module ActsAsList
  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 ActsAsList::InstanceMethods

        def acts_as_list_class
          ::#{self.name}
        end

        def position_column
          '#{configuration[:column]}'
        end

        #{scope_condition_method}

        before_destroy :remove_from_list_without_saving
        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
      lower = lower_item
      return unless lower
      acts_as_list_class.transaction do
        self.update_attribute(position_column, lower.send(position_column))
        lower.decrement_position
      end
    end

    # Swap positions with the next higher item, if one exists.
    def move_higher
      higher = higher_item
      return unless higher
      acts_as_list_class.transaction do
        self.update_attribute(position_column, higher.send(position_column))
        higher.increment_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

    # Removes the item from the list.
    def remove_from_list(save = true)
      if in_list?
        decrement_positions_on_lower_items
        update_attribute(position_column, nil) if save
      end
    end

    def remove_from_list_without_saving
      self.remove_from_list(false)
    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_s}", :order => "#{position_column} DESC"
      )
    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_s}", :order => "#{position_column} ASC"
      )
    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.find(:first, :conditions => conditions, :order => "#{position_column} DESC")
      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