# encoding: utf-8

module Veritas
  module SQL
    module Generator
      class Relation

        # Generates an SQL statement for a set relation
        class Set < Binary

          DIFFERENCE   = 'EXCEPT'.freeze
          INTERSECTION = 'INTERSECT'.freeze
          UNION        = 'UNION'.freeze

          # Visit a Union
          #
          # @param [Algebra::Union] union
          #
          # @return [self]
          #
          # @api private
          def visit_veritas_algebra_union(union)
            set_operation(UNION)
            set_operands(union)
            set_name
            self
          end

          # Visit an Intersection
          #
          # @param [Algebra::Intersection] intersection
          #
          # @return [self]
          #
          # @api private
          def visit_veritas_algebra_intersection(intersection)
            set_operation(INTERSECTION)
            set_operands(intersection)
            set_name
            self
          end

          # Visit an Difference
          #
          # @param [Algebra::Difference] difference
          #
          # @return [self]
          #
          # @api private
          def visit_veritas_algebra_difference(difference)
            set_operation(DIFFERENCE)
            set_operands(difference)
            set_name
            self
          end

          # Return the SQL for the set relation
          #
          # @example
          #   sql = set_relation.to_s
          #
          # @return [#to_s]
          #
          # @api public
          def to_s
            generate_sql(:to_s)
          end

          # Return the SQL suitable for an subquery
          #
          # @return [#to_s]
          #
          # @api private
          def to_subquery
            generate_sql(:to_subquery)
          end

        private

          # Generate the SQL using the supplied method
          #
          # @param [Symbol] method
          #
          # @return [#to_s]
          #
          # @api private
          def generate_sql(method)
            return EMPTY_STRING unless visited?
            "(#{@left.send(method)}) #{@operation} (#{@right.send(method)})"
          end

          # Generates an SQL statement for base relation set operands
          class Base < Relation::Base; end

        end # class Set
      end # class Relation
    end # module Generator
  end # module SQL
end # module Veritas