# encoding: utf-8

module Veritas
  module SQL
    module Generator
      class Relation

        # Generates an SQL statement for a Binary relation
        class Binary < Relation

          JOIN       = 'NATURAL JOIN'.freeze
          PRODUCT    = 'CROSS JOIN'.freeze
          LEFT_NAME  = 'left'.freeze
          RIGHT_NAME = 'right'.freeze

          # Return the subquery for the generator and identifier
          #
          # @param [#to_subquery] generator
          #
          # @return [#to_s]
          #
          # @api private
          def self.subquery(generator, *)
            generator.kind_of?(Base) ? generator.to_subquery : super
          end

          # Visit an Join
          #
          # @param [Algebra::Join] join
          #
          # @return [self]
          #
          # @api private
          def visit_veritas_algebra_join(join)
            set_operation(JOIN)
            set_columns(join)
            set_operands(join)
            set_name
            self
          end

          # Visit an Product
          #
          # @param [Algebra::Product] product
          #
          # @return [self]
          #
          # @api private
          def visit_veritas_algebra_product(product)
            set_operation(PRODUCT)
            set_columns(product)
            set_operands(product)
            set_name
            self
          end

          # Return the SQL for the binary relation
          #
          # @example
          #   sql = binary_relation.to_s
          #
          # @return [#to_s]
          #
          # @api public
          def to_s
            generate_sql(@columns)
          end

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

        private

          # Generate the SQL using the supplied columns
          #
          # @param [String] columns
          #
          # @return [#to_s]
          #
          # @api private
          def generate_sql(columns)
            return EMPTY_STRING unless visited?
            "SELECT #{columns} FROM #{left_subquery} #{@operation} #{right_subquery}"
          end

          # Return the left subquery
          #
          # @return [#to_s]
          #
          # @api private
          def left_subquery
            self.class.subquery(@left, LEFT_NAME)
          end

          # Return the right subquery
          #
          # @return [#to_s]
          #
          # @api private
          def right_subquery
            self.class.subquery(@right, RIGHT_NAME)
          end

          # Set the operation
          #
          # @param [#to_s] operation
          #
          # @return [undefined]
          #
          # @api private
          def set_operation(operation)
            @operation = operation
          end

          # Set the columns from the relation
          #
          # @param [Relation::Operation::Binary] relation
          #
          # @return [undefined]
          #
          # @api private
          def set_columns(relation)
            @columns = columns_for(relation)
          end

          # Set the operands from the relation
          #
          # @param [Relation::Operation::Binary] relation
          #
          # @return [undefined]
          #
          # @api private
          def set_operands(relation)
            util   = self.class
            @left  = util.visit(relation.left)
            @right = util.visit(relation.right)
          end

          # Set the name using the operands' name
          #
          # @return [undefined]
          #
          # @api private
          def set_name
            @name = [ @left.name, @right.name ].uniq.join(UNDERSCORE).freeze
          end

          # Return a list of columns in a header
          #
          # @param [Veritas::Relation] relation
          #
          # @return [#to_s]
          #
          # @api private
          def columns_for(relation)
            relation.header.map { |attribute| dispatch(attribute) }.join(SEPARATOR)
          end

          # Generates an SQL statement for base relation binary operands
          class Base < Relation::Base

          private

            # Generate the SQL for this base relation
            #
            # @return [#to_s]
            #
            # @api private
            def generate_sql(*)
              visited? ? @from : EMPTY_STRING
            end

          end # class Base
        end # class Binary
      end # class Relation
    end # module Generator
  end # module SQL
end # module Veritas