module Virtus

  # Extensions common for both classes and instances
  module Extensions
    WRITER_METHOD_REGEXP   = /=\z/.freeze
    INVALID_WRITER_METHODS = %w[ == != === []= attributes= ].to_set.freeze

    # A hook called when an object is extended with Virtus
    #
    # @param [Object] object
    #
    # @return [undefined]
    #
    # @api private
    def self.extended(object)
      super
      object.instance_eval do
        extend InstanceMethods
        extend attribute_set
      end
    end
    private_class_method :extended

    # Defines an attribute on an object's class
    #
    # @example
    #    class Book
    #      include Virtus
    #
    #      attribute :title,        String
    #      attribute :author,       String
    #      attribute :published_at, DateTime
    #      attribute :page_count,   Integer
    #      attribute :index                   # defaults to Object
    #    end
    #
    # @param [Symbol] name
    #   the name of an attribute
    #
    # @param [Class] type
    #   the type class of an attribute
    #
    # @param [#to_hash] options
    #   the extra options hash
    #
    # @return [self]
    #
    # @see Attribute.build
    #
    # @api public
    def attribute(name, type, options = {})
      attribute = Attribute.build(name, type, merge_options(options))
      virtus_add_attribute(attribute)
      self
    end

    # The list of writer methods that can be mass-assigned to in #attributes=
    #
    # @return [Set]
    #
    # @api private
    def allowed_writer_methods
      @allowed_writer_methods ||=
        begin
          allowed_writer_methods  = allowed_methods.grep(WRITER_METHOD_REGEXP).to_set
          allowed_writer_methods -= INVALID_WRITER_METHODS
          allowed_writer_methods.freeze
        end
    end

  private

    # Return an attribute set for that instance
    #
    # @return [AttributeSet]
    #
    # @api private
    def attribute_set
      @attribute_set ||= AttributeSet.new
    end

    # Add an attribute to the attribute set
    #
    # @return [AttributeSet]
    #
    # @api private
    def virtus_add_attribute(attribute)
      attribute_set << attribute
    end

    # Merge default options
    #
    # @return [Hash]
    #
    # @api private
    def merge_options(options)
      { :coerce => Virtus.coerce }.merge(options)
    end

  end # module Extensions
end # module Virtus