From: Linus Sellberg <linse428@...>
Date: 2004-05-14T18:13:59+09:00
Subject: Re: How to make combinations of an array to produce all possible expressions?

> module Enumerable
>   def each_subset(skip_empty = false)
>     enum = respond_to?( :size ) ? self : to_a
> 
>     for n in (skip_empty ? 1 : 0) ... (1 << enum.size) do
>       subset = []
> 
>       enum.each_with_index do |elem, i|
>         subset << elem if n[i] == 1
>       end
> 
>       yield subset
>     end
> 
>     self
>   end
> 
>   def powerset(skip_empty = false)
>     subsets = []
> 
>     each_subset(skip_empty) { |s| subsets << s }
> 
>     return subsets
>   end

irb(main):013:0> def powerset( x )
irb(main):014:1> x.inject([[]]) {|m, n| m.map {|b| [n] + b} + m }
irb(main):015:1> end
irb(main):017:0> a = [1,2,3]
=> [1, 2, 3]
irb(main):018:0> powerset a
=> [[3, 2, 1], [3, 2], [3, 1], [3], [2, 1], [2], [1], []]
irb(main):019:0>

Only works for arrays, but it might be possible to generalize it?