module Axiom
  module Adapter
    module Arango
      # Base class for relation specific visitors
      class Visitor
        include AbstractType, Adamantium::Flat, Concord.new(:input, :context), AQL

        abstract_method :root

        # Return an AQL node from the relation
        #
        # @param [Relation] relation
        #
        # @return [AQL::Node]
        #
        # @api private
        #
        def self.run(relation)
          build(relation).root
        end

        REGISTRY = {}

        # Register handler for the axiom relation
        #
        # @param [Class] klass
        #
        # @return [undefined]
        #
        # @api private
        #
        def self.handle(klass)
          REGISTRY[klass]=self
        end

        private_class_method :handle

        # Return visitor for relation and context
        #
        # @param [Relation] relation
        # @param [Visitor] context
        #
        # @return [Visitor]
        #
        # @api private
        #
        def self.build(relation, context = nil)
          REGISTRY.fetch(relation.class).new(relation, context)
        end

      protected

        # Return depth
        #
        # @return [Integer]
        #
        # @api private
        #
        def depth
          context = self.context
          if context
            context.depth + 1
          else
            0
          end
        end
        memoize :depth

      private

        # Return a visitor for the given relation
        #
        # @param [Relation] relation
        # @param [Visitor] context
        #
        # @return [Visitor]
        #
        # @api private
        #
        def visitor(relation, context = self)
          self.class.build(relation, context)
        end

        # Return an AQL node from visiting axiom a given relation
        #
        # @param [Relation] relation
        # @param [Visitor] context
        #
        # @return [AQL::Node]
        #
        # @api private
        #
        def visit(relation, context = self)
          visitor(relation, context).root
        end

      end
    end
  end
end