class Async::Scheduler

def self.supported?
	true
end

def address_resolve(hostname)
	# On some platforms, hostnames may contain a device-specific suffix (e.g. %en0). We need to strip this before resolving.
	# See <https://github.com/socketry/async/issues/180> for more details.
	hostname = hostname.split("%", 2).first
	::Resolv.getaddresses(hostname)
end

def async(*arguments, **options, &block)
	Kernel::raise ClosedError if @selector.nil?
	
	task = Task.new(Task.current? || 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)
	
	# Console.logger.debug "Initial execution of task #{fiber} complete (#{result} -> #{fiber.alive?})..."
	return task
end

def block(blocker, timeout)
	# $stderr.puts "block(#{blocker}, #{Fiber.current}, #{timeout})"
	fiber = Fiber.current
	
	if timeout
		timer = @timers.after(timeout) do
			if fiber.alive?
				fiber.transfer(false)
			end
		end
	end
	
	begin
		@blocked += 1
		@selector.transfer
	ensure
		@blocked -= 1
	end
ensure
	timer&.cancel
end

def close
	# It's critical to stop all tasks. Otherwise they might be holding on to resources which are never closed/released correctly.
	until self.terminate
		self.run_once
	end
	
	Kernel.raise "Closing scheduler with blocked operations!" if @blocked > 0
	
	# We depend on GVL for consistency:
	# @guard.synchronize do
	
	# We want `@selector = nil` to be a visible side effect from this point forward, specifically in `#interrupt` and `#unblock`. If the selector is closed, then we don't want to push any fibers to it.
	selector = @selector
	@selector = nil
	
	selector&.close
	
	# end
	
	consume
end

def closed?
	@selector.nil?
end

def fiber(...)
	return async(...).fiber
end

def initialize(parent = nil, selector: nil)
	super(parent)
	
	@selector = selector || ::IO::Event::Selector.new(Fiber.current)
	@interrupted = false
	
	@blocked = 0
	
	@timers = ::Timers::Group.new
end

def interrupt
	@interrupted = true
	@selector&.wakeup
end

def io_read(io, buffer, length, offset = 0)
	@selector.io_read(Fiber.current, io, buffer, length, offset)
end

def io_wait(io, events, timeout = nil)
	fiber = Fiber.current
	
	if timeout
		timer = @timers.after(timeout) do
			fiber.raise(TimeoutError)
		end
	end
	
	# Console.logger.info(self, "-> io_wait", fiber, io, events)
	events = @selector.io_wait(fiber, io, events)
	# Console.logger.info(self, "<- io_wait", fiber, io, events)
	
	return events
rescue TimeoutError
	return false
ensure
	timer&.cancel
end

def io_write(io, buffer, length, offset = 0)
	@selector.io_write(Fiber.current, io, buffer, length, offset)
end

def kernel_sleep(duration = nil)
	if duration
		self.block(nil, duration)
	else
		self.transfer
	end
end

def process_wait(pid, flags)
	return @selector.process_wait(Fiber.current, pid, flags)
end

def push(fiber)
	@selector.push(fiber)
end

def raise(*arguments)
	@selector.raise(*arguments)
end

def resume(fiber, *arguments)
	@selector.resume(fiber, *arguments)
end

def run(...)
	Kernel::raise ClosedError if @selector.nil?
	
	initial_task = self.async(...) if block_given?
	
	@interrupted = false
	
	while self.run_once
		if @interrupted
			break
		end
	end
	
	return initial_task
ensure
	Console.logger.debug(self) {"Exiting run-loop because #{$! ? $! : 'finished'}."}
end

def run_once(timeout = nil)
	Kernel::raise "Running scheduler on non-blocking fiber!" unless Fiber.blocking?
	
	# If we are finished, we stop the task tree and exit:
	if self.finished?
		return false
	end
	
	interval = @timers.wait_interval
	
	# 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
	
	begin
		@selector.select(interval)
	rescue Errno::EINTR
		# Ignore.
	end
	
	@timers.fire
	
	# The reactor still has work to do:
	return true
end

def scheduler_close
	self.run
ensure
	self.close
end

def timeout_after(duration, exception, message, &block)
	with_timeout(duration, exception, message) do |timer|
		yield duration
	end
end

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

def transfer
	@selector.transfer
end

def unblock(blocker, fiber)
	# $stderr.puts "unblock(#{blocker}, #{fiber})"
	
	# This operation is protected by the GVL:
	if selector = @selector
		selector.push(fiber)
		selector.wakeup
	end
end

def with_timeout(duration, exception = TimeoutError, message = "execution expired", &block)
	fiber = Fiber.current
	
	timer = @timers.after(duration) do
		if fiber.alive?
			fiber.raise(exception, message)
		end
	end
	
	yield timer
ensure
	timer.cancel if timer
end

def yield
	@selector.yield
end