aivika-4.0: examples/MachRep2.hs

-- It corresponds to model MachRep2 described in document 
-- Introduction to Discrete-Event Simulation and the SimPy Language
-- [http://heather.cs.ucdavis.edu/~matloff/156/PLN/DESimIntro.pdf]. 
-- SimPy is available on [http://simpy.sourceforge.net/].
--   
-- The model description is as follows.
--   
-- Two machines, but sometimes break down. Up time is exponentially 
-- distributed with mean 1.0, and repair time is exponentially distributed 
-- with mean 0.5. In this example, there is only one repairperson, so 
-- the two machines cannot be repaired simultaneously if they are down 
-- at the same time.
--
-- In addition to finding the long-run proportion of up time as in
-- model MachRep1, let’s also find the long-run proportion of the time 
-- that a given machine does not have immediate access to the repairperson 
-- when the machine breaks down. Output values should be about 0.6 and 0.67. 

import Control.Monad
import Control.Monad.Trans

import Simulation.Aivika

meanUpTime = 1.0
meanRepairTime = 0.5

specs = Specs { spcStartTime = 0.0,
                spcStopTime = 1000.0,
                spcDT = 1.0,
                spcMethod = RungeKutta4,
                spcGeneratorType = SimpleGenerator }
     
model :: Simulation Results
model =
  do -- number of times the machines have broken down
     nRep <- newRef 0 
     
     -- number of breakdowns in which the machine 
     -- started repair service right away
     nImmedRep <- newRef 0
     
     -- total up time for all machines
     totalUpTime <- newRef 0.0
     
     repairPerson <- newFCFSResource 1
     
     let machine :: Process ()
         machine =
           do upTime <-
                randomExponentialProcess meanUpTime
              liftEvent $
                modifyRef totalUpTime (+ upTime) 
              
              -- check the resource availability
              liftEvent $
                do modifyRef nRep (+ 1)
                   n <- resourceCount repairPerson
                   when (n == 1) $
                     modifyRef nImmedRep (+ 1)
                
              requestResource repairPerson
              repairTime <-
                randomExponentialProcess meanRepairTime
              releaseResource repairPerson
              
              machine

     runProcessInStartTime machine
     runProcessInStartTime machine

     let upTimeProp =
           do x <- readRef totalUpTime
              y <- liftDynamics time
              return $ x / (2 * y)

         immedProp :: Event Double
         immedProp =
           do n <- readRef nRep
              nImmed <- readRef nImmedRep
              return $
                fromIntegral nImmed /
                fromIntegral n

     return $
       results
       [resultSource
        "upTimeProp"
        "The long-run proportion of up time (~ 0.6)"
        upTimeProp,
        --
        resultSource
        "immedProp"
        "The proption of time of immediate access (~0.67)"
        immedProp]
  
main =
  printSimulationResultsInStopTime
  printResultSourceInEnglish
  model specs