From: MenTaLguY <mental@...>
Date: 2007-11-22T06:14:58+09:00
Subject: Re: Ordered Hash Usefulness

On Thu, 22 Nov 2007 05:25:07 +0900, Robert Klemme <shortcutter@googlemail.com> wrote:
>> In Ruby 1.8 the enumeration order of a Hash is undefined, in Ruby 1.9,
>> unless it's been backed out recently, it's defined as above.
> 
> Does Ruby 1.9 really impose the overhead to maintain insertion order for
> *every* Hash?

I asked matz about that on ruby-core.  The overhead turns out to be very
small; 1.9 maintains a doubly-linked list of hash elements, which has
constant-time insertion and removal.  Memory overhead is two words per
hash element.

We may implement the 1.9 ordering in JRuby soon, as it solves a number of
problems with concurrency and iterator stability.

-mental