##
# EventLoop Module, providing main loop for events
module EventLoop
  class << self
    # Config EvnetLoop, call by default if any other methods called
    # @param [NIO::Selector] selector an event selector
    def config(selector = NIO::Selector.new)
      # Raw NIO Selector
      @selector = selector
      # Array of active timers
      @timers = []
      # Hash of io and its callback
      @ios = Hash.new
      # IO queue
      @queue = Hash.new
      # Root fiber
      @root_fiber = Fiber.current
      # Thread for timer
      @timer_thread = nil
      nil
    end
  end
end

EventLoop.config

module EventLoop
  class << self
    # Add timer in event loop
    # @param [EventLoop::Timer] timer timer to insert
    # @return [nil] nil
    def add_timer(timer)
      timer.start_time = Time.now.to_f + timer.time
      @timers << timer
      nil
    end

    def remove_timer(timer)
      @timers.delete(timer)
    end

    # Register I/O event with queue protection
    # @param [IO] io io to register
    # @param [Symbol] interest :r for read only, :w for write only, and :rw for both
    # @yield what to run when io callbacks
    # @return [nil] nil
    def register(io, interest=(:rw), &callback)
      if @queue[io.to_i].nil?
        @queue[io.to_i] = Array.new
        register_raw(io, interest, callback)
      else
        @queue[io.to_i] << [io, interest, callback]
      end
      nil
    end

    # Register I/O event directly, without any queue protection
    # @param [IO] io io to register
    # @param [Symbol] interest :r for read only, :w for write only, and :rw for both
    # @param [Proc] callback what to run when io callbacks
    # @return [nil] nil
    def register_raw(io, interest=(:rw), callback)
      @selector.register(io, interest)
      @ios[io] = { callback: callback }
      nil
    end

    # Deregister I/O event
    # @param [IO] io io to deregister
    # @return [nil] nil
    def deregister(io)
      @selector.deregister(io)
      @ios.delete(io)
      unless io.closed?
        # If the I/O closed accidentally, it should manually release the queue
        # Otherwise, there would be a memory leak
        fd = io.fileno
        next_register = @queue[fd].shift
        next_register.nil? ? @queue.delete(fd) : register_raw(*next_register)
      end
      nil
    end

    # Manually release queues of closed I/O
    # if the I/O has accidentally closed
    # @param [IO] io io to release
    # @return [nil] nil
    def release_queue(io)
      fd = io.fileno
      @queue.delete(fd)
      nil
    end

    # Run I/O selector once
    # @return [nil] nil
    def run_once
      @selector.select(0.2) do |monitor| # Timeout for 0.2 secs
        @ios[monitor.io][:callback].call(monitor)
      end
      nil
    end

    # Run timer once
    # @return [nil] nil
    def timer_once
      now_time = Time.now.to_f
      callbacks = []
      @timers.reject! do |timer|
        ticked = timer.start_time < now_time
        callbacks << timer.callback if ticked
        ticked
      end
      callbacks.each { |c| c.call }
      nil
    end

    # Start the event loop
    # @return [nil] nil
    def start
      return if running?
      @stop = false
      @timer_thread = Thread.new { loop { sleep 0.5; timer_once }}
      until @stop
        run_once
      end
      @stop = nil
    end

    # Set the stop flag
    # @return [nil] nil
    def stop
      @stop = true
      Thread.kill(@timer_thread)
      nil
    end

    # Detect the stop flag
    # @return [Boolean] return if eventloop is set to be stopped
    def running?
      @stop = true if @stop.nil?
      !@stop
    end

    def root_fiber
      @root_fiber
    end
  end
end