From: Christian Szegedy <szegedy@...>
Date: 2002-09-25T18:42:31+09:00
Subject: Re: constructing closures on the fly

Bulat Ziganshin wrote:
> Hello ruby-talk,
> 
>   ok, that's my problem: i had large list of filenames (100k strings,
>   for example) and need to quickly sort them based on constructed
>   criterion. This criterion gived by user in form of string containing
>   letters 'ensgi3', where each letter maps to expression, parameterized
>   by filename, so:
> 
>   filenames.sort_files "en"
>   must be equal to
>   filenames.sort_by {|name| [ext(name), name(name)]}
> 
>   filenames.sort_files "ges"
>   must be equal to
>   filenames.sort_by {|name| [group(name), ext(name), size(name)]}
> 
>   and so on. Currently i use non-functional approach:
> 
>   def sort_files(crit)
>     sort_by {|name| construct(crit,name)}
>   end
> 
>   def construct(crit,name)
>     arr = []
>     crit.each_byte {|c|
>       case c
>       when ?g then arr << group(name)
>       ....
>       end
>     }
>     arr
>   end
> 
>   while faster approach will be:
> 
>   def sort_files(crit)
>     sort_by convert_crit(crit)
>   end

I think that your approach is quite OK.

The only way to create the closure on fly (and
omit the iteration in it) is by eval (not tested):

def apply_crit
   a=[]
   crit.each_byte { |b| a << {?g => group,...}[b] }
   a.join(",")
end

eval "sort_by {|n| [#{apply_crit}] }"

Of course this is not a very good style, but may be
somewhat faster.

Regards, Christian