From: "Gerard A.W. Vreeswijk" <gv@...>
Date: 2003-06-03T01:06:48+09:00
Subject: Programming generators with threads

Hi, in Rubytalk 55895 [
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/55895 ] Matz
wrote that generators can be written with Threads as well as with
Callcc.  He then gave an example with Calcc.  Works fine.  Here is my
example with Threads.  But I'm not sure whether it is good programming.
Is it free of race conditions, for example?  Thanks.

module Math
   def Math.divisible(number)
      square_root = Math.sqrt(number).floor
      for divisor in 2..square_root
         return true if number % divisor==0
      end
      return false
   end
end

class PrimeGenerator

   def initialize
      @thread = Thread.new { # create a separate thread
         loop do # this thread is an infinite loop..

            Thread.stop # going to sleep, goodnight everybody

            # Uh oh ... Brrr.  Where am I?
            # I have been woken up and scheduled to run.
            # See (*)

            Thread.critical = true # take exclusive right to run
            # this right is necessary, because I must be kept awake
            # by scheduler until next prime is computed
            complex_computation # do something interesting..
            Thread.critical = false # release exclusive right
         end
      }
      @number = 1
   end

   def next
      @thread.run
      return @number
   end

   def complex_computation
      @number += 1
      @number += 1 while Math.divisible(@number)
   end

end

p = PrimeGenerator.new
puts p.next #> 2
puts p.next #> 3
puts p.next #> 5
puts p.next #> 7