module CanCan

  # This module is designed to be included into an Ability class. This will
  # provide the "can" methods for defining and checking abilities.
  #
  #   class Ability
  #     include CanCan::Ability
  #
  #     def initialize(user)
  #       if user.admin?
  #         can :manage, :all
  #       else
  #         can :read, :all
  #       end
  #     end
  #   end
  #
  module Ability
    attr_accessor :user

    # Use to check the user's permission for a given action and object.
    #
    #   can? :destroy, @project
    #
    # You can also pass the class instead of an instance (if you don't have one handy).
    #
    #   can? :create, Project
    #
    # Not only can you use the can? method in the controller and view (see ControllerAdditions),
    # but you can also call it directly on an ability instance.
    #
    #   ability.can? :destroy, @project
    #
    # This makes testing a user's abilities very easy.
    #
    #   def test "user can only destroy projects which he owns"
    #     user = User.new
    #     ability = Ability.new(user)
    #     assert ability.can?(:destroy, Project.new(:user => user))
    #     assert ability.cannot?(:destroy, Project.new)
    #   end
    #
    def can?(action, noun, *can_params)
      (@can_definitions || []).reverse.each do |base_behavior, defined_action, defined_noun, defined_block|
        defined_actions = expand_actions(defined_action)
        defined_nouns = [defined_noun].flatten
        if includes_action?(defined_actions, action) && includes_noun?(defined_nouns, noun)
          result = can_perform_action?(action, noun, defined_actions, defined_nouns, defined_block, can_params)
          return base_behavior ? result : !result
        end
      end
      false
    end

    # Convenience method which works the same as "can?" but returns the opposite value.
    #
    #   cannot? :destroy, @project
    #
    def cannot?(*args)
      !can?(*args)
    end

    # Defines which abilities are allowed using two arguments. The first one is the action
    # you're setting the permission for, the second one is the class of object you're setting it on.
    #
    #   can :update, Article
    #
    # You can pass an array for either of these parameters to match any one.
    #
    #   can [:update, :destroy], [Article, Comment]
    #
    # In this case the user has the ability to update or destroy both articles and comments.
    #
    # You can pass a block to provide logic based on the article's attributes.
    #
    #   can :update, Article do |article|
    #     article && article.user == user
    #   end
    #
    # If the block returns true then the user has that :update ability for that article, otherwise he
    # will be denied access. It's possible for the passed in model to be nil if one isn't specified,
    # so be sure to take that into consideration.
    #
    # You can pass :all to reference every type of object. In this case the object type will be passed
    # into the block as well (just in case object is nil).
    #
    #   can :read, :all do |object_class, object|
    #     object_class != Order
    #   end
    #
    # Here the user has permission to read all objects except orders.
    #
    # You can also pass :manage as the action which will match any action. In this case the action is
    # passed to the block.
    #
    #   can :manage, Comment do |action, comment|
    #     action != :destroy
    #   end
    #
    # You can pass custom objects into this "can" method, this is usually done through a symbol
    # and is useful if a class isn't available to define permissions on.
    #
    #   can :read, :stats
    #   can? :read, :stats # => true
    #
    def can(action, noun, &block)
      @can_definitions ||= []
      @can_definitions << [true, action, noun, block]
    end

    # Define an ability which cannot be done. Accepts the same arguments as "can".
    #
    #   can :read, :all
    #   cannot :read, Comment
    #
    # A block can be passed just like "can", however if the logic is complex it is recommended
    # to use the "can" method.
    #
    #   cannot :read, Product do |product|
    #     product.invisible?
    #   end
    #
    def cannot(action, noun, &block)
      @can_definitions ||= []
      @can_definitions << [false, action, noun, block]
    end

    # Alias one or more actions into another one.
    #
    #   alias_action :update, :destroy, :to => :modify
    #   can :modify, Comment
    #
    # Then :modify permission will apply to both :update and :destroy requests.
    #
    #   can? :update, Comment # => true
    #   can? :destroy, Comment # => true
    #
    # This only works in one direction. Passing the aliased action into the "can?" call
    # will not work because aliases are meant to generate more generic actions.
    #
    #   alias_action :update, :destroy, :to => :modify
    #   can :update, Comment
    #   can? :modify, Comment # => false
    #
    # Unless that exact alias is used.
    #
    #   can :modify, Comment
    #   can? :modify, Comment # => true
    #
    # The following aliases are added by default for conveniently mapping common controller actions.
    #
    #   alias_action :index, :show, :to => :read
    #   alias_action :new, :to => :create
    #   alias_action :edit, :to => :update
    #
    # This way one can use params[:action] in the controller to determine the permission.
    def alias_action(*args)
      target = args.pop[:to]
      aliased_actions[target] ||= []
      aliased_actions[target] += args
    end

    # Returns a hash of aliased actions. The key is the target and the value is an array of actions aliasing the key.
    def aliased_actions
      @aliased_actions ||= default_alias_actions
    end

    # Removes previously aliased actions including the defaults.
    def clear_aliased_actions
      @aliased_actions = {}
    end

    private

    def default_alias_actions
      {
        :read => [:index, :show],
        :create => [:new],
        :update => [:edit],
      }
    end

    def expand_actions(actions)
      [actions].flatten.map do |action|
        if aliased_actions[action]
          [action, *aliased_actions[action]]
        else
          action
        end
      end.flatten
    end

    def can_perform_action?(action, noun, defined_actions, defined_nouns, defined_block, can_params)
      if defined_block.nil?
        true
      else
        block_args = []
        block_args << action if defined_actions.include?(:manage)
        block_args << (noun.class == Class ? noun : noun.class) if defined_nouns.include?(:all)
        block_args << (noun.class == Class ? nil : noun)
        block_args += can_params
        return defined_block.call(*block_args)
      end
    end

    def includes_action?(actions, action)
      actions.include?(:manage) || actions.include?(action)
    end

    def includes_noun?(nouns, noun)
      nouns.include?(:all) || nouns.include?(noun) || nouns.any? { |c| c.kind_of?(Class) && noun.kind_of?(c) }
    end
  end
end