module ActiveRecord
  class Attribute # :nodoc:
    class << self
      def from_database(name, value, type)
        FromDatabase.new(name, value, type)
      end

      def from_user(name, value, type, original_attribute = nil)
        FromUser.new(name, value, type, original_attribute)
      end

      def with_cast_value(name, value, type)
        WithCastValue.new(name, value, type)
      end

      def null(name)
        Null.new(name)
      end

      def uninitialized(name, type)
        Uninitialized.new(name, type)
      end
    end

    attr_reader :name, :value_before_type_cast, :type

    # This method should not be called directly.
    # Use #from_database or #from_user
    def initialize(name, value_before_type_cast, type, original_attribute = nil)
      @name = name
      @value_before_type_cast = value_before_type_cast
      @type = type
      @original_attribute = original_attribute
    end

    def value
      # `defined?` is cheaper than `||=` when we get back falsy values
      @value = type_cast(value_before_type_cast) unless defined?(@value)
      @value
    end

    def original_value
      if assigned?
        original_attribute.original_value
      else
        type_cast(value_before_type_cast)
      end
    end

    def value_for_database
      type.serialize(value)
    end

    def changed?
      changed_from_assignment? || changed_in_place?
    end

    def changed_in_place?
      has_been_read? && type.changed_in_place?(original_value_for_database, value)
    end

    def forgetting_assignment
      with_value_from_database(value_for_database)
    end

    def with_value_from_user(value)
      type.assert_valid_value(value)
      self.class.from_user(name, value, type, self)
    end

    def with_value_from_database(value)
      self.class.from_database(name, value, type)
    end

    def with_cast_value(value)
      self.class.with_cast_value(name, value, type)
    end

    def with_type(type)
      self.class.new(name, value_before_type_cast, type, original_attribute)
    end

    def type_cast(*)
      raise NotImplementedError
    end

    def initialized?
      true
    end

    def came_from_user?
      false
    end

    def has_been_read?
      defined?(@value)
    end

    def ==(other)
      self.class == other.class &&
        name == other.name &&
        value_before_type_cast == other.value_before_type_cast &&
        type == other.type
    end
    alias eql? ==

    def hash
      [self.class, name, value_before_type_cast, type].hash
    end

    protected

    attr_reader :original_attribute
    alias_method :assigned?, :original_attribute

    def initialize_dup(other)
      if defined?(@value) && @value.duplicable?
        @value = @value.dup
      end
    end

    def changed_from_assignment?
      assigned? && type.changed?(original_value, value, value_before_type_cast)
    end

    def original_value_for_database
      if assigned?
        original_attribute.original_value_for_database
      else
        _original_value_for_database
      end
    end

    def _original_value_for_database
      value_for_database
    end

    class FromDatabase < Attribute # :nodoc:
      def type_cast(value)
        type.deserialize(value)
      end

      def _original_value_for_database
        value_before_type_cast
      end
    end

    class FromUser < Attribute # :nodoc:
      def type_cast(value)
        type.cast(value)
      end

      def came_from_user?
        true
      end
    end

    class WithCastValue < Attribute # :nodoc:
      def type_cast(value)
        value
      end

      def changed_in_place_from?(old_value)
        false
      end
    end

    class Null < Attribute # :nodoc:
      def initialize(name)
        super(name, nil, Type::Value.new)
      end

      def value
        nil
      end

      def with_type(type)
        self.class.with_cast_value(name, nil, type)
      end

      def with_value_from_database(value)
        raise ActiveModel::MissingAttributeError, "can't write unknown attribute `#{name}`"
      end
      alias_method :with_value_from_user, :with_value_from_database
    end

    class Uninitialized < Attribute # :nodoc:
      def initialize(name, type)
        super(name, nil, type)
      end

      def value
        if block_given?
          yield name
        end
      end

      def value_for_database
      end

      def initialized?
        false
      end
    end
    private_constant :FromDatabase, :FromUser, :Null, :Uninitialized, :WithCastValue
  end
end