-- It corresponds to model MachRep1 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, which sometimes break down.
-- Up time is exponentially distributed with mean 1.0, and repair time is
-- exponentially distributed with mean 0.5. There are two repairpersons,
-- so the two machines can be repaired simultaneously if they are down
-- at the same time.
--
-- Output is long-run proportion of up time. Should get value of about
-- 0.66.

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 Double
model =
  do totalUpTime <- newRef 0.0
     
     let machine :: Process ()
         machine =
           do upTime <-
                liftParameter $
                randomExponential meanUpTime
              holdProcess upTime
              liftEvent $ 
                modifyRef totalUpTime (+ upTime)
              repairTime <-
                liftParameter $
                randomExponential meanRepairTime
              holdProcess repairTime
              machine

     runProcessInStartTime machine
     runProcessInStartTime machine
     
     runEventInStopTime $
       do x <- readRef totalUpTime
          y <- liftParameter stoptime
          return $ x / (2 * y)
  
main = runSimulation model specs >>= print