Added sptf.rb demo

demos/sptf.rb

 #!/usr/bin/env ruby
 
 require_relative '../lib/simplekit'
-require_relative '../lib/priority_queue'
-
-# Customer class to store arrival times and processing times.
-class Customer
-  attr_reader :arrival_time, :processing_time
-
-  def initialize(arrival, processing)
-    @arrival_time = arrival
-    @processing_time = processing
-  end
 
+# Create a Customer Struct that will store arrival times and processing times.
+Customer = Struct.new(:arrival_time, :processing_time) do
   include Comparable
+  # rank customers by their processing times, smallest first.
   def <=>(other)
     processing_time <=> other.processing_time
   end
@@ -41,6 +34,7 @@ class SPTF
     @numAvailableServers = @maxServers
     @q = PriorityQueue.new
     schedule(:arrival, 0.0)
+    schedule(:close_doors, @closeTime)
   end
 
 
@@ -69,21 +63,24 @@ class SPTF
   # the desired closing time, halt the simulation.
   def endService
     @numAvailableServers += 1
-    if (@q.empty?)
-      halt if model_time >= @closeTime
-    else
+    unless @q.empty?
       schedule(:beginService, 0.0)
     end
   end
 
+  # Commence shutdown by denying the next :arrival
+  def close_doors
+    cancel :arrival
+  end
+
   # Exponential random variate generator.
   # param: rate - The rate (= 1 / mean) of the distribution.
   # returns: A realization of the specified distribution.
   def exponential(rate)
     -Math.log(rand) / rate
   end
-
 end
 
 # Instantiate an SPTF object with a particular parameterization and run it.
-SPTF.new(4.5, 1.0, 5, 100.0).run
+srand(9876543)  # set seed for repeatability
+SPTF.new(6, 1.0, 5, 20.0).run