# frozen_string_literal: true
# encoding: utf-8

module Mongoid
  class Criteria
    module Queryable

      # Contains behavior 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

        # Clear the current strategy and negating flag, used after cloning.
        #
        # @example Reset the strategies.
        #   mergeable.reset_strategies!
        #
        # @return [ Criteria ] self.
        #
        # @since 1.0.0
        def reset_strategies!
          self.strategy = nil
          self.negating = nil
          self
        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 $and/$or/$nor criteria to a copy of this selection.
        #
        # Each of the criteria can be a Hash of key/value pairs or MongoDB
        # operators (keys beginning with $), or a Selectable object
        # (which typically will be a Criteria instance).
        #
        # @api private
        #
        # @example Add the criterion.
        #   mergeable.__multi__([ 1, 2 ], "$in")
        #
        # @param [ Array<Hash | Criteria> ] criteria Multiple key/value pair
        #   matches or Criteria objects.
        # @param [ String ] operator The MongoDB operator.
        #
        # @return [ Mergeable ] The new mergeable.
        #
        # @since 1.0.0
        def __multi__(criteria, operator)
          clone.tap do |query|
            sel = query.selector
            criteria.flatten.each do |expr|
              next unless expr
              result_criteria = sel[operator] || []
              if expr.is_a?(Selectable)
                expr = expr.selector
              end
              normalized = _mongoid_expand_keys(expr)
              sel.store(operator, result_criteria.push(normalized))
            end
          end
        end

        # Combines criteria into a MongoDB selector.
        #
        # Criteria is an array of criterions which will be flattened.
        #
        # Each criterion can be:
        # - A hash
        # - A Criteria instance
        # - nil, in which case it is ignored
        #
        # @api private
        private def _mongoid_add_top_level_operation(operator, criteria)
          # Flatten the criteria. The idea is that predicates in MongoDB
          # are always hashes and are never arrays. This method additionally
          # allows Criteria instances as predicates.
          # The flattening is existing Mongoid behavior but we could possibly
          # get rid of it as applications can splat their predicates, or
          # flatten if needed.
          clone.tap do |query|
            sel = query.selector
            _mongoid_flatten_arrays(criteria).each do |criterion|
              if criterion.is_a?(Selectable)
                expr = _mongoid_expand_keys(criterion.selector)
              else
                expr = _mongoid_expand_keys(criterion)
              end
              if sel.empty?
                sel.store(operator, [expr])
              elsif sel.keys == [operator]
                sel.store(operator, sel[operator] + [expr])
              else
                operands = [sel.dup] + [expr]
                sel.clear
                sel.store(operator, operands)
              end
            end
          end
        end

        # Calling .flatten on an array which includes a Criteria instance
        # evaluates the criteria, which we do not want. Hence this method
        # explicitly only expands Array objects and Array subclasses.
        private def _mongoid_flatten_arrays(array)
          out = []
          pending = array.dup
          until pending.empty?
            item = pending.shift
            if item.nil?
              # skip
            elsif item.is_a?(Array)
              pending += item
            else
              out << item
            end
          end
          out
        end

        # Takes a criteria hash and expands Key objects into hashes containing
        # MQL corresponding to said key objects.
        #
        # Ruby does not permit multiple symbol operators. For example,
        # {:foo.gt => 1, :foo.gt => 2} is collapsed to {:foo.gt => 2} by the
        # language. Therefore this method never has to deal with multiple
        # identical operators.
        #
        # Similarly, this method should never need to expand a literal value
        # and an operator at the same time.
        #
        # @param [ Hash ] Criteria including Key instances.
        #
        # @return [ Hash ] Expanded criteria.
        private def _mongoid_expand_keys(expr)
          unless expr.is_a?(Hash)
            raise ArgumentError, 'Argument must be a Hash'
          end

          result = {}
          expr.each do |field, value|
            field.__expr_part__(value.__expand_complex__).each do |k, v|
              if result[k]
                if result[k].is_a?(Hash)
                  # Existing value is an operator.
                  # If new value is also an operator, ensure there are no
                  # conflicts and add
                  if v.is_a?(Hash)
                    # The new value is also an operator.
                    # If there are no conflicts, combine the hashes, otherwise
                    # add new conditions to top level with $and.
                    if (v.keys & result[k].keys).empty?
                      result[k].update(v)
                    else
                      raise NotImplementedError, 'Ruby does not allow same symbol operator with different values'
                      result['$and'] ||= []
                      result['$and'] << {k => v}
                    end
                  else
                    # The new value is a simple value.
                    # If there isn't an $eq operator already in the query,
                    # transform the new value into an $eq operator and add it
                    # to the existing hash. Otherwise add the new condition
                    # with $and to the top level.
                    if result[k].key?('$eq')
                      raise NotImplementedError, 'Ruby does not allow same symbol operator with different values'
                      result['$and'] ||= []
                      result['$and'] << {k => v}
                    else
                      result[k].update('$eq' => v)
                    end
                  end
                else
                  # Existing value is a simple value.
                  # If we are adding an operator, and the operator is not $eq,
                  # convert existing value into $eq and add the new operator
                  # to the same hash. Otherwise add the new condition with $and
                  # to the top level.
                  if v.is_a?(Hash) && !v.key?('$eq')
                    result[k] = {'$eq' => result[k]}.update(v)
                  else
                    raise NotImplementedError, 'Ruby does not allow same symbol operator with different values'
                    result['$and'] ||= []
                    result['$and'] << {k => v}
                  end
                end
              else
                result[k] = v
              end
            end
          end
          result
        end

        # Adds the criterion to the existing selection.
        #
        # @api private
        #
        # @example Add the criterion.
        #   mergeable.__override__([ 1, 2 ], "$in")
        #
        # @param [ Hash | Criteria ] criterion The criteria.
        # @param [ String ] operator The MongoDB operator.
        #
        # @return [ Mergeable ] The new mergeable.
        #
        # @since 1.0.0
        def __override__(criterion, operator)
          if criterion.is_a?(Selectable)
            criterion = criterion.selector
          end
          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__, {field_name: [ 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", "$gt", 10)
        #
        # @param [ String ] field The name of the field.
        # @param [ Object ] value The value.
        #
        # @return [ Object ] The serialized value.
        #
        # @since 1.0.0
        def prepare(field, operator, value)
          unless operator =~ /exists|type|size/
            value = value.__expand_complex__
            field = field.to_s
            name = aliases[field] || field
            serializer = serializers[name]
            value = serializer ? serializer.evolve(value) : value
          end
          selection = { operator => value }
          negating? ? { "$not" => selection } : selection
        end
      end
    end
  end
end