aivika-0.1: 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 Random
import Control.Monad
import Control.Monad.Trans

import Simulation.Aivika.Dynamics

upRate = 1.0 / 1.0       -- reciprocal of mean up time
repairRate = 1.0 / 0.5   -- reciprocal of mean repair time

specs = Specs { spcStartTime = 0.0,
                spcStopTime = 1000.0,
                spcDT = 1.0,
                spcMethod = RungeKutta4 }
        
exprnd :: Double -> IO Double
exprnd lambda =
  do x <- getStdRandom random
     return (- log x / lambda)
     
model :: Dynamics (Dynamics (Double, Double))
model =
  do queue <- newQueue
     
     -- number of times the machines have broken down
     nRep <- newRef queue 0 
     
     -- number of breakdowns in which the machine 
     -- started repair service right away
     nImmedRep <- newRef queue 0
     
     -- total up time for all machines
     totalUpTime <- newRef queue 0.0
     
     repairPerson <- newResource queue 1
     
     pid1 <- newPID queue
     pid2 <- newPID queue
     
     let machine :: DynamicsProc ()
         machine =
           do startUpTime <- liftD time
              upTime <- liftIO $ exprnd upRate
              holdProc upTime
              finishUpTime <- liftD time
              liftD $ modifyRef' totalUpTime 
                (+ (finishUpTime - startUpTime))
              
              -- check the resource availability
              liftD $ modifyRef' nRep (+ 1)
              n <- resourceCount repairPerson
              when (n == 1) $
                liftD $ modifyRef' nImmedRep (+ 1)
                
              requestResource repairPerson
              repairTime <- liftIO $ exprnd repairRate
              holdProc repairTime
              releaseResource repairPerson
              
              machine
         
     runProc machine pid1 starttime
     runProc machine pid2 starttime
     
     let system :: Dynamics (Double, Double)
         system =
           do x <- readRef totalUpTime
              y <- stoptime
              n <- readRef nRep
              nImmed <- readRef nImmedRep
              return (x / (2 * y), 
                      fromIntegral nImmed / fromIntegral n)
     
     return system
  
main =         
  do a <- runDynamics1 model specs
     print a