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module ActiveRecord

  module Acts #:nodoc:

    module List #:nodoc:

      def self.included(base)

        base.extend(ClassMethods)

      end

      # This +acts_as+ extension provides the capabilities for sorting and reordering a number of objects in a list.

      # The class that has this specified needs to have a +position+ column defined as an integer on

      # the mapped database table.

      #

      # Todo list example:

      #

      #   class TodoList < ActiveRecord::Base

      #     has_many :todo_items, :order => "position"

      #   end

      #

      #   class TodoItem < ActiveRecord::Base

      #     belongs_to :todo_list

      #     acts_as_list :scope => :todo_list

      #   end

      #

      #   todo_list.first.move_to_bottom

      #   todo_list.last.move_higher

      module ClassMethods

        # Configuration options are:

        #

        # * +column+ - specifies the column name to use for keeping the position integer (default: +position+)

        # * +scope+ - restricts what is to be considered a list. Given a symbol, it'll attach <tt>_id</tt> 

        #   (if it hasn't already been added) and use that as the foreign key restriction. It's also possible 

        #   to give it an entire string that is interpolated if you need a tighter scope than just a foreign key.

        #   Example: <tt>acts_as_list :scope => 'todo_list_id = #{todo_list_id} AND completed = 0'</tt>

        def acts_as_list(options = {})

          configuration = { :column => "position", :scope => "1 = 1" }

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

          configuration[:scope] = "#{configuration[:scope]}_id".intern if configuration[:scope].is_a?(Symbol) && configuration[:scope].to_s !~ /_id$/

          if configuration[:scope].is_a?(Symbol)

            scope_condition_method = %(

              def scope_condition

                if #{configuration[:scope].to_s}.nil?

                  "#{configuration[:scope].to_s} IS NULL"

                else

                  "#{configuration[:scope].to_s} = \#{#{configuration[:scope].to_s}}"

                end

              end

            )

          else

            scope_condition_method = "def scope_condition() \"#{configuration[:scope]}\" end"

          end

          class_eval <<-EOV

            include ActiveRecord::Acts::List::InstanceMethods

            def acts_as_list_class

              ::#{self.name}

            end

            def position_column

              '#{configuration[:column]}'

            end

            #{scope_condition_method}

            before_destroy :remove_from_list

            before_create  :add_to_list_bottom

          EOV

        end

      end

      # All the methods available to a record that has had <tt>acts_as_list</tt> specified. Each method works

      # by assuming the object to be the item in the list, so <tt>chapter.move_lower</tt> would move that chapter

      # lower in the list of all chapters. Likewise, <tt>chapter.first?</tt> would return +true+ if that chapter is

      # the first in the list of all chapters.

      module InstanceMethods

        # Insert the item at the given position (defaults to the top position of 1).

        def insert_at(position = 1)

          insert_at_position(position)

        end

        # Swap positions with the next lower item, if one exists.

        def move_lower

          return unless lower_item

          acts_as_list_class.transaction do

            lower_item.decrement_position

            increment_position

          end

        end

        # Swap positions with the next higher item, if one exists.

        def move_higher

          return unless higher_item

          acts_as_list_class.transaction do

            higher_item.increment_position

            decrement_position

          end

        end

        # Move to the bottom of the list. If the item is already in the list, the items below it have their

        # position adjusted accordingly.

        def move_to_bottom

          return unless in_list?

          acts_as_list_class.transaction do

            decrement_positions_on_lower_items

            assume_bottom_position

          end

        end

        # Move to the top of the list. If the item is already in the list, the items above it have their

        # position adjusted accordingly.

        def move_to_top

          return unless in_list?

          acts_as_list_class.transaction do

            increment_positions_on_higher_items

            assume_top_position

          end

        end

        # Move to the given position

        def move_to=(pos)

          case pos.to_s

          when 'highest'

            move_to_top

          when 'higher'

            move_higher

          when 'lower'

            move_lower

          when 'lowest'

            move_to_bottom

          end

          reset_positions_in_list

        end

        def reset_positions_in_list

          acts_as_list_class.where(scope_condition).reorder("#{position_column} ASC, id ASC").each_with_index do |item, i|

            unless item.send(position_column) == (i + 1)

              acts_as_list_class.update_all({position_column => (i + 1)}, {:id => item.id})

            end

          end

        end

        # Removes the item from the list.

        def remove_from_list

          if in_list?

