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# Factorial States Example

Files:

- statemachine/factorial/main.cpp
- statemachine/factorial/factorial.pro

The Factorial States example shows how to use The State Machine Framework to calculate the factorial of an integer.

The statechart for calculating the factorial looks as follows:

In other words, the state machine calculates the factorial of 6 and prints the result.

     class Factorial : public QObject
     {
         Q_OBJECT
         Q_PROPERTY(int x READ x WRITE setX)
         Q_PROPERTY(int fac READ fac WRITE setFac)
     public:
         Factorial(QObject *parent = 0)
             : QObject(parent), m_x(-1), m_fac(1)
         {
         }

         int x() const
         {
             return m_x;
         }

         void setX(int x)
         {
             if (x == m_x)
                 return;
             m_x = x;
             emit xChanged(x);
         }

         int fac() const
         {
             return m_fac;
         }

         void setFac(int fac)
         {
             m_fac = fac;
         }

     Q_SIGNALS:
         void xChanged(int value);

     private:
         int m_x;
         int m_fac;
     };

The Factorial class is used to hold the data of the computation, `x` and `fac`. It also provides a signal that's emitted whenever the value of `x` changes.

     class FactorialLoopTransition : public QSignalTransition
     {
     public:
         FactorialLoopTransition(Factorial *fact)
             : QSignalTransition(fact, SIGNAL(xChanged(int))), m_fact(fact)
         {}

         virtual bool eventTest(QEvent *e)
         {
             if (!QSignalTransition::eventTest(e))
                 return false;
             QStateMachine::SignalEvent *se = static_cast<QStateMachine::SignalEvent*>(e);
             return se->arguments().at(0).toInt() > 1;
         }

         virtual void onTransition(QEvent *e)
         {
             QStateMachine::SignalEvent *se = static_cast<QStateMachine::SignalEvent*>(e);
             int x = se->arguments().at(0).toInt();
             int fac = m_fact->property("fac").toInt();
             m_fact->setProperty("fac",  x * fac);
             m_fact->setProperty("x",  x - 1);
         }

     private:
         Factorial *m_fact;
     };

The FactorialLoopTransition class implements the guard (`x` > 1) and calculations (`fac` = `x` * `fac`; `x` = `x` - 1) of the factorial loop.

     class FactorialDoneTransition : public QSignalTransition
     {
     public:
         FactorialDoneTransition(Factorial *fact)
             : QSignalTransition(fact, SIGNAL(xChanged(int))), m_fact(fact)
         {}

         virtual bool eventTest(QEvent *e)
         {
             if (!QSignalTransition::eventTest(e))
                 return false;
             QStateMachine::SignalEvent *se = static_cast<QStateMachine::SignalEvent*>(e);
             return se->arguments().at(0).toInt() <= 1;
         }

         virtual void onTransition(QEvent *)
         {
             fprintf(stdout, "%d\n", m_fact->property("fac").toInt());
         }

     private:
         Factorial *m_fact;
     };

The FactorialDoneTransition class implements the guard (`x` <= 1) that terminates the factorial computation. It also prints the final result to standard output.

     int main(int argc, char **argv)
     {
         QCoreApplication app(argc, argv);
         Factorial factorial;
         QStateMachine machine;

The application's main() function first creates the application object, a Factorial object and a state machine.

         QState *compute = new QState(&machine);
         compute->assignProperty(&factorial, "fac", 1);
         compute->assignProperty(&factorial, "x", 6);
         compute->addTransition(new FactorialLoopTransition(&factorial));

The `compute` state is created, and the initial values of `x` and `fac` are defined. A FactorialLoopTransition object is created and added to the state.

         QFinalState *done = new QFinalState(&machine);
         FactorialDoneTransition *doneTransition = new FactorialDoneTransition(&factorial);
         doneTransition->setTargetState(done);
         compute->addTransition(doneTransition);

A final state, `done`, is created, and a FactorialDoneTransition object is created with `done` as its target state. The transition is then added to the `compute` state.

         machine.setInitialState(compute);
         QObject::connect(&machine, SIGNAL(finished()), &app, SLOT(quit()));
         machine.start();

         return app.exec();
     }

The machine's initial state is set to be the `compute` state. We connect the QStateMachine::finished() signal to the QCoreApplication::quit() slot, so the application will quit when the state machine's work is done. Finally, the state machine is started, and the application's event loop is entered.