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Datei: Till.vdmpp Sprache: VDM

Original von: VDM^©

\section{The Class Till}

This class models a till. A till is connected to a central resource
and holds a number of retained cards, which have not been returned to
a user of the till. It may hold a current card and it has an attribute
to say whether the current card and its PIN code have been validated
successfully. In this version of the till we assume that the central
resource will always become available within a reasonable time limit.

\begin{vdm_al}
class Till

instance variables
  curCard : [Card] := nil;
  cardOk : bool := false;
  retainedCards : set of Card := {};
  resource : CentralResource;

  inv curCard = nil => not cardOk;
\end{vdm_al}

The invariant says that if there is no card in the till then the
validation status of the current card shall be false.

\begin{vdm_al}
operations
  public Till: CentralResource ==> Till
  Till(res) ==
    resource := res;
\end{vdm_al}

The Create operation is used once in a till object's lifetime in order to set up a connection to a central resource.

\begin{vdm_al}
  public InsertCard : Card ==> ()
  InsertCard(c) ==
    curCard := c
  pre not CardInside();
\end{vdm_al}

The InsertCard operation models the activity of inserting a card into
the till. This cannot be done if the till holds a card already, which
is documented in the precondition.

\begin{vdm_al}
  public Validate : Card`PinCode ==> <PinOk> | <PinNotOk> | <Retained>
  Validate(pin) ==
    let cardId = curCard.GetCardId(),
        codeOk = curCard.GetCode() = Encode(pin),
        cardLegal = IsLegalCard()
    in
      (cardOk := codeOk and cardLegal;
       if not cardLegal then
         (retainedCards := retainedCards union {curCard};
          curCard := nil;
          return <Retained>)
       elseif codeOk then
         resource.ResetNumberOfTries(cardId)
       else
         (resource.IncrNumberOfTries(cardId);
          if resource.NumberOfTriesExceeded(cardId) then
            (retainedCards := retainedCards union {curCard};
             cardOk := false;
             curCard := nil;
             return <Retained>));
       return if cardOk
              then <PinOk>
              else <PinNotOk>)
  pre CardInside() and not cardOk;
\end{vdm_al}

The operation Validate is used to validate a PIN code and to check
that a card is not illegal. The precondition ensures that the till
currently holds a card which has not yet been validated. If a card
turns out to be illegal, the machine keeps the card.

\begin{vdm_al}
  public ReturnCard : () ==> ()
  ReturnCard() ==
    (cardOk := false;
     curCard:= nil)
  pre CardInside();
\end{vdm_al}

The ReturnCard operation is useful to end user sessions, though it is
not mentioned in the requirements. It allows the user to perform more
than one transaction in each session, e.g. to first view the balance
and then make a withdrawal.

The following three operations are listed in the requirements
document. They all require that the till holds a card which has been
validated successfully. The operations are mirrored in the central
resource. The GetBalance operation return the value nil if it is not
possible to get the balance.

\begin{vdm_al}
  public GetBalance : () ==> [nat]
  GetBalance() ==
    resource.GetBalance(curCard.GetAccountId())
  pre CardValidated();
\end{vdm_al}

The MakeWithdrawal and RequestStatement operations returns a boolean
indicating whether the requested transactions could be completed.

\begin{vdm_al}
  public MakeWithdrawal : nat ==> bool
  MakeWithdrawal(amount) ==
    resource.Withdrawal
      (curCard.GetAccountId(),curCard.GetCardId(),amount)
  pre CardValidated();

  public RequestStatement : () ==> bool
  RequestStatement() ==
    resource.PostStatement(curCard.GetAccountId(),curCard.GetCardId())
  pre CardValidated();
\end{vdm_al}

The IsLegalCard operation below is only used internally to validate a card.

\begin{vdm_al}
  pure public IsLegalCard : () ==> bool
  IsLegalCard() ==
    return
      resource.IsLegalCard(curCard.GetAccountId(),curCard.GetCardId())
  pre CardInside();

  pure public CardValidated: () ==> bool
  CardValidated() ==
    return curCard <> nil and cardOk;

  pure public CardInside: () ==> bool
  CardInside() ==
    return curCard <> nil;

functions
\end{vdm_al}

The requirements say that the till should encode the PIN code before
comparing it to the code on the card. We have documented this
requirement in the function below, but not yet made a choice of
approach to this.

\begin{vdm_al}
  Encode: Card`PinCode +> Card`Code
  Encode(pin) ==
    pin; -- NB! The actual encoding procedure has not yet been chosen

end Till
\end{vdm_al}

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