require "concurrent/executor/thread_pool_executor"
require "concurrent/executor/timer_set"
require "concurrent/scheduled_task"
require "concurrent/utility/processor_counter"

module GoodJob # :nodoc:
  #
  # Schedulers are generic thread pools that are responsible for
  # periodically checking for available tasks, executing tasks within a thread,
  # and efficiently scaling active threads.
  #
  # Every scheduler has a single {JobPerformer} that will execute tasks.
  # The scheduler is responsible for calling its performer efficiently across threads managed by an instance of +Concurrent::ThreadPoolExecutor+.
  # If a performer does not have work, the thread will go to sleep.
  # The scheduler maintains an instance of +Concurrent::TimerTask+, which wakes sleeping threads and causes them to check whether the performer has new work.
  #
  class Scheduler
    # Defaults for instance of Concurrent::ThreadPoolExecutor
    # The thread pool is where work is performed.
    DEFAULT_POOL_OPTIONS = {
      name: name,
      min_threads: 0,
      max_threads: Configuration::DEFAULT_MAX_THREADS,
      auto_terminate: true,
      idletime: 60,
      max_queue: Configuration::DEFAULT_MAX_THREADS,
      fallback_policy: :discard,
    }.freeze

    # @!attribute [r] instances
    #   @!scope class
    #   List of all instantiated Schedulers in the current process.
    #   @return [array<GoodJob:Scheduler>]
    cattr_reader :instances, default: [], instance_reader: false

    # Creates GoodJob::Scheduler(s) and Performers from a GoodJob::Configuration instance.
    # @param configuration [GoodJob::Configuration]
    # @param warm_cache_on_initialize [Boolean]
    # @return [GoodJob::Scheduler, GoodJob::MultiScheduler]
    def self.from_configuration(configuration, warm_cache_on_initialize: true)
      schedulers = configuration.queue_string.split(';').map do |queue_string_and_max_threads|
        queue_string, max_threads = queue_string_and_max_threads.split(':')
        max_threads = (max_threads || configuration.max_threads).to_i

        job_performer = GoodJob::JobPerformer.new(queue_string)
        GoodJob::Scheduler.new(
          job_performer,
          max_threads: max_threads,
          max_cache: configuration.max_cache,
          warm_cache_on_initialize: warm_cache_on_initialize
        )
      end

      if schedulers.size > 1
        GoodJob::MultiScheduler.new(schedulers)
      else
        schedulers.first
      end
    end

    # @param performer [GoodJob::JobPerformer]
    # @param max_threads [Numeric, nil] number of seconds between polls for jobs
    # @param max_cache [Numeric, nil] maximum number of scheduled jobs to cache in memory
    # @param warm_cache_on_initialize [Boolean] whether to warm the cache immediately
    def initialize(performer, max_threads: nil, max_cache: nil, warm_cache_on_initialize: true)
      raise ArgumentError, "Performer argument must implement #next" unless performer.respond_to?(:next)

      self.class.instances << self

      @performer = performer

      @max_cache = max_cache || 0
      @pool_options = DEFAULT_POOL_OPTIONS.dup
      if max_threads.present?
        @pool_options[:max_threads] = max_threads
        @pool_options[:max_queue] = max_threads
      end
      @pool_options[:name] = "GoodJob::Scheduler(queues=#{@performer.name} max_threads=#{@pool_options[:max_threads]})"

      create_pool
      warm_cache if warm_cache_on_initialize
    end

    # Shut down the scheduler.
    # This stops all threads in the pool.
    # If +wait+ is +true+, the scheduler will wait for any active tasks to finish.
    # If +wait+ is +false+, this method will return immediately even though threads may still be running.
    # Use {#shutdown?} to determine whether threads have stopped.
    # @param wait [Boolean] Wait for actively executing jobs to finish
    # @return [void]
    def shutdown(wait: true)
      return unless @pool&.running?

      instrument("scheduler_shutdown_start", { wait: wait })
      instrument("scheduler_shutdown", { wait: wait }) do
        @timer_set.shutdown

        @pool.shutdown
        @pool.wait_for_termination if wait
        # TODO: Should be killed if wait is not true
      end
    end

    # Tests whether the scheduler is shutdown.
    # @return [true, false, nil]
    def shutdown?
      !@pool&.running?
    end

