class NIO::Selector

def select(timeout = nil)
  selected_monitors = Set.new
  @lock.synchronize do
    readers = [@wakeup]
    writers = []
    @selectables.each do |io, monitor|
      readers << io if monitor.interests == :r || monitor.interests == :rw
      writers << io if monitor.interests == :w || monitor.interests == :rw
      monitor.readiness = nil
    end
    ready_readers, ready_writers = Kernel.select(readers, writers, [], timeout)
    return unless ready_readers # timeout
    ready_readers.each do |io|
      if io == @wakeup
        # Clear all wakeup signals we've received by reading them
        # Wakeups should have level triggered behavior
        @wakeup.read(@wakeup.stat.size)
      else
        monitor = @selectables[io]
        monitor.readiness = :r
        selected_monitors << monitor
      end
    end
    ready_writers.each do |io|
      monitor = @selectables[io]
      monitor.readiness = monitor.readiness == :r ? :rw : :w
      selected_monitors << monitor
    end
  end
  if block_given?
    selected_monitors.each { |m| yield m }
    selected_monitors.size
  else
    selected_monitors.to_a
  end
end