module IterationsSugar

  def extract
    result = ActiveArray.new
    each_with_index do |item, index|
      e = yield item, index
      result << e unless e.nil?
    end
    result
  end

  def extract_map
    result = Hash.new
    each_with_index do |item, index|
      e = yield item, index
      result[item] = e unless e.nil?
    end
    result
  end

  def extract_one
    hide_from_stack = true
    each_with_index do |item, index|
      result = yield item, index
      return result if result
    end
    nil
  end

  def expand(method)
    hide_from_stack = true
    result = ActiveArray.new
    each do |item|
      result.merge item.send(method)
    end
    result
  end

  def max_value
    max = nil
    each do |item|
      value = yield item
      if not max or (value and value > max)
        max = value
      end
    end
    max
  end

end

Set.class_eval do
  include IterationsSugar
end

Array.class_eval do
  include IterationsSugar


  alias some? any?
  alias index_of find_index
  alias filter select
  alias remove delete

  def merge(other)
    if other.is_a? Array
      self.concat other
    else
      self << other
    end
  end

  def merge_uniq(other)
    other.each do |element|
      self << element unless include? element
    end
  end

  def intersects?(other)
    not (self & other).empty?
  end

  def find!(&block)
    find(&block) || error("Not found")
  end

  # fix compact! to act properly
  alias original_compact! compact!
  def compact!
    original_compact!
    self
  end

  def add?(value)
    self << value if value
  end

  def single!
    r = take_the_only
    error 'Нет элемента, а нужен' if r.nil?
    r
  end

  def take_the_only
    if count > 1
      error "Много объектов, а нужно не больше одного: #{self}"
    end
    first
  end

  def find_class(cls)
    hide_from_stack = true
    find { |item| item.is_a? cls }
  end
  alias some_is_a? find_class
  alias has_class? find_class

end