module GraphQL
  class Query
    class SerialExecution
      class SelectionResolution
        attr_reader :target, :type, :selections, :execution_context

        def initialize(target, type, selections, execution_context)
          @target = target
          @type = type
          @selections = selections
          @execution_context = execution_context
        end

        def result
          flatten_and_merge_selections(selections)
            .values
            .reduce({}) { |result, ast_node|
              result.merge(resolve_field(ast_node))
            }
        rescue GraphQL::InvalidNullError => err
          err.parent_error? || execution_context.add_error(err)
          nil
        end

        private

        def flatten_selection(ast_node)
          strategy_method = STRATEGIES[ast_node.class]
          send(strategy_method, ast_node)
        end

        STRATEGIES = {
          GraphQL::Language::Nodes::Field => :flatten_field,
          GraphQL::Language::Nodes::InlineFragment => :flatten_inline_fragment,
          GraphQL::Language::Nodes::FragmentSpread => :flatten_fragment_spread,
        }

        def flatten_field(ast_node)
          result_name = ast_node.alias || ast_node.name
          { result_name => ast_node }
        end

        def flatten_inline_fragment(ast_node)
          return {} unless GraphQL::Query::DirectiveResolution.include_node?(ast_node, execution_context.query)
          flatten_fragment(ast_node)
        end

        def flatten_fragment_spread(ast_node)
          return {} unless GraphQL::Query::DirectiveResolution.include_node?(ast_node, execution_context.query)
          ast_fragment_defn = execution_context.get_fragment(ast_node.name)
          flatten_fragment(ast_fragment_defn)
        end

        def flatten_fragment(ast_fragment)
          if fragment_type_can_apply?(ast_fragment)
            flatten_and_merge_selections(ast_fragment.selections)
          else
            {}
          end
        end

        def fragment_type_can_apply?(ast_fragment)
          if ast_fragment.type.nil?
            true
          else
            child_type = execution_context.get_type(ast_fragment.type)
            resolved_type = GraphQL::Query::TypeResolver.new(target, child_type, type, execution_context.query.context).type
            !resolved_type.nil?
          end
        end

        def merge_fields(field1, field2)
          field_type = execution_context.get_field(type, field2.name).type.unwrap

          if field_type.kind.fields?
            # create a new ast field node merging selections from each field.
            # Because of static validation, we can assume that name, alias,
            # arguments, and directives are exactly the same for fields 1 and 2.
            GraphQL::Language::Nodes::Field.new(
              name: field2.name,
              alias: field2.alias,
              arguments: field2.arguments,
              directives: field2.directives,
              selections: field1.selections + field2.selections
            )
          else
            field2
          end
        end

        def resolve_field(ast_node)
          return {} unless GraphQL::Query::DirectiveResolution.include_node?(ast_node, execution_context.query)
          execution_context.strategy.field_resolution.new(
            ast_node,
            type,
            target,
            execution_context
          ).result
        end

        def merge_into_result(memo, selection)
          name = if selection.respond_to?(:alias)
            selection.alias || selection.name
          else
            selection.name
          end

          memo[name] = if memo.key?(name)
            merge_fields(memo[name], selection)
          else
            selection
          end
        end

        def flatten_and_merge_selections(selections)
          selections.reduce({}) do |result, ast_node|
            flattened_selections = flatten_selection(ast_node)
            flattened_selections.each do |name, selection|
              merge_into_result(result, selection)
            end
            result
          end
        end
      end
    end
  end
end