module RackGraphql
  class Middleware
    def initialize(schema:, context_handler: nil)
      @schema = schema
      @context_handler = context_handler || ->(_) {}
    end

    def call(env)
      return [406, {}, []] unless post_request?(env)

      params         = post_data(env)
      variables      = ensure_hash(params['variables'])
      operation_name = params['operationName']
      context        = context_handler.call(env)

      result = execute(params: params, operation_name: operation_name, variables: variables, context: context)

      [200, response_headers(result), [MultiJson.dump(result)]]
    rescue ArgumentError
      [400, { 'Content-Type' => 'application/json' }, [MultiJson.dump({})]]
    ensure
      ActiveRecord::Base.clear_active_connections! if defined?(ActiveRecord::Base)
    end

    private

    attr_reader :schema, :context_handler

    def post_request?(env)
      env['REQUEST_METHOD'] == 'POST'
    end

    def post_data(env)
      ::MultiJson.load(env['rack.input'].gets)
    rescue MultiJson::ParseError
      {}
    end

    # Handle form data, JSON body, or a blank value
    def ensure_hash(ambiguous_param)
      case ambiguous_param
      when String
        return {} if ambiguous_param.empty?

        begin
          ensure_hash(::MultiJson.load(ambiguous_param))
        rescue MultiJson::ParseError
          raise ArgumentError, "Unexpected parameter: #{ambiguous_param}"
        end
      when Hash
        ambiguous_param
      when nil
        {}
      else
        raise ArgumentError, "Unexpected parameter: #{ambiguous_param}"
      end
    end

    def execute(params:, operation_name:, variables:, context:)
      if valid_multiplex?(params)
        execute_multi(params['_json'], operation_name: operation_name, variables: variables, context: context)
      else
        execute_single(params['query'], operation_name: operation_name, variables: variables, context: context)
      end
    end

    def execute_single(query, operation_name:, variables:, context:)
      schema.execute(query, operation_name: operation_name, variables: variables, context: context)
    end

    def valid_multiplex?(params)
      params['_json'].is_a?(Array) && params['_json'].all? { |j| j.is_a?(Hash) }
    end

    def execute_multi(queries_params, operation_name:, variables:, context:)
      queries = queries_params.map do |param|
        {
          query: param['query'],
          operation_name: operation_name,
          variables: variables,
          context: context
        }
      end

      schema.multiplex(queries)
    end

    def response_headers(result = nil)
      {
        'Access-Control-Expose-Headers' => 'X-Subscription-ID',
        'Content-Type' => 'application/json'
      }.tap do |headers|
        headers['X-Subscription-ID'] = result.context[:subscription_id] if result_subscription?(result)
      end
    end

    def result_subscription?(result)
      return false unless result.is_a?(GraphQL::Query::Result)

      result.subscription?
    end
  end
end