module Gitlab
  module Metrics
    module Concern
      extend ActiveSupport::Concern
      class_methods do
        private

        def metrics_provider(type, name, docstring, options = {})
          @@_metrics_provider_mutex ||= Mutex.new

          if instance_methods(false).include?(name)
            raise ArgumentError, "metrics method #{name} already exists"
          end
          options[:base_labels] ||= {}

          args = [name.inspect, %{"#{docstring}"}, options[:base_labels].inspect]

          case type
            when :gauge
              options[:multiprocess_mode] ||= :all
              args << options[:multiprocess_mode].inspect
            when :histogram
              options[:buckets] ||= ::Prometheus::Client::Histogram::DEFAULT_BUCKETS
              args << options[:buckets].inspect
          end

          metric_fetching_code = %{Gitlab::Metrics.#{type}(#{args.join(', ')})}

          # optionally wrap in feature
          if options[:with_feature].is_a?(Symbol)
            metric_fetching_code = <<-FETCH.strip_heredoc
              if Feature.get(#{options[:with_feature].inspect}).enabled? 
                #{metric_fetching_code}
              else
                Gitlab::Metrics::NullMetric.new
              end
            FETCH
          end

          method_code, line = <<-METRIC, __LINE__ + 1
            @@_metric_provider_cached_#{name} = nil
            def #{name}
              return @@_metric_provider_cached_#{name} if @@_metric_provider_cached_#{name}

              @@_metrics_provider_mutex.synchronize do
                @@_metric_provider_cached_#{name} ||= #{metric_fetching_code}
              end
            end

            def reload_#{name}!
              @@_metric_provider_cached_#{name} = nil
            end
          METRIC

          instance_eval(method_code, __FILE__, line)
          module_eval(method_code, __FILE__, line)
        end

        # Declare a Counter
        # @param [Symbol] name
        # @param [String] docstring
        # @param [Hash] options
        def counter(name, docstring, options = {})
          metrics_provider(:counter, name, docstring, options)
        end

        # Declare a Gauge
        # @param [Symbol] name
        # @param [String] docstring
        # @param [Hash] options
        def gauge(name, docstring, options = {})
          metrics_provider(:counter, name, docstring, options)
        end

        # Declare a Histograam
        # @param [Symbol] name
        # @param [String] docstring
        # @param [Hash] options
        def histogram(name, docstring, options = {})
          metrics_provider(:histogram, name, docstring, options)
        end

        def summary(*args)
          raise NotImplementedError, "summary metrics are not currently supported"
        end
      end
    end
  end
end