Macruby and Rubymotion GCD Semaphore example (The Dining Philosophers Problem)

semaphore.rb

#!/usr/bin/env macruby -wKU

class Philosopher
  
  def initialize(name, left_chopstick, right_chopstick)
    @name = name
    @left_chopstick = left_chopstick
    @right_chopstick = right_chopstick
    @meals = 0
  end
 
  def run
    while @meals < 5
      think
      dine
    end
    puts "philosopher: #@name is full!"
  end
 
  def think
    puts "philosopher: #@name is thinking..."
    sleep(rand())
    puts "philosopher: #@name is hungry..."
  end
 
  def dine
    chopstick1, chopstick2 = @left_chopstick, @right_chopstick
    while true
      pickup(chopstick1, :wait => true)
      puts "philosopher: #@name has chopstick #{chopstick1.id}..."
      break if pickup(chopstick2, :wait => false)
      puts "philosopher: #@name cannot pickup second chopstick #{chopstick2.id}..."
      release(chopstick1)
      chopstick1, chopstick2 = chopstick2, chopstick1
    end
    puts "philosopher: #@name has the second chopstick #{chopstick2.id}..."
 
    puts "philosopher: #@name eats..."
    sleep(rand())
    puts "philosopher: #@name belches"
    @meals += 1
 
    release(@left_chopstick)
    release(@right_chopstick)
  end
 
  def pickup(chopstick, opt)
    puts "philosopher: #@name attempts to pickup chopstick #{chopstick.id}..."
    chopstick.wait(opt)
  end
 
  def release(chopstick)
    puts "philosopher: #@name releases chopstick #{chopstick.id}..."
    chopstick.release
  end
end

# we just wrappe the semaphores
class Chopstick < Struct.new(:id, :semaphore)
  def release
    semaphore.signal
  end
  
  def wait(opt={})
    semaphore.wait(opt[:wait] ? Dispatch::TIME_FOREVER : Dispatch::TIME_NOW)
  end
end


philosophers = %W{Aristotle Kant Spinoza Marx Russell}
size = philosophers.length

chopsticks = Array.new(size) { |i| Chopstick.new(i+1, Dispatch::Semaphore.new(1)) }
queue = Dispatch::Queue.concurrent("de.mateus.philosophers.problem")

group = Dispatch::Group.new
 
philosophers.each_with_index do |name, idx|
  queue.async(group) do
    cs1 = chopsticks[idx]
    cs2 = chopsticks[(idx+1)%size]
    Philosopher.new(name, cs1, cs2).run
  end
end

# since we want to see the out put, he have to wait till all the dispatch tasks are done
group.wait