Clone an Enumerator in Ruby?

Question

I have a tree that I'm trying to traverse. As I traverse it, I keep a stack of enumerators in which each enumerator is used to enumerate over a tree's children.

I'd like to be able to duplicate this stack of enumerators and hand it to another object so it may traverse the tree starting in the place indicated by the state of the stack.

When I attempt to call #dup on Enumerator, I get an error. Is it possible to duplicate an Enumerator? If not, how could I accomplish the same thing? (I've considered a stack of integers as indices, but am worried about the efficiency.

Here's some code to show what I'm seeing...

Once the first enumerator has started, you cannot duplicate it. That is my situation.

a = [1,2,3].each
 => #<Enumerator: [1, 2, 3]:each> 
a.next
 => 1 
b = a.dup
TypeError: can't copy execution context
    from (irb):3:in `initialize_copy'
    from (irb):3:in `initialize_dup'
    from (irb):3:in `dup'
    from (irb):3

Answer 1

Implement your own enumerator class.

There’s not much magic to an enumerator beyond incrementing an internal counter.

class MyArrayEnumerator
  def initialize(array)
    @ary,@n=array,0
  end
  def next
    raise StopIteration if @n == @ary.length
    a=@ary[@n];@n+=1;a
  end
end

class Array
  def my_each
    MyArrayEnumerator.new(self)
  end
end

a = [1,2,3].my_each # => #<MyArrayEnumerator:0x101c96588 @n=0, @array=[1, 2, 3]>
a.next # => 1
b = a.dup # => #<MyArrayEnumerator:0x101c95ae8 @n=1, @array=[1, 2, 3]>
a.next # => 2
b.next # => 2