From: Tanaka Akira <akr@...>
Date: 2009-02-14T12:40:07+09:00
Subject: Re: If you are unhappy with the direction of Ruby 1.8.7+, respond

In article <a245658d0902131727k241af80ai195a5fe0b534a060@mail.gmail.com>,
  Pit Capitain <pit.capitain@gmail.com> writes:

> This is an excerpt from a previous version of your code:
>
>   DICTIONARY = RegexpGroup.new({
>     :OPERATOR => /return|typeof|[\[(\^=,{}:;&|!*?]/.source,
>     :CONDITIONAL => /\/\*@\w*|\w*@\*\/|\/\/@\w*|@\w+/.source,
>     :COMMENT1 => /\/\/[^\n]*/.source,
>     :COMMENT2 => /\/\*[^*]*\*+([^\/][^*]*\*+)*\//.source,
>     :REGEXP => /\/(\\[\/\\]|[^*\/])(\\.|[^\/\n\\])*\/[gim]*/.source,
>     :STRING1 => /\'(\\.|[^\'\\])*\'/.source,
>     :STRING2 => /"(\\.|[^"\\])*"/.source
>   })

Symbol#hash is fragile.

% ruby-1.8.6p287 -e 'p :a.hash' 
104738
% ruby-1.8.6p287 -e ':x; p :a.hash'
104818

The :x expression, which seems have no side-effect, affects
:a.hash.

This is because Symbol#hash depends on how many symbols
known when the symbol first appeared.

This value affects the bucket of a hash.  So it affects the
iteration order, until ruby 1.8.

It also depends on ruby version.

% all-ruby -e 'p :a.hash'
ruby-1.4.6(2000-08-16)         8593
ruby-1.6.0(2000-09-19)         2281742
ruby-1.6.1(2000-09-27)         2281742
ruby-1.6.2(2000-12-25)         2291982
ruby-1.6.3(2001-03-19)         2294030
ruby-1.6.4(2001-06-04)         2294030
ruby-1.6.5(2001-09-19)         2294030
ruby-1.6.6(2001-12-26)         2296078
ruby-1.6.7(2002-03-01)         2302222
ruby-1.6.8(2002-12-24)         2308366
ruby-1.8.0(2003-08-04)         2605326
ruby-1.8.1(2003-12-25)         2609422
ruby-1.8.2(2004-12-25)         2619662
ruby-1.8.3(2005-09-21)         2625806
ruby-1.8.4(2005-12-24)         2625806
ruby-1.8.5(2006-08-25)         104018
ruby-1.8.5p52(2007-06-07)      104178
ruby-1.8.5p113(2007-09-23)     104178
ruby-1.8.5p115(2008-03-03)     104178
ruby-1.8.5p231(2008-06-20)     104178
ruby-1.8.5head(2008-06-20p231) 104178
ruby-1.8.6p0(2007-03-13)       104738
ruby-1.8.6p36(2007-06-07)      104738
ruby-1.8.6p110(2007-09-23)     104738
ruby-1.8.6p111(2007-09-24)     104738
ruby-1.8.6p114(2008-03-03)     104738
ruby-1.8.6p230(2008-06-20)     104738
ruby-1.8.6p287(2008-08-11)     104738
ruby-1.8.6head(2009-02-13p330) 104738
ruby-1.8.7p0(2008-05-31)       109218
ruby-1.8.7p17(2008-06-09)      109218
ruby-1.8.7p22(2008-06-20)      109218
ruby-1.8.7p72(2008-08-11)      109218
ruby-1.8.7head(2009-02-13p116) 109218
ruby-1.8(2009-02-05r22080)     109938
ruby-1.9.1p0(2009-01-30r21907) 117848
ruby-1.9(2009-02-14r22297)     118088

It seems ruby-1.8.7 changes the value, as well as other many
versions.

The changes are because the number of symbols used
internally, algorithm changes, etc.

Symbol#hash is fragile.  Too dangerous to depend.

In general, the algorithm of #hash methods (not only
Symbol#hash) is ruby internal.  Don't depend on that.
-- 
Tanaka Akira