Generate a powerset of a set without keeping a stack in Erlang or Ruby

Question

I would like to generate a powerset of a rather big set (about 30-50 elements) and I know that it takes 2^n to store the powerset.

Is it possible to gen

Answer 1

This uses the standard "bit array" trick for generating power sets (and it uses the fact that Ruby's Integers behave as bit arrays). But more importantly, it uses an Enumerator to generate the sets lazily.

require 'set'

module Enumerable
  def powerset
    number_of_sets = 2 ** count

    Enumerator.new {|ps|
      number_of_sets.times {|i|
        ps << Set[*reject.with_index {|_, j| i[j].zero? }]
      }
    }
  end
end

This works perfectly fine even for thousands of elements:

enum = (1..10_000).powerset
enum.next # => #
enum.next # => #
enum.next # => #
enum.next # => #
enum.next # => #
enum.next # => #
enum.next # => #
enum.next # => #
enum.next # => #
enum.next # => #
enum.next # => #
enum.next # => #
enum.next # => #
enum.next # => #
enum.next # => #
enum.next # => #
enum.next # => #
# ...

EDIT: This is based on @steenslag's solution. I totally forgot about Array#combination, since I was too focused on finding a solution that would work for any Enumerable. However, my solution requires that the Enumerable be finite anyway, and any finite Enumerable should probably be representable as an Array, so that's not much of a restriction.

module Enumerable
  def powerset
    ary = to_a

    Enumerator.new {|ps|
      ary.size.times {|n|
        ary.combination(n).each(&ps.method(:yield))
      }
    }
  end
end