cached_enumerable.rb

class CachedEnumerable
  include Enumerable

  def initialize(wrapped = nil)
    @enumerator = case wrapped
    when Enumerator
      wrapped
    when Enumerable
      wrapped.to_enum
    when NilClass
      Enumerator.new { |e| yield(e) }
    else
      raise ArgumentError, "#{self.class.name} must be initialized with an Enumerator, an Enumerable or a block, not a #{wrapped.class.name}"
    end
    @cache = []
  end

  def each(&block)
    @cache.each(&block)
    while next_item = @enumerator.next
      @cache << next_item
      block.call(next_item)
    end
  rescue StopIteration
    # nothing
  end
end