# encoding: utf-8
module Mongoid
  class Criteria
    module Queryable

      # Contains behaviour for merging existing selection with new selection.
      module Mergeable

        # @attribute [rw] strategy The name of the current strategy.
        attr_accessor :strategy

        # Instruct the next mergeable call to use intersection.
        #
        # @example Use intersection on the next call.
        #   mergeable.intersect.in(field: [ 1, 2, 3 ])
        #
        # @return [ Mergeable ] The intersect flagged mergeable.
        #
        # @since 1.0.0
        def intersect
          use(:__intersect__)
        end

        # Instruct the next mergeable call to use override.
        #
        # @example Use override on the next call.
        #   mergeable.override.in(field: [ 1, 2, 3 ])
        #
        # @return [ Mergeable ] The override flagged mergeable.
        #
        # @since 1.0.0
        def override
          use(:__override__)
        end

        # Instruct the next mergeable call to use union.
        #
        # @example Use union on the next call.
        #   mergeable.union.in(field: [ 1, 2, 3 ])
        #
        # @return [ Mergeable ] The union flagged mergeable.
        #
        # @since 1.0.0
        def union
          use(:__union__)
        end

        # Reset the stratgies to nil, used after cloning.
        #
        # @example Reset the strategies.
        #   mergeable.reset_strategies!
        #
        # @return [ nil ] nil.
        #
        # @since 1.0.0
        def reset_strategies!
          self.strategy, self.negating = nil, nil
        end

        private

        # Adds the criterion to the existing selection.
        #
        # @api private
        #
        # @example Add the criterion.
        #   mergeable.__add__({ name: 1 }, "$in")
        #
        # @param [ Hash ] criterion The criteria.
        # @param [ String ] operator The MongoDB operator.
        #
        # @return [ Mergeable ] The new mergeable.
        #
        # @since 1.0.0
        def __add__(criterion, operator)
          with_strategy(:__add__, criterion, operator)
        end

        # Adds the criterion to the existing selection.
        #
        # @api private
        #
        # @example Add the criterion.
        #   mergeable.__expanded__([ 1, 10 ], "$within", "$center")
        #
        # @param [ Hash ] criterion The criteria.
        # @param [ String ] outer The outer MongoDB operator.
        # @param [ String ] inner The inner MongoDB operator.
        #
        # @return [ Mergeable ] The new mergeable.
        #
        # @since 1.0.0
        def __expanded__(criterion, outer, inner)
          selection(criterion) do |selector, field, value|
            selector.store(field, { outer => { inner => value }})
          end
        end

        # Perform a straight merge of the criterion into the selection and let the
        # symbol overrides do all the work.
        #
        # @api private
        #
        # @example Straight merge the expanded criterion.
        #   mergeable.__merge__(location: [ 1, 10 ])
        #
        # @param [ Hash ] criterion The criteria.
        #
        # @return [ Mergeable ] The cloned object.
        #
        # @since 2.0.0
        def __merge__(criterion)
          selection(criterion) do |selector, field, value|
            selector.merge!(field.__expr_part__(value))
          end
        end

        # Adds the criterion to the existing selection.
        #
        # @api private
        #
        # @example Add the criterion.
        #   mergeable.__intersect__([ 1, 2 ], "$in")
        #
        # @param [ Hash ] criterion The criteria.
        # @param [ String ] operator The MongoDB operator.
        #
        # @return [ Mergeable ] The new mergeable.
        #
        # @since 1.0.0
        def __intersect__(criterion, operator)
          with_strategy(:__intersect__, criterion, operator)
        end

        # Adds the criterion to the existing selection.
        #
        # @api private
        #
        # @example Add the criterion.
        #   mergeable.__multi__([ 1, 2 ], "$in")
        #
        # @param [ Hash ] criterion The criteria.
        # @param [ String ] operator The MongoDB operator.
        #
        # @return [ Mergeable ] The new mergeable.
        #
        # @since 1.0.0
        def __multi__(criterion, operator)
          clone.tap do |query|
            sel = query.selector
            criterion.flatten.each do |expr|
              next unless expr
              criteria = sel[operator] || []
              normalized = expr.inject({}) do |hash, (field, value)|
                hash.merge!(field.__expr_part__(value.__expand_complex__))
                hash
              end
              sel.store(operator, criteria.push(normalized))
            end
          end
        end

        # Adds the criterion to the existing selection.
        #
        # @api private
        #
        # @example Add the criterion.
        #   mergeable.__override__([ 1, 2 ], "$in")
        #
        # @param [ Hash ] criterion The criteria.
        # @param [ String ] operator The MongoDB operator.
        #
        # @return [ Mergeable ] The new mergeable.
        #
        # @since 1.0.0
        def __override__(criterion, operator)
          selection(criterion) do |selector, field, value|
            expression = prepare(field, operator, value)
            existing = selector[field]
            if existing.respond_to?(:merge!)
              selector.store(field, existing.merge!(expression))
            else
              selector.store(field, expression)
            end
          end
        end

        # Adds the criterion to the existing selection.
        #
        # @api private
        #
        # @example Add the criterion.
        #   mergeable.__union__([ 1, 2 ], "$in")
        #
        # @param [ Hash ] criterion The criteria.
        # @param [ String ] operator The MongoDB operator.
        #
        # @return [ Mergeable ] The new mergeable.
        #
        # @since 1.0.0
        def __union__(criterion, operator)
          with_strategy(:__union__, criterion, operator)
        end

        # Use the named strategy for the next operation.
        #
        # @api private
        #
        # @example Use intersection.
        #   mergeable.use(:__intersect__)
        #
        # @param [ Symbol ] strategy The strategy to use.
        #
        # @return [ Mergeable ] The existing mergeable.
        #
        # @since 1.0.0
        def use(strategy)
          tap do |mergeable|
            mergeable.strategy = strategy
          end
        end

        # Add criterion to the selection with the named strategy.
        #
        # @api private
        #
        # @example Add criterion with a strategy.
        #   mergeable.with_strategy(:__union__, [ 1, 2, 3 ], "$in")
        #
        # @param [ Symbol ] strategy The name of the strategy method.
        # @param [ Object ] criterion The criterion to add.
        # @param [ String ] operator The MongoDB operator.
        #
        # @return [ Mergeable ] The cloned query.
        #
        # @since 1.0.0
        def with_strategy(strategy, criterion, operator)
          selection(criterion) do |selector, field, value|
            selector.store(
              field,
              selector[field].send(strategy, prepare(field, operator, value))
            )
          end
        end

        # Prepare the value for merging.
        #
        # @api private
        #
        # @example Prepare the value.
        #   mergeable.prepare("field", "$gt", 10)
        #
        # @param [ String ] field The name of the field.
        # @param [ Object ] value The value.
        #
        # @return [ Object ] The serialized value.
        #
        # @since 1.0.0
        def prepare(field, operator, value)
          unless operator =~ /exists|type|size/
            value = value.__expand_complex__
            serializer = serializers[field]
            value = serializer ? serializer.evolve(value) : value
          end
          selection = { operator => value }
          negating? ? { "$not" => selection } : selection
        end
      end
    end
  end
end