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module ActiveRecord

  module Acts

    module Tree

      def self.included(base)

        base.extend(ClassMethods)

      end

      # Specify this +acts_as+ extension if you want to model a tree structure by providing a parent association and a children

      # association. This requires that you have a foreign key column, which by default is called +parent_id+.

      #

      #   class Category < ActiveRecord::Base

      #     acts_as_tree :order => "name"

      #   end

      #

      #   Example:

      #   root

      #    \_ child1

      #         \_ subchild1

      #         \_ subchild2

      #

      #   root      = Category.create("name" => "root")

      #   child1    = root.children.create("name" => "child1")

      #   subchild1 = child1.children.create("name" => "subchild1")

      #

      #   root.parent   # => nil

      #   child1.parent # => root

      #   root.children # => [child1]

      #   root.children.first.children.first # => subchild1

      #

      # In addition to the parent and children associations, the following instance methods are added to the class

      # after calling <tt>acts_as_tree</tt>:

      # * <tt>siblings</tt> - Returns all the children of the parent, excluding the current node (<tt>[subchild2]</tt> when called on <tt>subchild1</tt>)

      # * <tt>self_and_siblings</tt> - Returns all the children of the parent, including the current node (<tt>[subchild1, subchild2]</tt> when called on <tt>subchild1</tt>)

      # * <tt>ancestors</tt> - Returns all the ancestors of the current node (<tt>[child1, root]</tt> when called on <tt>subchild2</tt>)

      # * <tt>root</tt> - Returns the root of the current node (<tt>root</tt> when called on <tt>subchild2</tt>)

      module ClassMethods

        # Configuration options are:

        #

        # * <tt>foreign_key</tt> - specifies the column name to use for tracking of the tree (default: +parent_id+)

        # * <tt>order</tt> - makes it possible to sort the children according to this SQL snippet.

        # * <tt>counter_cache</tt> - keeps a count in a +children_count+ column if set to +true+ (default: +false+).

        def acts_as_tree(options = {})

          configuration = { :foreign_key => "parent_id", :dependent => :destroy, :order => nil, :counter_cache => nil }

          configuration.update(options) if options.is_a?(Hash)

          belongs_to :parent, :class_name => name, :foreign_key => configuration[:foreign_key], :counter_cache => configuration[:counter_cache]

          has_many :children, :class_name => name, :foreign_key => configuration[:foreign_key], :order => configuration[:order], :dependent => configuration[:dependent]

          scope :roots, where("#{configuration[:foreign_key]} IS NULL").order(configuration[:order])

          send :include, ActiveRecord::Acts::Tree::InstanceMethods

        end

      end

      module InstanceMethods

        # Returns list of ancestors, starting from parent until root.

        #

        #   subchild1.ancestors # => [child1, root]

        def ancestors

          node, nodes = self, []

          nodes << node = node.parent while node.parent

          nodes

        end

        # Returns list of descendants.

        #

        #   root.descendants # => [child1, subchild1, subchild2]

        def descendants(depth=nil)

          depth ||= 0

          result = children.dup

          unless depth == 1

            result += children.collect {|child| child.descendants(depth-1)}.flatten

          end

          result

        end

        # Returns list of descendants and a reference to the current node.

        #

        #   root.self_and_descendants # => [root, child1, subchild1, subchild2]

        def self_and_descendants(depth=nil)

          [self] + descendants(depth)

        end

        # Returns the root node of the tree.

        def root

          node = self

          node = node.parent while node.parent

          node

        end

        # Returns all siblings of the current node.

        #

        #   subchild1.siblings # => [subchild2]

        def siblings

          self_and_siblings - [self]

        end

        # Returns all siblings and a reference to the current node.

        #

        #   subchild1.self_and_siblings # => [subchild1, subchild2]

        def self_and_siblings

          parent ? parent.children : self.class.roots

        end

      end

    end

  end

end