# frozen_string_literal: true

module Gitlab
  module Cache
    module Helpers
      # @return [ActiveSupport::Duration]
      DEFAULT_EXPIRY = 1.day

      # @return [ActiveSupport::Cache::Store]
      def cache
        Rails.cache
      end

      def render_cached(obj_or_collection, with:, cache_context: -> (_) { current_user&.cache_key }, expires_in: Gitlab::Cache::Helpers::DEFAULT_EXPIRY, **presenter_args)
        json =
          if obj_or_collection.is_a?(Enumerable)
            cached_collection(
              obj_or_collection,
              presenter: with,
              presenter_args: presenter_args,
              context: cache_context,
              expires_in: expires_in
            )
          else
            cached_object(
              obj_or_collection,
              presenter: with,
              presenter_args: presenter_args,
              context: cache_context,
              expires_in: expires_in
            )
          end

        render Gitlab::Json::PrecompiledJson.new(json)
      end

      private

      # Optionally uses a `Proc` to add context to a cache key
      #
      # @param object [Object] must respond to #cache_key
      # @param context [Proc] a proc that will be called with the object as an argument, and which should return a
      #                       string or array of strings to be combined into the cache key
      # @return [String]
      def contextual_cache_key(presenter, object, context)
        return object.cache_key if context.nil?

        [presenter.class.name, object.cache_key, context.call(object)].flatten.join(":")
      end

      # Used for fetching or rendering a single object
      #
      # @param object [Object] the object to render
      # @param presenter [Grape::Entity]
      # @param presenter_args [Hash] keyword arguments to be passed to the entity
      # @param context [Proc]
      # @param expires_in [ActiveSupport::Duration, Integer] an expiry time for the cache entry
      # @return [String]
      def cached_object(object, presenter:, presenter_args:, context:, expires_in:)
        misses = 0

        json = cache.fetch(contextual_cache_key(presenter, object, context), expires_in: expires_in) do
          time_action(render_type: :object) do
            misses += 1

            Gitlab::Json.dump(presenter.represent(object, **presenter_args).as_json)
          end
        end

        increment_cache_metric(render_type: :object, total_count: 1, miss_count: misses)

        json
      end

      # Used for fetching or rendering multiple objects
      #
      # @param objects [Enumerable<Object>] the objects to render
      # @param presenter [Grape::Entity]
      # @param presenter_args [Hash] keyword arguments to be passed to the entity
      # @param context [Proc]
      # @param expires_in [ActiveSupport::Duration, Integer] an expiry time for the cache entry
      # @return [Array<String>]
      def cached_collection(collection, presenter:, presenter_args:, context:, expires_in:)
        misses = 0

        json = fetch_multi(presenter, collection, context: context, expires_in: expires_in) do |obj|
          time_action(render_type: :collection) do
            misses += 1

            Gitlab::Json.dump(presenter.represent(obj, **presenter_args).as_json)
          end
        end

        increment_cache_metric(render_type: :collection, total_count: collection.length, miss_count: misses)

        json.values
      end

      # An adapted version of ActiveSupport::Cache::Store#fetch_multi.
      #
      # The original method only provides the missing key to the block,
      # not the missing object, so we have to create a map of cache keys
      # to the objects to allow us to pass the object to the missing value
      # block.
      #
      # The result is that this is functionally identical to `#fetch`.
      def fetch_multi(presenter, *objs, context:, **kwargs)
        objs.flatten!
        map = multi_key_map(presenter, objs, context: context)

        # TODO: `contextual_cache_key` should be constructed based on the guideline https://docs.gitlab.com/ee/development/redis.html#multi-key-commands.
        Gitlab::Instrumentation::RedisClusterValidator.allow_cross_slot_commands do
          cache.fetch_multi(*map.keys, **kwargs) do |key|
            yield map[key]
          end
        end
      end

      # @param objects [Enumerable<Object>] objects which _must_ respond to `#cache_key`
      # @param context [Proc] a proc that can be called to help generate each cache key
      # @return [Hash]
      def multi_key_map(presenter, objects, context:)
        objects.index_by do |object|
          contextual_cache_key(presenter, object, context)
        end
      end

      def increment_cache_metric(render_type:, total_count:, miss_count:)
        return unless caller_id

        metric_name = :cached_object_operations_total
        hit_count = total_count - miss_count

        current_transaction&.increment(
          metric_name,
          hit_count,
          { caller_id: caller_id, render_type: render_type, cache_hit: true }
        )

        current_transaction&.increment(
          metric_name,
          miss_count,
          { caller_id: caller_id, render_type: render_type, cache_hit: false }
        )
      end

      def time_action(render_type:, &block)
        real_start = Gitlab::Metrics::System.monotonic_time

        presented_object = yield

        real_duration_histogram(render_type).observe({}, Gitlab::Metrics::System.monotonic_time - real_start)

        presented_object
      end

      def real_duration_histogram(render_type)
        Gitlab::Metrics.histogram(
          :gitlab_presentable_object_cacheless_render_real_duration_seconds,
          'Duration of generating presentable objects to be cached in real time',
          { caller_id: caller_id, render_type: render_type },
          [0.1, 0.5, 1, 2]
        )
      end

      def current_transaction
        @current_transaction ||= ::Gitlab::Metrics::WebTransaction.current
      end

      def caller_id
        @caller_id ||= Gitlab::ApplicationContext.current_context_attribute(:caller_id)
      end
    end
  end
end