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
