6.096 10-08-97 RECITATION:
Finite State Machines (FSM)

scribe: Aparna Das


FSM
- finite number of ststaes
- finite number of possible inputs
- finite number of possible outputs
- transition function
- output function

Soda Machine
- accepts coins
- waits for enough coins
- lights the soda buttons
- accepts press of soda button
- releases soda
- returns change

Inputs:
- money
- soda button

Outputs:
- light
- soda
- return coins
- display current money status
- processing: count money


Soda Machine:

public interface SodaMachine
{
      // constants
      public static final int SODA_BTN = 0;
      public static final int PRICE = 60;

      /*
      method: getInput() returns the next input
      if the return value > 0, it refers to the amount of money
      if there is no available input right now, then getInput waits
      */
      public int getInput();

      public void showLights (boolean on);
      public void showDisplay(String s) ;
      public void giveChange (int howMuch);
      public void releaseSoda();
}



Soda Controller:

public class SodaCtrl implements Runnable
{
      //fields
     	private Thread sprite;
      private SodaMachine soda;

      private int currentMoney = 0;
      private int currentState = WAIT_STATE;
      // constants for state
      private static final int WAIT_STATE = 0;
      private static final int TAKING_STATE = 1;
      private static final int ENOUGH_STATE = 2;

      /* constructor allows SodaCtrl class to access the 
       * methods of the SodaMachine class
       */
      public SodaCtrl (SodaMachine s)
      {
		      //label soda gets stuck on SodaMachine with value s
         	this.soda = s;

            this.sprite = new Thread (this);
            /* new Thread named sprite created so sprite can 
             * execute the self-animated SodaCtrl class sequence
             */
            this.sprite.start();
		      //start Thread sprite
      } 
          
      public void run()
      {
            while (true)
		      //i.e., always
            {
                	switch (currentState)
                  {
				            case WAIT_STATE:
                              actWaitState();
                              break;
                        case TAKING_STATE:
                              actTakingState();
                              break;
                        case ENOUGH_STATE:
                              actEnoughState();
                              break;
                        default: 
                              System.out.println("invalid state");
                  }
           	}
    	}

	   private void actWaitState()
      {
            // display outputs
          	this.soda.showLights (false);
            this.soda.showDisplay ("BUY ME");

            //get input
            int input = soda.getInput();

            // process input
            if (input > 0)
            {
                  //got money

                  //add money to currentMoney
                	currentMoney += input;
                  if (currentMoney >= SodaMachine.PRICE)
                  {
                    	currentState = ENOUGH_STATE;
                  } 
			         else
                  {
                     currentState = TAKING_STATE;
                  }
            }
            else
            {
                  // do nothing and stay in the same state
            }
	   }

      private void actTakingState()
      {
            //display output
            this.soda.showLights (false);
            this.soda.showDisplay (currentMoney + "");

            //get input
            int input = soda.getInput();

            // process input
            if (input > 0)
            {
                  //got money

                  //add money to currentMoney
                	currentMoney += input;
                  if (currentMoney >= SodaMachine.PRICE)
                  {
                    	currentState = ENOUGH_STATE;
                  } 
			         else
                  {
                     currentState = TAKING_STATE;
                  }
            }
            else
            {
                  // do nothing and stay in the same state
            }
    	}            

      private void actEnoughState()
      {
            //display outputs
          	this.soda.showLights (true);
            this.soda.showDisplay (currentMoney + "");

            //get input
            int input = soda.getInput();

            //process input
            if (input == SodaMachine.SODA_BTN)
            {
                  // soda button was pressed
                	//add money to currentMoney
                  if (currentMoney >= SodaMachine.PRICE)
                  {
                     	soda.releaseSoda();
                        soda.giveChange(currentMoney-SodaMachine.PRICE);
                        currentMoney = 0;
                        currentState = WAIT_STATE;
                  }
            } 
            else if (input > 0)
            {
                  //got money

                  currentMoney += input;
                 	currentState = ENOUGH_STATE;
            }
            else
            {
                  // do nothing and stay in the same state
            }
     	}
}