module ActiveRecord
  # Declare an enum attribute where the values map to integers in the database, but can be queried by name. Example:
  #
  #   class Conversation < ActiveRecord::Base
  #     enum status: %i( active archived )
  #   end
  #
  #   Conversation::STATUS # => { active: 0, archived: 1 }
  #
  #   # conversation.update! status: 0
  #   conversation.active!
  #   conversation.active? # => true
  #   conversation.status  # => :active
  #   
  #   # conversation.update! status: 1
  #   conversation.archived!
  #   conversation.archived? # => true
  #   conversation.status    # => :archived
  #   
  #   # conversation.update! status: 1
  #   conversation.status = :archived
  #
  # You can set the default value from the database declaration, like:
  #
  #   create_table :conversation do
  #     t.column :status, :integer, default: 0
  #   end
  # 
  # Good practice is to let the first declared status be the default.
  module Enum
    def enum(definitions)
      definitions.each do |name, values|
        const_name = name.to_s.upcase

        # DIRECTION = { }
        const_set const_name, {}

        # def direction=(value) self[:direction] = DIRECTION[value] end
        class_eval "def #{name}=(value) self[:#{name}] = #{const_name}[value] end"

        # def direction() DIRECTION.key self[:direction] end
        class_eval "def #{name}() #{const_name}.key self[:#{name}] end"

        values.each_with_index do |value, i|
          # DIRECTION[:incoming] = 0
          const_get(const_name)[value] = i

          # scope :incoming, -> { where direction: 0 }
          scope value, -> { where name => i }

          # def incoming?() direction == 0 end
          class_eval "def #{value}?() self[:#{name}] == #{i} end"

          # def incoming! update! direction: :incoming end
          class_eval "def #{value}!() update! #{name}: :#{value} end"
        end
      end
    end
  end
end