Hash#hash and detecting changes?

Hello,

I am writing a Rack session class. I want to detect changes to the
session so that I don’t write the data to the session store if the
underlying hash has not changed. Right now I’m doing the following but
it doesn’t seem right:

  def call(env)
    load_session(env)
    status, headers, body = @app.call(env)
    commit_session(env, status, headers, body)
  end

load_session loads env[“rack.session”] from the cookie, and also saves
env[“rack.session”].hash into env[“myframework.session.hash”]

commit_session checks env[“rack.session”].hash against the saved one
(env[“myframework.session.hash”]) and if it is different, it writes the
changes to disk.

The reason to do this is that writing data is expensive in this
particular case, since it happens for many kinds of requests.

However my understanding is that while x.hash != y.hash implies that x
and y are different, x.hash == y.hash does not imply that x and y are
the same. Can someone comment on this?

Kind regards,
Samuel

On 3/20/2010 7:41 PM, Space Ship T. wrote:

load_session loads env[“rack.session”] from the cookie, and also saves env[“rack.session”].hash into env[“myframework.session.hash”]

commit_session checks env[“rack.session”].hash against the saved one (env[“myframework.session.hash”]) and if it is different, it writes the changes to disk.

The reason to do this is that writing data is expensive in this particular case, since it happens for many kinds of requests.

However my understanding is that while x.hash != y.hash implies that x and y are different, x.hash == y.hash does not imply that x and y are the same. Can someone comment on this?

Kind regards,
Samuel

From http://ruby-doc.org/core/classes/Hash.html#M002847

hsh == other_hash => true or false

Equality—Two hashes are equal if they each
http://ruby-doc.org/core/classes/Hash.html#M002861 contain the same
number of keys http://ruby-doc.org/core/classes/Hash.html#M002866 and
if each http://ruby-doc.org/core/classes/Hash.html#M002861 key-value
pair is equal to (according to Object
http://ruby-doc.org/core/classes/Object.html#==) the corresponding
elements in the other hash.

h1 = { “a” => 1, “c” => 2 }
h2 = { 7 => 35, “c” => 2, “a” => 1 }
h3 = { “a” => 1, “c” => 2, 7 => 35 }
h4 = { “a” => 1, “d” => 2, “f” => 35 }
h1 == h2 #=> false
h2 == h3 #=> true
h3 == h4 #=> false

How much more equal do you need?

On 3/21/2010 12:16 AM, Walton H. wrote:

  end

Samuel
pair is equal to (according to Object

How much more equal do you need?

Sorry for all the URLs, I just blindly pasted from my browser, not
thinking about what would happen when Thunderbird changed it to plain
text. It should read:

Equality—Two hashes are equal if they each
http://ruby-doc.org/core/classes/Hash.html#M002861 contain the same
number of keys and if each key-value pair is equal to (according to
Object http://ruby-doc.org/core/classes/Object.html#==) the
corresponding elements in the other hash.

h1 = { “a” => 1, “c” => 2 }
h2 = { 7 => 35, “c” => 2, “a” => 1 }
h3 = { “a” => 1, “c” => 2, 7 => 35 }
h4 = { “a” => 1, “d” => 2, “f” => 35 }
h1 == h2 #=> false
h2 == h3 #=> true
h3 == h4 #=> false

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