From: Sam Duncan <sduncan@...>
Date: 2011-11-14T05:14:27+09:00
Subject: Re: model "lift-passengers" and concurrent programming

On 14/11/11 08:51, Roman Faceless wrote:
> Hi,
>
> I'm trying to write a model of an elevator with one button. Elevator can
> take passengers along the way when going down.
>
> But in my code some error. Because passengers press a button only when
> the elevator carries passengers. And no passengers push the button when
> the elevator is free.
>
> I took a producer-consumer model.
>
>
> require 'semaphore'
> t0 = Time.now
>
> max=5#the maximum number of passengers
> c = 10#number of iterations
>
> tp = []#producer
> tc = []#consumer
>
>
> ffrom=Array.new(max)#floor, where the button is pressed
> fto=Array.new(max)#floor, which requires the passenger
>
> prod_from=[]#producer: floor where the passenger has been taken
> prod_to=[]#producer:floor where the passenger has arrived
> cons_from=[]#consumer: floor where the passenger has been taken
> cons_to=[]#floor where the passenger has arrived
>
> buf = Array.new(max)#buffer of lift
> s1 = Semaphore.new(buf.size)#overflow protection
> s2 = Semaphore.new(0)
> m  = Semaphore.new(1)
> first=true
>
> producer = Thread.new do#producer
>    i = 0
>    j=0
>    c.times do
>      #sleep(rand(5)%5)
>      good_pas=false
>      m.P
>      d = rand(8)+1
>      puts "button is pressed on the #{d}th floor" unless(first)
>      unless(first)then
>        if((d>fto[j])and(d<=ffrom[j]))then
>          good_pas=true
>          puts "passenger can be allowed\n"
>        else
>          puts "passenger can't be allowed\n"
>        end
>      end
>      j = ( j + 1 ) % buf.size unless(first)
>      m.V unless(good_pas)
>      ffrom[i]=d
>      fto[i]=rand(8)+1;
>      first=false
>      while(fto[i]==ffrom[i])do fto[i]=rand(8)+1;  end
>      prod_from<<ffrom[i]
>      prod_to<<fto[i]
>      tp<<  d
>      s1.P
>      m.P unless(good_pas)
>      puts "\nfrom the #{ffrom[i]}th floor passenger came and asked for
> #{fto[i]}th floor"
>      buf[i] = d
>      m.V
>      s2.V
>      i = ( i + 1 ) % buf.size
>    end
> end
>
> consumer = Thread.new do#consumer
>    i = 0
>    c.times do
>      s2.P
>      m.P
>      d = buf[i]
>      puts "\npassenger arrived from the #{ffrom[i]}th floor to the
> #{fto[i]}th floor"
>      cons_from<<ffrom[i]
>      cons_to<<fto[i]
>      m.V
>      s1.V
>      tc<<  d
>      i = ( i + 1 ) % buf.size
>    end
> end
>
> producer.join
> consumer.join
>
> puts "\nproducer - floors where the passenger went into the
> lift:\n\t["+prod_from.join(',')+"]"
> puts "produser - floor where the passenger came out of the
> lift:\n\t["+prod_to.join(',')+"]"
> puts "consumer - floor where the passenger went into the
> lift:\n\t["+cons_from.join(',')+"]"
> puts "consumer - floor where the passenger came out of the
> lift:\n\t["+cons_to.join(',')+"]"
>
> puts "\n\n:prod_from==cons_from: #{prod_from==cons_from}"
> puts "\nprod_to==cons_to: #{prod_to==cons_to}"
> puts "time: #{Time.now - t0}"
>
>
> Can someone give me some advice about rhis problem?
>
Can you post your semaphore implementation?

Sam