    # Restart the Scheduler.
    # When shutdown, start; or shutdown and start.
    # @param wait [Boolean] Wait for actively executing jobs to finish
    # @return [void]
    def restart(wait: true)
      instrument("scheduler_restart_pools") do
        shutdown(wait: wait) unless shutdown?
        create_pool
        warm_cache
      end
    end

    # Wakes a thread to allow the performer to execute a task.
    # @param state [nil, Object] Contextual information for the performer. See {Performer#next?}.
    # @return [nil, Boolean] Whether work was started.
    #   Returns +nil+ if the scheduler is unable to take new work, for example if the thread pool is shut down or at capacity.
    #   Returns +true+ if the performer started executing work.
    #   Returns +false+ if the performer decides not to attempt to execute a task based on the +state+ that is passed to it.
    def create_thread(state = nil)
      return nil unless @pool.running?

      if state
        return false unless @performer.next?(state)

        if state[:scheduled_at]
          scheduled_at = if state[:scheduled_at].is_a? String
                           Time.zone.parse state[:scheduled_at]
                         else
                           state[:scheduled_at]
                         end
          delay = [(scheduled_at - Time.current).to_f, 0].max
        end
      end

      delay ||= 0
      run_now = delay <= 0.01
      if run_now
        return nil unless @pool.ready_worker_count.positive?
      elsif @max_cache.positive?
        return nil unless remaining_cache_count.positive?
      end

      future = Concurrent::ScheduledTask.new(delay, args: [@performer], executor: @pool, timer_set: timer_set) do |performer|
        output = nil
        Rails.application.executor.wrap { output = performer.next }
        output
      end
      future.add_observer(self, :task_observer)
      future.execute

      run_now ? true : nil
    end

    # Invoked on completion of ThreadPoolExecutor task
    # @!visibility private
    # @return [void]
    def task_observer(time, output, thread_error)
      GoodJob.on_thread_error.call(thread_error) if thread_error && GoodJob.on_thread_error.respond_to?(:call)
      instrument("finished_job_task", { result: output, error: thread_error, time: time })
      create_task if output
    end

    def warm_cache
      return if @max_cache.zero?

      @performer.next_at(
        limit: @max_cache,
        now_limit: @pool_options[:max_threads]
      ).each do |scheduled_at|
        create_thread({ scheduled_at: scheduled_at })
      end
    end

    def stats
      {
        name: @performer.name,
        max_threads: @pool_options[:max_threads],
        active_threads: @pool_options[:max_threads] - @pool.ready_worker_count,
        available_threads: @pool.ready_worker_count,
        max_cache: @max_cache,
        active_cache: cache_count,
        available_cache: remaining_cache_count,
      }
    end

    private

    attr_reader :timer_set

    def create_pool
      instrument("scheduler_create_pool", { performer_name: @performer.name, max_threads: @pool_options[:max_threads] }) do
        @timer_set = Concurrent::TimerSet.new
        @pool = ThreadPoolExecutor.new(@pool_options)
      end
    end

    def create_task(delay = 0)
      future = Concurrent::ScheduledTask.new(delay, args: [@performer], executor: @pool, timer_set: timer_set) do |performer|
        output = nil
        Rails.application.executor.wrap { output = performer.next }
        output
      end
      future.add_observer(self, :task_observer)
      future.execute
    end

    def instrument(name, payload = {}, &block)
      payload = payload.reverse_merge({
                                        scheduler: self,
                                        process_id: GoodJob::CurrentExecution.process_id,
                                        thread_name: GoodJob::CurrentExecution.thread_name,
                                      })

      ActiveSupport::Notifications.instrument("#{name}.good_job", payload, &block)
    end

    def cache_count
      timer_set.instance_variable_get(:@queue).length
    end

    def remaining_cache_count
      @max_cache - cache_count
    end

    # Custom sub-class of +Concurrent::ThreadPoolExecutor+ to add additional worker status.
    # @private
    class ThreadPoolExecutor < Concurrent::ThreadPoolExecutor
      # Number of inactive threads available to execute tasks.
      # https://github.com/ruby-concurrency/concurrent-ruby/issues/684#issuecomment-427594437
      # @return [Integer]
      def ready_worker_count
        synchronize do
          if Concurrent.on_jruby?
            @executor.getMaximumPoolSize - @executor.getActiveCount
          else
            workers_still_to_be_created = @max_length - @pool.length
            workers_created_but_waiting = @ready.length
            workers_still_to_be_created + workers_created_but_waiting
          end
        end
      end
    end
  end
end