

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

h = { X.new("foo") => :bar}
h[X.new("foo")] = :baz
h["foo"] = :bat

h # => {"foo"=>:bat, X:"foo"=>:baz, X:"foo"=>:bar}

The reason is that the value of #hash is not the only thing needed to
distinguish hash keys. All objects with the same hash value are mapped
to the same hash 'bucket' hash collisions are resolved by using the
eql? method,

X.new("a").eql?(X.new("a")) # => false

class X
  def eql?(other)
    other.class == X  && # One of the rare occurrences when checking
the class is a good thing
    name.eql?(other.name)
  end
end

h = { X.new("foo") => :bar}
h[X.new("foo")] = :baz
h["foo"] = :bat

h # => {"foo"=>:bat, X:"foo"=>:baz}

X.new("a").eql?(X.new("a")) # => true

Note that the Hash class depends on a relationship between eql? and hash, that

a.eql?(b) => a.hash == b.hash

where => here is logical implication, that is

a => b is the same as

b ||| !a


-- 
Rick DeNatale

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