From: Artem Voroztsov <artem.voroztsov@...>
Date: 2008-03-19T03:37:07+09:00
Subject: Re: Questions on Enumerator#skip_first and Enumerable#first

I remember Matz talked about LazyArray's in  Ruby 2.0
(http://www.ruby-forum.com/topic/129792#579690),
which, i guest should solve performance issues of current drop and take.

The question is not only in :drop and :take methods, but also in :map,
:inject and etc.

I prefer method 'Enumerator#map' does not start from the beginning of
the collection.
In fact I think restarting is not possible for abstract Enumerator.

After some (crude, breaking semantics) redefinitions we can compose :take,
:drop and :map without creating intermediate arrays.

# a.rb
class Object
  def singleton_class
    class <<self; self; end
  end
end
module Enumerable
  class Enumerator
    def drop(n=1)
      n.times { self.next }
      self
    end

    def take(n=1)
      enum = Enumerator.new(self, :each)
      enum.instance_variable_set('@parent', self)
      enum.instance_variable_set('@left_elmts', n)

      enum.singleton_class.class_eval do
        def next
          @left_elmts -= 1
          if @left_elmts >= 0
            @parent.next
          else
            raise StopIteration, "Take limit reached"
          end
        end
      end
      enum
    end

    def map(&block)
      res = []
      begin
        loop {res << block[self.next] }
      rescue StopIteration=>e;
      end
      res
    end
  end
end

p( (1..1000000000).each.drop(2).take(3).map{|i| i}  )

$ time ruby_dev a.rb
[3, 4, 5]

real    0m0.017s
user    0m0.006s
sys     0m0.002s