From: Rick DeNatale <rick.denatale@...>
Date: 2009-11-03T23:06:09+09:00
Subject: Re: Merging hashes using both symbols and strings as keys

On Tue, Nov 3, 2009 at 8:40 AM, Josh Cheek <josh.cheek@gmail.com> wrote:
> On Tue, Nov 3, 2009 at 7:27 AM, Paul Smith <paul@pollyandpaul.co.uk> wrote:
>
>>
>> A big difference here is that a pastie is only available through
>> pastie.org, a gist is a git repository that can be cloned.
>>
>
>
> Here is the code in a gist http://gist.github.com/225048

It might be of interest to note that this code basically take the
opposite tack from the HashWithIndifferentAccess which is part of
ActiveSupport and therefore Rails.

HashWithIndifferentAccess (which I'll abbreviate to HWIA) converts
symbol keys to strings in the access methods ([], []= etc.)  Contrary
to some people's thinking (including mine when I first encountered it)
the actual keys in the hash are strings, not symbols.

The trade-offs between the two approaches include:

1) Symbols once created can't be garbage collected. Since Rails uses
HWIA for things like the params hash, and the keys come from parts of
arbitrary client provided URIs this could be an opening for a DOS
attack which creates tons of useless extra parameters which result in
non-GCable symbols.  Since HWIA uses strings, for the keys, keys
coming from query parameters and the like never get turned into
symbols.  The only symbols will be those coming from the application
itself.

2) Using strings for the keys is slightly slower on access because
computing the hash value of a string is O(length of the string) while
the hash value of a symbol is computed once and is O(1) subsequently.
On the other hand this only becomes significant if the same key is
used to access the hash multiple times, since interning a string as a
symbol requires computing the hash of the string anyway.

In practice, and particularly for the typical usage in Rails apps, I
doubt that there's any real effect on performance from storing strings
rather than symbols for the keys.  The main advantage of HWIA is that
it allows you to save a keystroke

   params[:id]
vs
  params['id']

and some think the former looks a little bit nicer.  Although this is
no doubt a potential source of controversy.  I for one have a slight
preference for the :id form, but that might be the result of Stockholm
syndrome having worked on so many Rails apps.
-- 
Rick DeNatale

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