require "concurrent/executor/thread_pool_executor"
require "concurrent/timer_task"
require "concurrent/utility/processor_counter"

module GoodJob
  class Scheduler
    DEFAULT_TIMER_OPTIONS = {
      execution_interval: 1,
      timeout_interval: 1,
      run_now: true,
    }.freeze

    DEFAULT_POOL_OPTIONS = {
      name: 'good_job',
      min_threads: 0,
      max_threads: Concurrent.processor_count,
      auto_terminate: true,
      idletime: 0,
      max_queue: 0,
      fallback_policy: :abort, # shouldn't matter -- 0 max queue
    }.freeze

    def initialize(performer, timer_options: {}, pool_options: {})
      raise ArgumentError, "Performer argument must implement #next" unless performer.respond_to?(:next)

      @performer = performer
      @pool = Concurrent::ThreadPoolExecutor.new(DEFAULT_POOL_OPTIONS.merge(pool_options))
      @timer = Concurrent::TimerTask.new(DEFAULT_TIMER_OPTIONS.merge(timer_options)) do
        idle_threads = @pool.max_length - @pool.length
        create_thread if idle_threads.positive?
      end
      @timer.add_observer(self, :timer_observer)
      @timer.execute
    end

    def execute
    end

    def shutdown(wait: true)
      @_shutdown = true

      ActiveSupport::Notifications.instrument("scheduler_start_shutdown.good_job", { wait: wait })
      ActiveSupport::Notifications.instrument("scheduler_shutdown.good_job", { wait: wait }) do
        if @timer.running?
          @timer.shutdown
          @timer.wait_for_termination if wait
        end

        if @pool.running?
          @pool.shutdown
          @pool.wait_for_termination if wait
        end
      end
    end

    def shutdown?
      @_shutdown
    end

    def create_thread
      future = Concurrent::Future.new(args: [@performer], executor: @pool) do |performer|
        result = nil
        Rails.application.executor.wrap { result = performer.next }
        result
      end
      future.add_observer(self, :task_observer)
      future.execute
    end

    def timer_observer(time, executed_task, error)
      ActiveSupport::Notifications.instrument("finished_timer_task.good_job", { result: executed_task, error: error, time: time })
    end

    def task_observer(time, result, error)
      ActiveSupport::Notifications.instrument("finished_job_task.good_job", { result: result, error: error, time: time })
      create_thread if result
    end
  end
end