From: Robert Klemme <shortcutter@...>
Date: 2008-12-02T07:19:35+09:00
Subject: Re: Multi-Index Containers

On 01.12.2008 16:53, Avdi Grimm wrote:
> Does anyone know of a multi-index container library for Ruby?  I just
> want to check before I put it on my personal TODO list.
> 
> By multi-index, I mean a container which will index its contained
> objects by more than one attribute.  E.g. something along these lines:
> 
>   mi = MultiIndex.new
>   mi.add_index(:prefix) {|val| val[0,3]}
>   mi.add_index(:size)
> 
>   mi << "foobar"
>   mi << "foobaz"
>   mi << "fuzz"
>   mi << "fuzzbizz"
> 
>   mi.by_prefix("foo") # => ["foobar", "foobaz"]
>   mi.by_prefix("fuz") # => ["fuzz", "fuzzbizz"]
>   mi.by_size(4) # => ["fuzz"]
>   mi.by_size(8) #= ["fuzzbizz"]
> 
> Yes, I know I can just uses #select on Ruby's existing containers, but
> the idea here is that a MultiIndex would index each attribute on
> insert, making lookups faster than using a #select.
> 
> So: does this exist?

I haven't seen it so far.  Basically this would mimic what a relational 
DB does in memory.

Not difficult to write though.

class Table
   def initialize(index_keys)
     @indexes =
       index_keys.inject({}) do |idx,key|
         idx[key]= Hash.new {|h,k| h[k]=[]}
         idx
       end
   end

   def add(o)
     @indexes.each do |key,idx|
       if Array === key
         idx[o.send(key)] << o
       else
         idx[key.map{|k| o.send(k)}] << o
       end
     end
     self
   end
end

Kind regards

	robert