queue_select.rb

require 'thwait'

# Synchronous hand-off between two threads.
# Both threads will block until a transfer is successful.
class Rendevous
  def initialize
    @mutex = Mutex.new
    @send_flag = ConditionVariable.new
    @recv_flag = ConditionVariable.new
    @container = []
  end

  def value
    @mutex.synchronize do
      while @container.empty?
        ::Kernel.raise @exception if instance_variable_defined? :@exception
        @recv_flag.wait @mutex
      end
      # can do this because the signal will only be acted after the mutex is
      # released, which is only after @container.pop
      @send_flag.signal
      @container.pop
    end
  end

  def value=( rhs )
    @mutex.synchronize do
      while !@container.empty?
        @send_flag.wait @mutex
      end
      ::Kernel.raise @exception if instance_variable_defined? :@exception
      @container << rhs
      @recv_flag.signal
    end
  end

  def raise( *args )
    @mutex.synchronize do
      @exception = (::Kernel.raise *args rescue $!)
      @recv_flag.broadcast
    end
  end
end

# maybe select is the wrong name, seeing as in posix, select is non-blocking
def Queue.select(*qs, &receiver)
  return enum_for(__method__, *qs) unless block_given?

  xchg = Rendevous.new

  consumer_threads = qs.map do |q|
    Thread.new do
      while item = q.pop
        xchg.value = item, q
      end
    end
  end

  # all queues finished, so wake threads waiting on xchg
  Thread.new do
    ThreadsWait.all_waits *consumer_threads
    xchg.raise StopIteration
  end

  # only need to make this decision once
  to_receiver =
  case receiver.arity
  when 1;     ->(item, _){receiver.call item}
  when -1, 2; ->(item, q){receiver.call item, q}
  else
    raise "unknown arity for receiver #{receiver.arity}"
  end

  loop do
    item, q = xchg.value
    to_receiver.call item, q
  end
end

def exercise
  q1 = SizedQueue.new 9
  q2 = SizedQueue.new 3
  q3 = SizedQueue.new 11

  prod1 = Thread.new{12.times{|i| item = "q1 << #{i}"; q1 << item ; i += 1}; q1 << nil}
  prod2 = Thread.new{15.times{|i| item = "q2 << #{i}"; q2 << item ; i += 1}; q2 << nil}
  prod3 = Thread.new{7.times{|i| item = "q3 << #{i}"; q3 << item ; i += 1}; q3 << nil}

  Queue.select(q1,q2,q3).each_with_index{|(item,q),index| puts [index,item,q].inspect}
end