A mostly equivalent version of Array#flatten in Ruby, including checks for recursive arrays

my_array_flatten.rb

require 'set'

class MyArray < Array 
  def my_flatten(level = -1)
    level = Integer(level)
    return self.dup if level == 0

    flattened_array = self.class.new
    recursively_flatten(self, flattened_array, level)
    flattened_array
  end
  
  def my_flatten!(level = -1)
    level = Integer(level)
    return nil if level == 0

    flattened_array = self.class.new
    if recursively_flatten(self, flattened_array, level)
      self.replace flattened_array
    end
  end
  
  private
  
  # Adapted and simplified from Array#recursively_flatten of Rubinius
  # https://github.com/rubinius/rubinius/blob/b7a755c83f3dd3f0c1f5e546f0e58fb61851ea44/core/array.rb#L2089
  def recursively_flatten(array, out, max_levels = -1, memo = Set.new)
    # Strict equality since < 0 means 'infinite'
    if max_levels == 0
      out.concat(array)
      return false
    end

    max_levels -= 1
    modified = false

    id = array.object_id
    raise ArgumentError, 'tried to flatten recursive array' if memo.include?(id)

    memo.add id
    array.each do |element|
      if element.respond_to?(:to_ary)
        modified = true
        recursively_flatten(element.to_ary, out, max_levels, memo)
      else
        out << element
      end
    end
    memo.delete id

    modified
  end
end