eventizzle.rb

class EventList
  Event = Struct.new :type, :time
  
  attr_reader :events
  
  def initialize
    @events = []
  end
  
  def push type, time
    @events.push Event.new type, time
  end
  
  def next
    @events = @events.sort_by &:time
    @events.shift
  end
end

class Host
  attr_reader :packets_dropped, :event_list
  attr_reader :buffer_length, :buffer_maximum
  
  def initialize mu, lambda, buffer_maximum = nil
    @mu, @lambda = mu, lambda
    @event_list     = EventList.new
    @buffer_maximum = buffer_maximum
    
    @time           = 0.0
    @buffer_length  = @packets_dropped = 0
    @last_departure = @time_previous   = 0.0
    @utilization    = @buffer_stat     = 0.0
    
    arrival # seed the first arrival
  end
  
  def nedt rate
    -1 / rate * Math.log(1 - rand)
  end
  
  def arrival
    event_list.push :arrival, @time + nedt(@lambda) # schedule the next arrival
    
    if buffer_free?
      @last_departure = next_departure
      event_list.push :departure, @last_departure # schedule the departure for this packet
      @buffer_length += 1
    else
      @packets_dropped += 1
    end
  end
  
  def departure
    @buffer_length -= 1
  end
  
  def next_departure
    [@time, @last_departure].max + nedt(@mu)
  end
  
  def buffer_free?
    !@buffer_maximum or @buffer_length <= @buffer_maximum
  end
  
  def buffer_used?
    @buffer_length > 0
  end
  
  def process_event
    event = event_list.next
    
    @time = event.time
    gather_statistics
    @time_previous = @time;
    
    send event.type
  end
  
  def gather_statistics
    @utilization += @time - @time_previous if buffer_used?
    @buffer_stat += @buffer_length * (@time - @time_previous);
  end
  
  def server_utilization
    @utilization / @time * 100
  end
  
  def mean_buffer_length
    @buffer_stat / @time
  end
end

host = Host.new 1, 0.3, 1
100_000.times do
  host.process_event
end

puts "Server utilization: #{host.server_utilization}"
puts "Mean buffer length: #{host.mean_buffer_length}"
puts "Dropped packets:    #{host.packets_dropped}"