class Async::Reactor

def self.run(*arguments, **options, &block)
	if current = Task.current?
		reactor = current.reactor
		
		return reactor.async(*arguments, **options, &block)
	else
		reactor = self.new
		
		begin
			return reactor.run(*arguments, **options, &block)
		ensure
			reactor.close
		end
	end
end

def self.selector
	if backend = ENV['ASYNC_BACKEND']&.to_sym
		if NIO::Selector.backends.include?(backend)
			return NIO::Selector.new(backend)
		else
			warn "Could not find ASYNC_BACKEND=#{backend}!"
		end
	end
	
	return NIO::Selector.new
end

def << fiber
	@ready << fiber
end

def async(*arguments, **options, &block)
	task = Task.new(self, **options, &block)
	
	# I want to take a moment to explain the logic of this.
	# When calling an async block, we deterministically execute it until the
	# first blocking operation. We don't *have* to do this - we could schedule
	# it for later execution, but it's useful to:
	# - Fail at the point of the method call where possible.
	# - Execute determinstically where possible.
	# - Avoid scheduler overhead if no blocking operation is performed.
	task.run(*arguments)
	
	# logger.debug "Initial execution of task #{fiber} complete (#{result} -> #{fiber.alive?})..."
	return task
end

def block(blocker, timeout)
	fiber = Fiber.current
	
	if timeout
		timer = self.after(timeout) do
			if fiber.alive?
				fiber.resume(false)
			end
		end
	end
	
	begin
		@blocked += 1
		Fiber.yield
	ensure
		@blocked -= 1
	end
ensure
	timer&.cancel
end

def close
	# This is a critical step. Because tasks could be stored as instance variables, and since the reactor is (probably) going out of scope, we need to ensure they are stopped. Otherwise, the tasks will belong to a reactor that will never run again and are not stopped.
	self.stop(false)
	
	@selector.close
	@selector = nil
end

def closed?
	@selector.nil?
end

def fiber(&block)
	if @scheduler
		Fiber.new(blocking: false, &block)
	else
		Fiber.new(&block)
	end
end

def finished?
	# TODO I'm not sure if checking `@running.empty?` is really required.
	super && @ready.empty? && @running.empty? && @blocked.zero?
end

def initialize(parent = nil, selector: self.class.selector, logger: nil)
	super(parent)
	
	@selector = selector
	@timers = Timers::Group.new
	@logger = logger
	
	@ready = []
	@running = []
	
	if Scheduler.supported?
		@scheduler = Scheduler.new(self)
	else
		@scheduler = nil
	end
	
	@interrupted = false
	@guard = Mutex.new
	@blocked = 0
	@unblocked = []
end

def interrupt
	@guard.synchronize do
		unless @interrupted
			@interrupted = true
			@selector.wakeup
		end
	end
end

def logger
	@logger || Console.logger
end

def register(io, interest, value = Fiber.current)
	monitor = @selector.register(io, interest)
	monitor.value = value
	
	return monitor
end

def run(*arguments, **options, &block)
	raise RuntimeError, 'Reactor has been closed' if @selector.nil?
	
	@scheduler&.set!
	
	initial_task = self.async(*arguments, **options, &block) if block_given?
	
	while self.run_once
		# Round and round we go!
	end
	
	return initial_task
ensure
	@scheduler&.clear!
	logger.debug(self) {"Exiting run-loop because #{$! ? $! : 'finished'}."}
end

def run_once(timeout = nil)
	# logger.debug(self) {"@ready = #{@ready} @running = #{@running}"}
	
	if @ready.any?
		# running used to correctly answer on `finished?`, and to reuse Array object.
		@running, @ready = @ready, @running
		
		@running.each do |fiber|
			fiber.resume if fiber.alive?
		end
		
		@running.clear
	end
	
	unless @blocked.zero?
		unblocked = Array.new
		
		@guard.synchronize do
			unblocked, @unblocked = @unblocked, unblocked
		end
		
		while fiber = unblocked.pop
			fiber.resume if fiber.alive?
		end
	end
	
	if @ready.empty?
		interval = @timers.wait_interval
	else
		# if there are tasks ready to execute, don't sleep:
		interval = 0
	end
	
	# If we are finished, we stop the task tree and exit:
	if self.finished?
		return false
	end
	
	# If there is no interval to wait (thus no timers), and no tasks, we could be done:
	if interval.nil?
		# Allow the user to specify a maximum interval if we would otherwise be sleeping indefinitely:
		interval = timeout
	elsif interval < 0
		# We have timers ready to fire, don't sleep in the selctor:
		interval = 0
	elsif timeout and interval > timeout
		interval = timeout
	end
	
	# logger.info(self) {"Selecting with #{@children&.size} children with interval = #{interval ? interval.round(2) : 'infinite'}..."}
	if monitors = @selector.select(interval)
		monitors.each do |monitor|
			monitor.value.resume
		end
	end
	
	@timers.fire
	
	# We check and clear the interrupted flag here:
	if @interrupted
		@guard.synchronize do
			@interrupted = false
		end
		
		return false
	end
	
	# The reactor still has work to do:
	return true
end

def sleep(duration)
	fiber = Fiber.current
	
	timer = @timers.after(duration) do
		if fiber.alive?
			fiber.resume
		end
	end
	
	Task.yield
ensure
	timer.cancel if timer
end

def stop(later = true)
	@children&.each do |child|
		# We don't want this process to propagate `Async::Stop` exceptions, so we schedule tasks to stop later.
		child.stop(later)
	end
end

def stopped?
	@children.nil?
end

def to_s
	"\#<#{self.description} #{@children&.size || 0} children (#{stopped? ? 'stopped' : 'running'})>"
end

def unblock(blocker, fiber)
	@guard.synchronize do
		@unblocked << fiber
		@selector.wakeup
	end
end

def with_timeout(timeout, exception = TimeoutError)
	fiber = Fiber.current
	
	timer = @timers.after(timeout) do
		if fiber.alive?
			error = exception.new("execution expired")
			fiber.resume error
		end
	end
	
	yield timer
ensure
	timer.cancel if timer
end

def yield(fiber = Fiber.current)
	@ready << fiber
	
	Fiber.yield
end