            decrement_positions_on_lower_items

            update_attribute position_column, nil

          end

        end

        # Increase the position of this item without adjusting the rest of the list.

        def increment_position

          return unless in_list?

          update_attribute position_column, self.send(position_column).to_i + 1

        end

        # Decrease the position of this item without adjusting the rest of the list.

        def decrement_position

          return unless in_list?

          update_attribute position_column, self.send(position_column).to_i - 1

        end

        # Return +true+ if this object is the first in the list.

        def first?

          return false unless in_list?

          self.send(position_column) == 1

        end

        # Return +true+ if this object is the last in the list.

        def last?

          return false unless in_list?

          self.send(position_column) == bottom_position_in_list

        end

        # Return the next higher item in the list.

        def higher_item

          return nil unless in_list?

          acts_as_list_class.find(:first, :conditions =>

            "#{scope_condition} AND #{position_column} = #{(send(position_column).to_i - 1).to_s}"

          )

        end

        # Return the next lower item in the list.

        def lower_item

          return nil unless in_list?

          acts_as_list_class.find(:first, :conditions =>

            "#{scope_condition} AND #{position_column} = #{(send(position_column).to_i + 1).to_s}"

          )

        end

        # Test if this record is in a list

        def in_list?

          !send(position_column).nil?

        end

        private

          def add_to_list_top

            increment_positions_on_all_items

          end

          def add_to_list_bottom

            self[position_column] = bottom_position_in_list.to_i + 1

          end

          # Overwrite this method to define the scope of the list changes

          def scope_condition() "1" end

          # Returns the bottom position number in the list.

          #   bottom_position_in_list    # => 2

          def bottom_position_in_list(except = nil)

            item = bottom_item(except)

            item ? item.send(position_column) : 0

          end

          # Returns the bottom item

          def bottom_item(except = nil)

            conditions = scope_condition

            conditions = "#{conditions} AND #{self.class.primary_key} != #{except.id}" if except

            acts_as_list_class.where(conditions).reorder("#{position_column} DESC").first

          end

          # Forces item to assume the bottom position in the list.

          def assume_bottom_position

            update_attribute(position_column, bottom_position_in_list(self).to_i + 1)

          end

          # Forces item to assume the top position in the list.

          def assume_top_position

            update_attribute(position_column, 1)

          end

          # This has the effect of moving all the higher items up one.

          def decrement_positions_on_higher_items(position)

            acts_as_list_class.update_all(

              "#{position_column} = (#{position_column} - 1)", "#{scope_condition} AND #{position_column} <= #{position}"

            )

          end

          # This has the effect of moving all the lower items up one.

          def decrement_positions_on_lower_items

            return unless in_list?

            acts_as_list_class.update_all(

              "#{position_column} = (#{position_column} - 1)", "#{scope_condition} AND #{position_column} > #{send(position_column).to_i}"

            )

          end

          # This has the effect of moving all the higher items down one.

          def increment_positions_on_higher_items

            return unless in_list?

            acts_as_list_class.update_all(

              "#{position_column} = (#{position_column} + 1)", "#{scope_condition} AND #{position_column} < #{send(position_column).to_i}"

            )

          end

          # This has the effect of moving all the lower items down one.

          def increment_positions_on_lower_items(position)

            acts_as_list_class.update_all(

              "#{position_column} = (#{position_column} + 1)", "#{scope_condition} AND #{position_column} >= #{position}"

           )

          end

          # Increments position (<tt>position_column</tt>) of all items in the list.

          def increment_positions_on_all_items

            acts_as_list_class.update_all(

              "#{position_column} = (#{position_column} + 1)",  "#{scope_condition}"

            )

          end

          def insert_at_position(position)

            remove_from_list

            increment_positions_on_lower_items(position)

            self.update_attribute(position_column, position)

          end

      end 

    end

  end

end