13.2 Lotto example sketch

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The next example is an experiment on playing Lotto. In this simplified version of Lotto you have to guess seven different numbers in the interval from 1 to 34. You may submit one bet each week. At the end of the week the Lotto system chooses randomly seven different winner numbers, and the winning players are those that have submitted a bet with these winner numbers.

We use a MonitorSystem to represent the players and the Lotto. Lotto is represented by an object with MonitorProcess as superclass. A player is represented by an instance of class Player that is a subclass of MonitorProcess.

The Lotto system has the following overall structure:

LottoExperiment: obj MonitorSystem
   mnoOfPlayers: val 500
   betSize: val 7
   class Hand:
      :::
   class Bet(thePlayer: ref Player, theHand: ref Hand):
      :::
   class Player(inx: var integer): MonitorProcess
      :::
   Lotto: obj MonitorProcess
      :::
   Lotto.start
   generatePlayers: do
      ...

LottoExperiment has the following main attributes:

  • Constants noOfPlayers and betSize, which defines the number of Player objects and the size of a Hand in a Bet.
  • Class Hand which represents a hand of seven numbers.
  • Class Bet which represents a bet. It has parameters thePlayer, which is the Player that has submitted the Bet and theHand, which is the Hand of the Bet.
  • Class Player which represents a player.
  • An object Lotto, which represents the Lotto.
  • A statement that starts the Lotto process.
  • An object generatePlayers which is an object that generates the players.

Class Player has the following structure:

   class Player: MonitorProcess  
      cycle
         Lotto.submit(Bet(this(Player),Hand)
         doSomethingElse
  • A Player repeatedly submits a Bet by invoking Lotto.submit.
  • The argument of submit is the object Bet(this(Player), Hand).
  • The first argument of Bet is a reference to the Player submitting the Bet.
  • The second argument of Bet is a Hand which randomly generates 7 numbers.
  • After submitting a Bet, it executes doSomethingElse before playing the next time; doSomethingElse is not specified here.

The Lotto is represented by an object Lotto. This object keeps the bets being submitted, it has the winning bet, and the deadline for submitting bets. As players in parallel submit bets by calling the method submit, the Lotto object is defined as a MonitorProcess with submit as an entry method — similar to en entry-method of a Monitor object.

The structure of Lotto is as follows:

   Lotto: obj MonitorProcess
      submit(B: ref Bet): entry
         bets.insert(B)
      clearBets:
         bets.clear	 
      findWinningBets:
         :::
      bets: obj Set(#Bet)
      :::

Lotto has the following attributes:

  • An entry-method submit, which may be used by Player objects to submit bets. As mentioned, it works in the same way as an entry method of a Monitor. This means that at most one submit method may be executed at a given time.
  • The Bets being submitted are stored in bets which is a Set.
  • Methods clearBets and findWinningBets that are private to Lotto in the sense that they may not be invoked by objects (here Players) outside Lotto. As for Monitor, only methods being submethods of entry may be invoked from outside Lotto.

As said, a Lotto-period is a week. During a week the Players may submit bets. At the end of a week, Lotto stops accepting Bets via submit. It then invokes findWinningBets to find the possible winners of the week.

When possible winners have been found, Lotto informs the winners. It invokes clearBets to remove all elements from bets and open up for a new round of submissions. This is represented by the following statements of Lotto:

Lotto: obj MonitorProcess
   -"-
  run: do
      waitAndAccept(aWeek)
      findWinningBets
      clearBets
      restart(run)

The object run is executed forever:

  • When a MonitorProcess (here Lotto) is executing, it is not possible to execute an entry-method from outside.
  • The method waitAndAccept is defined as an attribute of MonitorProcess.
  • When waitAndAccept(aWeek), the MonitorProcess (Lotto) waits for a period – here defined by aWeek and while waiting it accepts entry-methods like submit.
  • When Lotto has waited for a week, waitAndAccept resumes execution and closes for execution of entry-methods like submit.
  • It then executes findWinningBets followed by clearBets.

When Lotto executes waitAndAccept, there may be Players that have invoked Lotto.submit. These invocations may be executed before waitAndAccept closes for execution of entry-methods. This means that a Player may submit a Bet after a week has passed. From a modeling point-of-view this may be ok since the invocation of submit has happened before a week has passed. If one does not consider this to be ok, the implementation of waitAndSubmit has to be able to block Players waiting to execute a submit.

We now show the details of findWinningBets:

      findWinningBets:     
         winningBet := Bet(Player("Winner"), Hand)
         bets.scan
             if (current.equal(winningBet)) :then
                "Winner: ".print
                current.print		
         newline
  • As mentioned, it starts by assigning possibleToSubmit to false.
  • It then generates a winningBet with a hypothetical Player called “Winner” and a Hand with seven random numbers as arguments
  • It then scans the submitted bets in bets and checks whether or not the current Bet is equal to the winningBet
  • If a winner is found, the Player of this Bet is printed.

Finally we show the structure of class Hand and class Bet:

   class Hand:
      numbers: obj Array(betSize,#integer)
      for (1):to(betSize):repeat
           numbers.put(random(1,34)):at[inx]

   class Bet(thePlayer: ref Player, theHand: ref Hand):
      equal(aBet ref Bet) -> B: var boolean:
         ...
  • Class Hand has an array numbers that contains the seven random numbers of the Hand.
  • It uses a for:to:repeat to generate and store seven random numbers in numbers.
  • Class Bet has an equal method in addition to its two parameters.

For completeness, all the code for the Lotto example is shown below:

LottoExperiment: obj MonitorSystem
   mnoOfPlayers: val 500
   betSize: val 7
   class Hand:
      numbers: obj Array(betSize,#integer)
      for (1):to(betSize):repeat
           numbers.put(random(1,34)):at[inx]
   class Bet(thePlayer: ref Player, theHand: ref Hand):
      equal(aBet ref Bet) -> B: var boolean:
         ...
   class Player(inx: var integer): MonitorProcess
       cycle
          Lotto.submit(Bet(this(Player),Hand)
          doSomethingElse
   Lotto: obj MonitorProcess
      submit(B: ref Bet): entry
         bets.insert(B)
      clearBets:
         bets.clear	 
      findWinningBets:
         winningBet := Bet(Player("Winner"), Hand)
            bets.scan
              if (current.equal(winningBet)) :then
                 "Winner: ".print
                 current.print		
              newline
      bets: obj Set(#Bet)
      run: do
         waitAndAccept(aWeek)
         findWinningBets
         clearBets
         restart(run)
   Lotto.start
   generatePlayers: do
      ...

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