require 'delayed_job'

module JobsAutoscaling
  class Monitor
    IDLE = :idle
    BUSY = :busy

    def initialize(action: )
      @action = action
    end

    def activate!
      Delayed::Worker.lifecycle.after(:work_queue_pop, &method(:work_queue_pop))
      load_preexisting_jobs
      change_state(preexisting_jobs_running? ? BUSY : IDLE)
    end

    protected

    def load_preexisting_jobs
      @process_mtime_map = {}
      Delayed::Job.processes_locked_locally.each do |pid|
        mtime = Delayed::Worker::ProcessHelper.mtime(pid)
        @process_mtime_map[pid] = mtime if mtime
      end
    end

    def preexisting_jobs_running?
      @process_mtime_map.each do |pid, mtime|
        unless Delayed::Worker::ProcessHelper.process_is_still_running?(pid, mtime)
          @process_mtime_map.delete(pid)
        end
      end
      @process_mtime_map.any?
    end

    def work_queue_pop(work_queue, _worker_config)
      unless work_queue.respond_to?(:all_workers_idle?)
        return
      end

      new_state = work_queue.all_workers_idle? && !preexisting_jobs_running? ? IDLE : BUSY
      if new_state != @worker_state
        change_state(new_state)
      end
    end

    def change_state(new_state)
      @worker_state = new_state
      @action.public_send(new_state)
    end
  end
end